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ELEMENTARY TREATISE

PHOTOTOPOGRAPHIC METHODS

AND INSTRUMENTS

INCLUDING A CONCISE REVIEW OF EXECUTED

PHOTOTOPOGRAPHIC SURVEFS AND OF

PUBLICATIONS ON THIS SUBJECT

J. A. FLEMER

Topographical EugiHoor

FIRST EDITION
HRST THOUSAND

NEW YORK

JOHN WILEY & SONS

London: CHAPMAN & HALL, Limited

1906

J. A. FLEMER

ROBBRT DRUMMOND, PRINTER,- NBW YORK

c^ 1-

. ^

98097

JUL 17 1906

SGcB

-6Z

PREFACE.

Light-rays, in addition to their heating and illuminating
qualities, have a chemical or so-called " actinic " power, char-
acterized by a general tendency to decompose certain chemical
compounds, some of which, for that very reason, being utilized
in photography.

The term photography, composed of two Greek words
phos, or phota, and grapho, means light-drawing or sun-
picture, and a photograph may be defined as a picture pro-
duced by the actinic action of light-rays (emanating from the
object to be pictured) upon a surface chemically prepared for
this particular purpose.

Although the origin of photography may be ascribed to the
alchemists of the sixteenth century, who had observed that chlo-
ride of silver becomes blackened when subjected sufficiently
long to the action of light-rays, still, photography as we know
that art to-day is not so very old.

In 1777 the Swedish chemist Scheele discovered that the
intensity of the actinic power of light-rays changed with the
quality of the light, inasmuch as argentic chloride would turn
black quicker when exposed to the rays of the violet end of the
solar spectrum.

Wedgewood and Davy probably produced the first '* sun-
pictures" obtained by the action of light-rays upon surfaces

IV PREFACE.

sensitized with nitrate of silver. Their pictures, however, were
not permanent — they were not " fixed."

Joseph Nicephore Niepce produced the first permanent sun-
pictures in 1 814 by a process known under the name of heliog-
raphy, and in 1824 Daguerre commenced his researches and
experiments which eventually (after Niepce and Daguerre had
formed a copartnership in 1829) ^^ ^^ ^^ perfection of the
so-called " daguerreotj^pe."

In 1839, while alluding to the details of the daguerreotype
process before the Chamber of Deputies, in Paris, Arago declared:
..." Nous pourrions, par exemplc, parler de quelques id^cs
qu'on a eu sur les moyens rapides d'investigation, que le topo-
graphe pourra emprunter k la photographic." . . .

Gay Lussac expressed himself in a similar manner when
he had occasion to refer to the possibilities in adapting photog-
raphy to topographical sur\'eys. At about the same period, or
possibly a little earlier. Fox Talbot read a paper on photogenic
drawings before the Royal Society in London, describing his
method for obtaining silhouettes, or shadowgraphs, of objects
on paper that first had been treated with a solution of common
table salt, then dried and immersed in a solution of nitrate of
silver. The salt absorbed by the paper converts a part of the
nitrate of silver deposit into chloride of silver, some of the silver
nitrate remaining unaffected in the paper. Talbot fixed these
silhouettes on the paper by treating the outline pictures, with a
solution of bromide of potassium.

In 1 841 Talbot had perfected his method to reproduce objects
in general, and his method thereafter became known as the
Calotype or Talbotypc.

In 1 85 1 Scott Archer introduced the so-called '' wet collo-
dion process," which remained in general use during the fol-
lowing twenty-five years. The wet collodion process required
the plates to be coated immediately before use, first with collo-
dion iodide and again with the sensitizing silver bath, and imme-
diately after exposure they had to be developed and fixed. For

PREFACE. 7

outdoor photography a dark- tent or a dark-room wagon had to
be provided and the necessary chemicals had to be carried along
in the field, together with the camera, plates, and plate-holders.
The old photographic cameras, moreover, were unwieldy and
cumbersome, and to apply photography on exploring expeditions
and travels generally required such an increase in baggage,
not to mention the need of special expert knowledge in photo-
graphic chemistry, that amateur photography was practically
unknown during the period when the wet-plate process flourished.

All these difficulties, restricting the practice of photography
to professionals, were overcome when, in 1871, Dr. Maddox
discovered the so-called " dry-plate process " and published the
details of his " gelatine-emulsion dry-plate coating " for photo-
graphic plates.

Dr. Maddox had succeeded in preparing an emulsion of
bromide of silver in gelatine, with which plates could be coated
and dried in the dark-room until the film had become quite
hard, such plates remaining sensitive to the action of light-rays
long after the sensitized film had been applied. The present
general popularity of photography and its extensive application
in various branches of the sciences as well as in the arts dates
from Dr. Maddox's invention, which, in 1873, was improved by
Kennett.

Collodion has now been entirely replaced by gelatine for all
outdoor work and gelatino- bromide dry-plates are manufactured
in large quantities and in such variety as to answer all demands
and requirements of the different applications of photography
of the present day.

The method of photographic surveying as developed by
Col. A. Laussedat, who in his first experimental work used the
" camera clara,'' now became more widely known and Laussc-
dat's methods found practical application in several countries,
where they now receive a general recognition as valuable adjuncts
to the instrumental topographic methods.

Notwithstanding the recent rise in favor of photography,

VI PREFACE.

applied to the sciences in general and to topographic surveys in
particular, we still meet with many surveyors who seriously
question the practical value and general accuracy of photographic
surveys, either from misconception of their guiding principles,
from defective results so often obtained from a mere mechanical
application of phototopographic rules and methods, or more fre-
quently still, from extreme conservatism.

Others, again, may have failed to become interested in photo-
graphic surveying, believing that a thorough familiarity with
the theories and laws of optics, photochemistry, descriptive
geometry, perspective and general cartography are essentials
without which no practical knowledge and understanding of
photographic surveying may be obtained.

It should, of course, be admitted that such knowledge will
enable the student to master phototopography in a rapid and
easy manner, giving him an unquestioned advantage in, and an
enlarged field for, the practical application of photography to
surveying, or in teaching photogrammetric methods to others,
yet the fundamental principles underlying these methods arc so
simple that it is believed any topographer (with the knowledge
that he should have as such) may readily acquire enough of the
theory to become fully able to apply photography quite success-
fully to practical surveying, especially if he is familiar with the
methods of the plane table.

This treatise has been written primarily with a view towards
overcoming some of the existing prejudices against photographic
surveying, and if it aids to demonstrate that this branch of
surveying may rightly be assigned a legitimate place in the cur-
riculum of every modem topographer, filling as it does a par-
ticular gap in the general series of topographic methods here-
tofore recognized, its existence will be justified.

In the following pages reference will be made chiefly to
those photogrammetric methods which find application to topo-
graphic surveys, although the same principles are also used when
applying photography to:

PREFACE. Vll

Geological SurveySy made for the study of volcanic eruptions
and their effects; for the study of recurrent changes in sand-
dunes caused by winds blowing from certain directions at regu-
lar intervals; for noting changes in glaciers (glacial motion or
variation), based upon the comparison of glacier maps and
photographs obtained at stated time intervals from identical and
fixed camera stations, etc.

Meleorological Observations , for the study of the higher air-
currents and cloud altitudes, based upon iconometric cloud
charts obtained from photographic plates exposed at two or
more stations (simultaneously) at stated time intervals; for the
study of the paths of lightning, their lengths, forms, etc.

Hydrographic Surveys, for the location of rocks, buoys, cur-
rent floats, etc.; for the study of fluvial currents, riparian changes
due to corrasion, erosion, shoaling, or silting, etc.; for obtaining
coast views from points marked on the charts to serve for future
determinations of the positions of vessels that may sight the
land from the same locality in regions where fogs prevail; for
preliminary surveys of coastal belts while conducting a hydro-
graphic survey of the coast, harbors, etc.

Engineering Worksy for estimating the amount of work accom-
plished at any date, based on photographic records showing the
status of the work at specified dates, excavations, cuts, fills, struc-
tural buildings, etc.; for preliminary surveys to be used for the
location of roads, irrigation dams, canals, etc.

Architectural Surveys, for constructing the ground plans and
elevations of old buildings from their photographs; for purposes
of renovation, remodeling, publication, etc.

Military and Secret Surveys, for establishing the positions of
the enemy's forces; for locating fortifications; for establishing
range lines for artillery use; for obtaining topographic recon-
naissance maps, etc.

This treatise will indicate, in a general way, how photography
may be applied to topography by describing the simple processes
and methods, particularly those of a graphic character, that will

Vlll PREFACE.

suffice to direct beginners in their practical application, leaving
it to experience and subsequent special study to determine the
measure of success, the more so as several excellent works on
this subject have been published recently in the English, French,
German, Spanish, and Italian languages.

Preference should be given to the graphic solution of icono-
metric problems, inasmuch as topographic maps are primarily
graphical records of instrumental measurements made in the
field for locating the principal points of the terrene. Artificial
details and topographic features are largely sketched, or inter-
polated, when using the ordinary topographic surveying methods,
whereas this sketching may be reduced to a minimum when
applying photogranmietric methods by determining an increased
number of points that control the characteristic horizontal and
vertical changes in the terrene, which, in this case, are located
graphically upon the chart by means of iconometric transfers
from the photographic perspectives of the landscape.

The main control for a phototopographic survey should,
of course, be of trigonometric origin and the coordinates of the
triangulation points should be computed with a degree of accu-
racy commensurate with the degree of precision attained in the
field observations.

The writer, having consulted all available publications on
phototopographic methods and instruments in use, gladly acknowl-
edges his indebtedness for valuable information on this subject
to Col. A. Laussedat, Director of the Conservatoire des Arts
et Mftiers, Paris; Capt. E. Deville, Surveyor-general of Dominion
Lands, Ottawa, Canada; Dr. W. F. King, Alaskan Boundary
Commissioner to H. M., Ottawa; Signore L. P. Paganini, Engineer
Geographer of the Military Geographical Institute of Italy;
Dr. R. Doergens, Prof, of Geodesy, Royal Technical High School,
Charlottenburg, Prussia, and particularly to the following pub-
lications:

" La Fototopografia in Italia," L. P. Paganini, Rivista Marit-
tima, 1889;

PREFACE. IX

" Nuovi Appunti di Fototopografia," L. P. Paganini, Riv.
Marittima, 1894;

" Zeitschrift fuer Vermessungswesen,'* Stuttgart;

" Zeitschrift fuer Instrumentenkunde ";

" Lehrbuch der Praktischen Photographic," Dr. A. Miethe;

"Anthony's Photographic Journal";

" Die Photographische Messkunst," Prof. F. Schiffner,
Halle a./S., 1892;

" Die Anwendung der Photographie in der Practischen Mess-
kunst," E. Dolezal, Halle a./S., W. Knapp, 1896;

" Die Stereo-Photogrammetrie," Maj. von Huebl, Mitth.
des k. u. k. Militaer-Geographischen Inst., XII, 1903;

"Photographic Surveying," E. Deville, Ottawa, 1895;

" Photogrammetrie und Internationale Wolkenmessung," Dr.
C. Koppe, Braunschweig, 1896;

" Comptes Rendues de TAcad^mie des Sciences," Paris;

" Annales de TObservatoire M^t^orologique du Mont Blanc,"
J. Vallot, Paris, 1896;

" Recherches sur les Instruments, les M^thodes et le Dessin
Topographiques," Col. A. Laussedat, Paris, 1898.

CONTENTS.

PAOB

Introduction i

CHAPTER 1.

SHORT REVIEW OF EXECUTED PHOTOGRAPHIC SURVEYS AND USTS OP THE
MORE IMPORTANT PUBLICATIONS ON PHOTOGRAMMETRY AND PHOTO-
TOPOGRAPHY.

I. Photographic Surveying in France 5

Literature (French) 9

II Photographic Surveying in Germany 12

III. Photographic Surveying in Austria 15

Literature (German and Austrian) 16

IV. Photographic Surveying in Sweden 21

Literature (Swedish) 22

V. Photographic Survepng in Switzerland 23

VI. Photographic Surveying in Italy 25

VII. Photographic Surveying in Spain 27

V III Photographic Surveying in the Dominion of Canada and in Alaska . 29

Literature (English) ^^

CHAPTER II.

THE ELEB4ENTS OF PERSPECTIVE (CENTRAL PROJECTION).

I. Visual Seeing 35

II- Central Projection 36

m. Photographic Perspectives 36

XU CONTENTS.

CHAPTER in.

PIN-HOLE PHOTOGRAPHY.

PAGE

I. Diameter of the Pin-hole 42

II. Length of Exposure 43

III. Focal Lengths of Pin-hole Cameras 44

IV. Determination of the Values of the Pin-hole -camera Constants. . . 44

CHAPTER IV.

FUNDAMENTAL PRINCIPLES OF ICONOMETRIC MAP-PLOTTING ("ICONOMETRY")-

I. Orienting the Picture Traces on the Plotting-sheet 47

A. Iconometric Plotting when using a Surveying Camera only . 49

B. Plotting the Picture Traces when using a Camera or Photo-

theodolite "49

n. Arithmetical Determination of the Principal and of the Horizon

Line on the Photographic Perspectives 51

A. Determination of the Principal Point and Distance Line of

the Photographic Perspective 51

B. Determination of the Position of the Horizon Line on the

Perspective 5

in. Graphic Method for Determining the Positions of the Principal

and Horizon Lines on the Perspectives 54

IV. The "Five-point Problem" (by Prof. F. Steiner), or Locating
the Plotted Position of the Camera Station by means of the
Perspective when Five Triangulation Points are Pictured on
the Same Photographic Perspective 55

A. Determination of the Principal Point and Distance Line .... 55

B. Simplified Construction for Locating the Plotted Position

of the Camera Station by Means of the "Five-point
Problem" "..... 56

C. Application of the "Five-point Problem" to the Special Case,

where the Five Points range themselves into a Triangle

on the Working-sheet 57

D. To Find the Elevation of the Camera Horizon for a Station

that has been Located by Means of the "Five-point
Problem" 58

CONTENTS. xiii

PACE

V. The "Three-point Problem" 59

A. Mechanical Solution of the "Three-point Problem" (using a

Three-arm Protractor or Station-pointer) 60

B. Graphic Solution of the "Three-point Problem" 60

1. Using the so-called "Two-circle Method" 60

2. Using the Method of Bohnenberger and Bessel 6i

VI. The Orientation of Picture Traces, Based on Instrumental Measure-
ments Made in the Field 62

VII. Relations between Two Perspectives of the Same Object, Viewed

from Different Stations. (Prof. Guido Hauck's Method) . . 62

A. "Kernel Points" and "Kernel Planes" 64

B. Use of the "Perspective Axis" (Line of Intersection) of Two

Picture Planes that show Identical Objects Viewed from

Different Stations 66

VIII. To Plot a Figure, Situated in a Horizontal Plane, on the Ground

Plan by Means of its Perspective 68

IX. To Draw the Horizontal Projection of a Plane Figure on the Ground
Plan by Means of the so-called "Method of Squares," if its
Perspective in Vertical Plane and the Elements of the Per-
spective are given 72

X. The "Vanishing Scale" 74

CHAPTER V.

PHOTOGRAPHS ON INCLINED PLATES.

A. To Plot the Picture Trace of an Inclined Plate 78

B. Plotting the Lines of Direction to Points Pictured on an Inclined

Photographic Plate 79

C. Determination of the Altitudes of Points Pictured on an Inclined

Plate 80

D. Applications of Prof. Guido Hauck's Method 80

CHAPTER VI.

PHOTOTOPOGRAPHIC SURVEYING METHODS.

I. Analytical or Arithmetical Phototopographic Methods S^

A. Method of Prof W. Jordan S3

B. Method of Dr. G. Le Bon 86

XIV CONTENTS.

paob

G. Method of L. P. Paganini (Italian Method) 88

I. Determ'nation of the Focal Length of the Photographic

Perspective 88

a. When the Reference Point is Bisected by the Prin-

cipal Line of the Perspective 88

Example No. i 89

b. The Image of the Reference Point falls to either

Side of the Principal Line of the Photographic

Perspective 90

Example No. 2 91

Example No. 3 92

a. Orientation of the Picture Traces 92

3. Determination of the Elevations of Pictured Terrene

Points 95

Example No. 4 95

(of) Computation of the Focal Length (J) 98

(fi) Computation of the Abscissa for Plotting
Lines of Horizontal Directions to Pictured
Points of the Terrene and for Checking

the Position of the Principal Point 98

(f) Computation of the O d'nates of Pictured
Terrene Points of Known Elevations to
Check the Position of the Horizon Line on

the Negative 98

(d) Orienting a Panorama 99

4. Checking the Verticality of an Exposed Plate 100

Example No. 5 102

5. Application of Franz HaflFerrs Method for Finding the

Focal Length of a Photographic Perspective from

the Abscissae of Two Pictured Terrene Points 107

Example No. 6 108

Example No. 7 iii

6. Supplement 113

a. Forms Showing Arrangements of Field Records for

Panorama Views 113

b. Form used for Recording the Elevations of Second-

ary Points of the Panorama Views 1 15

D. General Arithmetical Method for Finding the Plotted Posi-

tions of Terrene Points when Pictured on Vertically Ex-
posed Picture Planes 115

E. General Arithmetical Method for Finding the Plotted Posi-

CONTENTS. XV

PAOB

tions of Terrene Points when Pictured on Inclined

Picture Planes 117

F. General Arithmetical Determination of the Elements of a

Photographic Perspective 119

XL Graphical Iconometrical Plotting Methods 121

A. Col. A. Laussedat's Method (French Method) 121

1. Orientation of the Picture Traces on the Plotting-sheet. 124

2. Locating Pomts on the Plotting-sheet that have been

Identified on Several Photographs 124

3. The Iconometric Determination of Elevations of Pic-

tured Terrene Points 125

4. Drawing the Plan Including Horizontal G>ntours 126 .

B. Method of Dr. A. Meydenbaur (German Method) 128

I. Determination of the Focal-length Value for the Photo-
graphic Perspective 128

3. Orientation of the Picture Traces on the Plotting-sheet. 130

3. Locating Points, Identified on Several Photographs, on

the Plotting-sheet 131

4. The Iconometric Determination of Elevations of Pic-

tiired Terrene Points 13a

C. Capt. E. Deville's Method (Canadian Method) 132

I. General Remarks on the Field-work 13a

a. General Remarks on the Iconometric Plotting of the

Survey 135

3. Orienting the Picture Traces on the Working-plan .... 137

4. The Identification of Pictured Points in Photographs

Representing Identical Points of the Terrene 138

5. Application of Prof. G. Hauck's Method for the Iden-

tification of Terrene Points Pictured on Several
Photographs 139

6. Plotting Pictured Terrene Points as Intersections of Lines

of Horizontal Directions. Iconometric Plotting of
Terrene Points in General. ("Horizontal Inter-
sections") 140

7. Iconometric Plotting of Pictured Terrene Points by so-

called " Vertical Intersections " 142

8. Iconometric Determination of the Elevations of Pic-

tured Terrene Points r45

9. Iconometric Determination of the Elevations of Pic-

tured Terrene Points by Means of the so-called
"Scale of Heights" 147

XVI CONTENTS.

PAGB

lo. The Use of the so-called "Photograph Board" 148

ri. Iconometric Plotting of the Trace of a Figure's Plane . . 149

12. Iconometric Contouring 151

13. The Use of the so-called "Photograph Protractor" 154

D. Method of Commandant V. Legros for Locating the Horizon

Line of a Vertically Exposed Plate 156

E. Prof. S. Finsteiwalder's Method for the Iconometric Plotting

of Horizontal Contours 157

CHAPTER Vn.

CAMERA LENSES.

A. The Refractive Index 159

B. Refraction of Light-rays 160

C. The Optical Lens 161

D. Optical Distortion 163

E. Nodal Points and Nodal Planes of a Lens 165

F. Principal Foci and Focal Planes of a Lens 168

G. The Focal Length of a Lens 169

H. The Biconvex or Positive Lens 170

I. Conjugate Foci and Conjugate Planes 171

K. To Find the Image of any Luminous Point for the Biconvex Lens. . 172

L. The Biconcave or Negative Lens 173

M. To Find the Image of a Luminous Point for a Biconcave Lens 174

N. Lens Combinations 174

O. Diaphragms or Lens Stops 176

P. Rapidity of a Lens 176

Q. Length of Exposure. . . . •. 177

R. Distortion Produced by Diaphragms 177

S. Chromatic Aberration of Light-rays 178

CHAPTER VIII.

PHOTOGRAMMETERS OR PHOTOGRAMMETRIC INSTRUMENTS.

General Requirements to be Fulfilled by a Topographic Surveying-
camera 183

I. Ordinary Cameras (with Extension Bellows) Converted into Sur-
veying-cameras 184

11. Special Surveying-cameras with Constant Focal Lengths 185

A. Dr. A. Meydenbaur's New Small Magazine Camera 185

CONTENTS. ' xvii

PAGB

B. Capt. E. Deville's Surveying-camera (New Model) i86

1. Determination of the Focal Length, the Horizon Line,

and the Principal Line 189

2. Adjustment of Camera Spirit-levels 192

3. Use of the Instruments Comprised in the Canadian

Phototopographic Outfit 193

C. The U. S Coast and Geodetic Survey Phototopographic

Cameras 196

D. L. P. Paganini's New Phototopographic Instrument for

Reconaissance Surveys on Scales of 1:50000 and

1 : 100,000 (Model of 1897) 200

1. The Phototopographic Camera Proper 201

2. The Horizontal Graduated Circle 203

3. The Azimuth Compass 203

4. The Tripod , 204

5. Adjustments and Use of the Instruments 204

m. Surveying-cameras Combined with Geodetic Instruments. (Pho-

totheodolites, Phototachymeters, Photographic Plane Tables,
etc.) 209

A. L. P. Paganini's Photogrammetric Instrument (Model of

1884) 211

Constant Focal Lengths of the Italian Cameras 215

B. L. P. Paganini's New Photo theodolite (Model of 1894) 219

C. L. P. Paganini's Photographic Azimuth Compass (Photo-

graphic " Azimutale ' ) 223

D. Photogrammetric Theodolite of Prof. S. Finsterwalder 227

E. Phototheodolite for Precise Work, by O. Ney 231

F. Phototheodolite of Dr. C. Koppe 234

G. Dr. C. Koppe's New Instrument and Method for Observing

Horizontal and Vertical Angles Directly on the Photo-
graphic Negative 236

H. Phototheodolite Devised by V. Pollack; Manufactured by

R. Lechner in Vienna, Austria 241

I. Phototheodolite Devised by Pollack and Hafferl 242

K. R. Lechner's Photogrammeter 243

L. Phototheodolite of Col. A. Laussedat (New Model) 244

M. Phototheodolite of Starke and Kammerer 245

N. Capt. von Huebl's Plane-table Photogrammeter 250

O. Phototheodolite ("Phototach^m^tre") Devised by J. and

H. Vallot 253

P. Phototheodolite Designed by J. Bridges-Lee 262

XVIU CONTENTS.

CHAPTER IX.

PANORAMA CAMERAS.

PAGE

I. The Photographic Plane Table Devised by A. Chevallier 270

II. The Rockwood-Shallenberger Panoramic Camera 271

ni. R. Moessard*s Topographic Cylindrograph 271

CHAPTER X.

ICONOMETERS AND PERSPECTOGRAPHS.

I. Graphic Protractor (of L. P. Paganini) 275

II. L. P. Paganini's Graphic Sector ("Settore Grafico") 278

in. L. P. Paganini's Graphic Hypsometer ("Squadro Grafico") 284

rV. The Centro-linead as Used by Capt. E. Deville 290

A. To Set the Arms / and /' of the Centro-linead if the Direc-

tion to the Vanishing Point be given by a Line in the
Ground Plan 293

B. To Set the Arms of the Centro-linead if the given Line

(VE) Belongs to the Perspective 294

V. The Perspectometer as Used by Capt. E. Deville 294

VI. The Perspectograph Devised by H. Ritter 298

Use of the Perspectograph 301

VII. Prof. G. Hauck's Trikolograph and its Use in Iconometric Plotting. 302
VIII. The Carl Zeiss Stereoscopic Telemeter and the Stereocomparator,
Including the Stereophotogrammetric Surveying Method De-
vised by Dr. C. Pulfrich 308

A. The Stereoscopic Telemeter, or Range-finder 309

B. The Stereocomparator and the Stereophotogranmietric Sur-

veying Method 318

CHAPTER XI.

PHOTOGRAPHIC OPERATIONS IN THE FIELD.

I. General Remarks on the Exposure of a Photographic Dry Plate . . 335

n. Orthochromatic Dry Plates and Ray-filters 339

A. Color-screens, or Ray-filters 340

B. Halation 342

III. Comparative Light Values and Exposures 343

Test Exposures and Trial Plates 346

CONTENTS. XIX

PAOB

IV, Development of Orthochromatic Dry Plates. . . : 348

A. Water and Water Tests 352

B. Developers 353

1. Developing with Ferrous Oxalate 353

a. Restraining the Ferrous Oxalate Development 356

h. Accelerating the Ferrous Oxalate Development. . . 357

2. Pyro Developer 358

3. Metol Developer 359

4. Metol-bicarbonate Developer 360

5. Hydrochinon Developer 361

6. Metol-hydrochinon Developer^ 363

7. Bromo-hydrochinon Developer 364

8. Eikonogen Developer 364

9. "Eiko-cum-hydro" Developer 365

10. Amidol Developer 366

C. Fixing the Negative 366

1. Tests for Presence of Hyposulphite of Soda 368

2. Drying the Finished Negative 369

3. Intensification of a Negative with the Aid of Metallic

Salts 3(59

4. Intensification with Silver Cyanide 370

5. Prof. R. E. Liesegang*s Intensifier 371

6. Intensifying Negatives without the Use of Metallic Salts . 372

7. Reducing the Density of a Negative 373

8. Cooling Solutions 374

D. Negative Varnish 375

V. Photographic Printing 376

A. Toning Photographic Prints 379

B. Fixing Photographic Prints 381

C. Formulas for Plain Toning-baths 382

D. Combined Toning- and Fixing-baths 383

CHAPTER XII.

OONCXtrSION AND REMARKS ON THE PRECISION OF TEE " POLAR ICONOMETRIC
method" and general remarks on TELEPHOTOGRAPHY.

I. General Remarks on Phototopography 387

H. Precision of the "Polar-iconometric" Method 391

m. General Remarks on Telephotography or Long-distance Pho-
tography 402

PHOTOTOPOGRAPHIC METHODS AND
INSTRUMENTS.

INTRODUCTION.

Topography is that branch of surveying which pictures
sections of the earth's surface, in reduced scale, as a horizontal
projection, showing the relative positions of points of the ter-
rene in both the horizontal and vertical sense. Under topog-
raphy, in the closer sense, we generally understand the represen-
tation of the terrene in the form of charts, drawn to the scales
of 1:5000 to 1:100000, showing not only the relative positions
of characteristic points of the earth's surface, but also clearly
delineating all natural and cultural details. Topographic charts
on scales smaller than 1:100000 partake of a geographic char-
acter, while surveys on scales larger than i : 5000 are generally
made for special technical purposes.

The works of filling in the details, topographic surveying
in the closer sense, may be accomplished by various methods,
differing in the matter of cost, time, and attainable accuracy.
One may be employed with advantage for one class of work,
while another may be preferable for another class, another locality,
or, to meet different conditions. The method best adapted to
any particular region should be employed to obtain the best
results.

2 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

The more important methods with their instrumental out-
fits are:

First — ^The direct plotting to scale, in the field, of all fea-
tures that are to be shown on the chart,

A — with a plane table and telemeter or stadia rods;

B — with a tachygraphometer and stadia rods;

C — with either of these instruments, but with a leveling
instrument in addition, for locating the horizontal
contours;

D — using an aneroid barometer in place of the level.
Second — ^The compilation of all available data (cadastral
surveys, public land, and county surveys, railroad
and canal surveys, etc.), giving principally the hori-
zontal distances, a supplementary survey being made
to furnish the missing data, which in this case are
principally elevations. They may be supplied by
trigonometric or spirit leveling, by interpolation and
sketching.
Third — The records of the survey may be obtained in the
shape of field notes and sketches (" tachymetry "),
the map being produced by plotting the recorded
data in the office.

A — with a surveyor's compass and steel tape the rela-
tive positions of characteristic points may be located
in the horizontal sense, while their relative eleva-
tions may be obtained by means of a level, minor
details being largely sketched;

B — with a transit and steel tape points are located, both
geographically and hypsometrically, minor details
are sketched;

C — with a transit and stadia rods;

D — with a tachymeter and stadia rods, elevations being
obtained automatically;

E — adding a leveling instrument for locating horizontal
contours;

INTRODUCTION. 3

F — ^using a specially constructed aneroid barometer
(" Goldschmidt's "), in place of the level for locating
and tracing the contours in the field.
Fourth — ^The field records for developing the terrene are
represented by photographic negatives, taken under
special conditions from stations of known positions
and elevations,
A — ^with a camera or phototheodolite, using telemeters
or other distance measures for obtaining the lengths
of base lines and a barometer for ascertaining the
elevations of tertiary points;
B — ^with a surveying camera, separate transit, telemeters,

and barometer;
C — ^with a photographic plane table and distance-meas-
uring alidade, using a barometer for obtaining the
elevations of detached camera stations;
D — with a sur\'eying camera, separate plane table, dis*

tance measure, and aneroid barometer;
E — with a specially constructed phototheodolite, the
iconometric plotting being done with the Zeiss stereo-
comparator;
F — with cameras designed to be used attached to kites,
to free or to captive balloons.
Minute and detailed methods with ensuing accurate results
should be applied to the surveys of cities and all densely settled
regions, to the coastal belts, large river valleys, and lakes, par-
ticularly when navigable, and this work should be plotted on a
large scale.

Arid, barren, and mountainous regions as well as prairies
and swamp lands, when sparsely settled, should be generalized
in their cartographic representations and they should be plotted
on a small scale.

In exemplification of the preceding suggestions we may
refer to the new topographic survey of Italy, where Paganini's
results not only fully proved the efficiency of photography applied

4 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

to surveys of alpine regions (plotted in 1:25000 and 1:50000),
but also led to the adoption of the phototheodolite as an auxiliary
instrument to the plane table. The latter was used for mapping
the areas below 2000 m. in altitude, while the phototheodolite
was depended on to delineate the terrene lying above that altitude.

After the area which is to be surveyed has been covered with
a net of triangles and polygons it will have been provided with a
framework of lines of known lengths and directions (being in
itself a skeleton survey of the country), and after the natural
details and artificial features have been filled in, by one of the
numerous topographic methods with more or less details and
accuracy, we will have a topographic sur\-ey of the area of more
or less precision.

The number of so-called control points for a given area,
determined in elevation and geographical position during a
topographic survey, should be increased in the same ratio as
the degree of accuracy, required for the survey, and also as the
amount of details, conditioned by the scale of the survey, may be
increased.

Photography has been extensively applied to surveys of rugged
mountain re^gions in Italy, Austria, Russia, Canada, and Alaska
with great success. The phototopographic method, originally
devised by Colonel Laussedat, found its first application in France
and in Germany. In its early stages it was practiced exclusively
under governmental and military auspices, being primarily used
for so-called secret and military surveys. Lately, however,
phototopography has found a wider and more general applica-
tion also in France and Germany.

More recently photographic survejrs have been executed in
Greece, Spain, Portugal, Norway, Belgium, Mexico, Chile, Peru,
Brazil, Argentine Republic, Switzerland, Australia, England,
Africa, and more recently still in the United States, although
Lieut. Henry A. Reed has, for several years past, taught photo-
graphic surveying, theoretically and practically, at the U. S..
Military Academy at West Point.

CHAPTER I.

SHORT REVIEW OF EXECUTED PHOTOGRAPfflC SURVEYS
AND LISTS OF THE MORE IMPORTANT PUBLICATIONS
ON PHOTOGRAMMETRY AND PHO TOTOPOGRAPHY.

I. Photographic Surveying in France.

Practical results from the early application of photography
to surveying failed to materialize for some time, partly owing to
the slowness and uncertainty of the old photographic process, and
partly due to the greatly increased efficiency of surveying instru-
ments and methods in general.

The chances of utilizmg landscape perspectives for a geo-
metrically true representation of the terrene in horizontal plan
became realized by the combination of surveying instruments
with the modem camera (with the reliable, uniform, and quick
dry-plate process). Of course, it is not necessary to merge the
camera and the geodetic instrument into a single apparatus.
For facilitating transportation and for other reasons they are
frequently used separately over the same station.

The theoretical principles upon which photogrammetric
methods are primarily based were known to J. H. Lambert (of
Zurich), who published a work on perspective in 1759, in which
reference is made to those identical principles. Still, Lambert's
suggestions were neither followed nor were his theories given a
practical application in this respect, until the well-known savant
and hydrographer Beautemps-Beaupr^, while on a scientific expe-
dition, in 1 791 to 1 793, made a series of free-hand sketches of the
regions skirting the shores of Van Diemansland (Tasmania) and
of Santa Cruz Island. After his return to France he attempted

6 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

the first practical application of Lambert's theory by constructing
topographic reconnaissance maps of the coastal regions just
referred to, based upon the outline sketches of the terrene.

Beautemps-Beaupr^ subsequently made frequent reference to
the feasibility of his cartographic methods, recommending them
particularly to explorers. Little or nothing, however, was under-
taken by others toward improving Beautemps-Beaupr^'s new
cartographic method, and it had practically fallen into oblivion
when Arago, in 1839, called attention to the possibilities of
photography when utilized in this connection.

Beautemps-Beaupr^'s suggestions probably met with so little
favor because it is not easy to make free-hand sketches of land-
scapes geometrically accurate enough to be used iconometrically in
place of the landscapes. Iconometry as applied to topographic
plotting rests upon the same principles as the plane-table method
of determining points by the intersections of lines of direction,
drawn to such points from known stations, only in iconometry
such lines are graphically deduced from the photographic per-
spectives and are drawn in the office.

Apparently Capt. Leblanc, of the French G6iie Corps, made
the only applications of Beautemps-Beaupr^'s method, in ante-
cedence of the year 1849, principally in connection with military
surveys.

Col. A. Laussedat had taken up the study of iconometric
map-plotting in 1850. In the early part of his investigations
he utilized a "camera clara" (invented by Wollaston in 1804)
for obtaining the necessary perspectives of the terrene, tracing
their outlines by hand. In 1852, however, he replaced that
instrument by the "camera obscura" (invented by Dom Panunce,
or by some attributed to G. della Porta). Laussedat's camera
obscura was modelled after Niepce's, but it was supplied with
special surveying devices.

Subsequently as Chef du Gfoie Corps, Laussedat executed
numerous experimental surveys, improving the surveying camera
and elaborating the methods. In 1858 he obtained a Bertaud

PHOTOGRAPHIC SURVEYING IN FRANCE. ^

lens, which was practically free from aberration and which he
used for the new phototheodolite made by Brunner in Paris.

In 1859 Laussedat felt justified, by the good results he had
obtained with this improved phototheodolite, to announce the
successful application of photography to surveying to the Academy
of Sciences in Paris. After a critical examination of Laussedat's
methods and results, by Daussy and Laugier, these two members
of the Academy approved and endorsed his methods. Laussedat
also made a few topographic maps with the aid of balloon-
photography, but soon returned to the exclusive use of the
station-camera.

At the exposition of 1867, in Paris, Laussedat exhibited the
first known phd to theodolite and some map specimens, based
upon photographic surveys, among others a plan of Paris (scale
1:6666) which compared very favorably with one that Emmery
had made in 1839 by means of instrumental surveys. The
survey for the phototopographic map of Faverges in Savoy,
scale 1:5000, was executed in 1867 by Capt. Javary and Lieut.
Garibaldy, of the Gfoie Corps, under Col. Laussedat's direction,
and it was based on 120 photographs. It covered an area of
about three square miles and the topography was controlled by
about 5000 points that had been identified on the pictures and
which were plotted iconometrically on the chart.

Col. Laussedat's work in this field has been so complete that
the guiding principles which he first laid down and subsequently
elaborated by numerous practical applications are still in use,
and his interest in this work continues unabated to this day.

From 1 85 1 to 1871 Col. Laussedat and his associates in this
work were frequently called away from the pursuance of photo-
topographic surveys, having other duties assigned them, and we
find that Laussedat's surveying methods did not become generally
known in France, and it was left to scientists and engineers of
other countries (Germany and Austria) to popularize this sur\'ey-
ing method and extend its application to various branches of the
sciences.

8 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

In 1858 Chevallier had an instrument patented under the
name " planchette photographique " which soon found much
favor, especially among members of the G^nie Corps. This
photographic plane table is described by Alophe (" Le pass^,
le pr&ent et Tavenir de la photographie," Paris, 1861), by
d'Abbadie (" Bulletin de la SociA^ de Gfographie de Paris,"
1862), by Pat^ (" Application de la photographie k la topographic
militaire," Paris, 1866), by Col. A. Laussedat (" Rechcrches
sur les Instruments, les Mfthodes et le Dessin topographiques,"
Paris, 1898), and others. It was manufactured by Dubosque,
in Paris, and used by Wiganowsky, Pat^, and A. Jouart.

Martens, in Paris, was probably the first to devise a " pano-
ramic camera " (1847), ^ which he used a cylindrically bent
daguerreotype plate.

Inclined plates for phototopography were also used at an early
date in France, notably by Th. Pujo and T. Fourcade, who
published their methods, under the title " Goniomftrie Photo-
graphique," in Les Mondes, No. 4, 1865.

France had an interesting exhibit at the World's Columbian
Exposition in Chicago, 1893, showing photographic surveying
instruments and map specimens, in illustration of topographic
and astronomical results, gained chiefly under the direction
of Col. A. Laussedat and taken from the collections of the Con-
ser\'atoire National des Arts et Metiers, Paris, of which insti-
tution Col. Laussedat is now director.

In recent years balloon 3urveying has been resumed in France
under the auspices of the Ministry of War, the camera being
used in connection with both the free and captive balloon. Bal-
loon surveying had been rather neglected, notwithstanding the
good results which had been obtained in the early stages of photo-
graphic surv^cying in Paris by Col. Laussedat and Nadar (1866).

Long-distance photography ('* telephotography ") seems to
have been first studied in France by Lacombe and Matthieu.
A r&um^ of their results has been embodied in an official report
to the French Government in 1887 by Commandant Fribourg,

PHOTOGRAPHIC SURVEYING IN FRANCE. 9

recommending the adoption of telephotography as a reconnoiter-
ing method in the G^nie Corps.

Guillemont and Jarret followed in the lines laid down by
Lacombe and Matthieu, but little reached the general public
regarding the practical results obtained by this method in France.
Still, it is now well known that the French officers stationed
at Grenoble have obtained excellent results in telephotography
as applied to military reconnaissance, showing the operations and
the disposition of troops in the field, depicting the effects of
cannon-shots upon bombarded fortifications, etc. The tele-
photographic negatives obtained at Grenoble clearly define
objects at distances from 2 to 6 km. The French telephoto-
graphic cameras are mostly made by Hondaide and Derogy in
Paris.

In 1893 H. Vallot commenced the mapping of the Mont
Blanc mountain group and its immediate vicinity. He is assisted
in this work by J. Vallot, who, in 1890, founded a meteorological
obsen-atory on Mont Blanc. This map is being dra^Ti in
1 : 20000 scale and the greater part of the topography is based
on photographs.

LITERATURE.

Other French publications on photography applied to sur-
veying, besides those already mentioned in the preceding para-
graphs, are as follows:

A. Laussedat. "Mtooire sur TEmploie de la Chambre Claire dans les

Reconnaissances topographiques.'' Memorial de I'Officier du G6iie,

No. 16, 1854.
A. Laussedat. "Analyse d'un M^moire sur TEmploie de la Photographie

dans le Levd des Plans. "Comptes rendus de I'Acadtoie des Sciences,

1859.
A. Laussedat. "Mtooire sur rEmploie de la Photographie dans le Lev^

des Plans,'' Memorial de TOfficier du G^nie, Nov. 17, 1864.
A. Laussedat. "Historique de 1' Application de la Photographie au Lever

des Plans," "Conference faite au Congr^s de 1' Association Franyaise

pour I'Avancement des Sciences," Septr. 17, 1892. Comptes rendus du

Congr^s de Pau et Revue scientifique de 1892.

10 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

A. Laussedat. "Llconom^trieetlaM^trophotographie." Conference faite
au Conservatoire desArtset Metiers, deuxifeme, S^rie, II, III, IV, 1892-93.

A. Laussedat. "Exposition Universelle de Chicago, 1893, Section Fran-
5aise, Instruments et Appareils Iconom^triques et Mdtrophotographiques,"
des Collections du Conservatoire National des Arts et Metiers. Paris,
1893, Imprimerie Nationale.

A. Laussedat. "Les Applications de la Perspective au Lever des Plans,'*
Annales du Conservatoire des Arts et Metiers. Paris, 1893-94.

A. Laussedat. "Reconnaissance faite k PAide de la Photographie, pour la
Delimitation de la Fronti^re entre PAlaska et la Cbjombie Britanique."
Comptes rendus de PAcademie des Sciences, Paris, 1894.

A. Laussedat. " Conference de Metrophotographie," Instructions et Ex-
plications sommaires k PEmploie de la Photographie dans les Recon-
naissances topographiques faites par les Voyageurs," Revue scientifique,
No. 26 I, No. 23 IL Paris, 1894.

A. Laussedat. "Recherches sur les Instruments, les Methodes et le Dessin
topographiques" (Tome II). Paris, 1903.

A. Laussedat. "De PEmploi du Stereoscope en Topographie et en Astro-
nomie." Comptes rendus des Seances de PAcademie des Sciences,
CXXXVI, p. 2^28, 1903.

A. Laussedat. "Sur un Moyen rapide d'obtenir le Plan d*un Terrain
en Pays de plaines, d*apr^ une Vue photographique prise en Ballon."
Comptes rendus, 137 Vol., pp. 24-30. Paris, 1903.

E. Pat6. "Applications de la Photographie h, la Topographie militaire.'*
Paris, 1862.

Laugier & Daussy. " Rapport sur le Memoire de M . Laussedat." Comptes
rendus de PAcademie des Sciences, i860.

A. Jouart. "Applications de la Photographie au Lever militaire." Paris,
1866.

Javary. "Le Memoire siu" les Applications de la Photographie aux Arts
militaires." Memorial de POfficier du Genie. Paris, No. 22, 1874.

Dr. G. Le Bon. "Les Civilizations de PInde." Firmin, Didot et Cie.
Paris, 1887.

Dr. G. Le Bon. "Les Levers photographiques et la Photographie en
Voyage." Gautier Villars et Fils. Paris, 1889.
Partie I: "Application de la Photographie k Pfoude geometrique des

monuments et ^ la Topographic."
Partie II: "Operations compiementaires des Levers topographiques."

P. Mol^ssARD. "Le Cylindrographe, Appareil panoramique."
Partie II: "Le Cylindrographe topographique." Gauthier, Villars et
Fib. Paris, 1889.

P. MoissARD. "Les Panoramas photographiques et les Appareils pane-

PHOTOGRAPHIC SURVEYING IN FRANCE. II

ramiques." Bulletin de la Socidt^ Fran^aise de Photographie, deuxi^me

Sdrie, Tome IX, 1893.
E. Monet. "Princtpes fondamentaux de la Photogramm^trie." Soci^t^

d'^ditions scientifiques. Paris, 1893.
E. Monet. "Application de la Photographie k la Topographic." Bulletin

d'Aoiit, 1894, de la Socidt^ des Ing^nieurs Civils de Fmnce.
J. GiRARD. " La Photographie et ses Applications aux f)tudes g^graphiques."

Paris, 1872.
Alfred Hanot. "La Photographie dans les Armies." Paris, 1875.
Fl. Dumas. "De la Photographie et ses Applications aux Besoins de

TArm^e." Parish 1872.
J. Bornecque. "La Photographie appliqu^e au Lever des Plans." Paris, 1886.
A. de la Fuye. "La Photographie k grande Distance." Revue du Cercle

militaire, 1895.
A. DE LA Fuye. "Mdmoiie sur TEmploi des Appareils photographiques

pour les Observations k grande et k petite Distances." Autographic k

r^cole de G6iie de Grenoble, 1891.
Capt. E. Deville. "Lever topographique des Montagnes Rocheuses, exe-
cute par la Photographie." BuUetin de la SociCtC frangaise de Photo-
graphie, deuxi^me SCrie, 1893.
R. CoLSON. "La Photographie sans Objective." Gauthier, Villars et Fils.

Paris, 1887.
Arthur Batut. "La Photographie a^enne par Cerf volant." Gauthier,

ViUars et Fils. Paris, 1890.
Le Col. Berthaud, Chef de la Section topographique. "La Carte de

France (i 750-1898). 6tude historique," 2 Vol., Imprimerie du Service

g^graphique de TArm^e, 1899.
Le Capt. du G^nie Bouttr^aux. "M^moire sur la Td^photographie,"

Revue du G^nie, Sept., 1897.
L. Cazes. "Stdrfoscopie de Precision. Thforie et Pratique," Librairie

Michelct, Ph. PelUer, ikiiteur, 1895.

E. Douglas-Archibald. "Les Cerfs volants militaires." Librairie Uni-

verselle. Paris, 1888.

F. Drouin. "Le St^rfoscope et la Photographie st^r^scopique." Ch.

Mendel. Paris, 1894.

Le Capt. HoUDAiLLE. "Sur une M^thode d'Essai sdentifique et pratique
des Objectifs photographiques " Gauthier, Villars et Fib. Paris, 1894.

J. Le Cornu. "Les Cerfs volants." Monie et Cie. Paris, 1902.

Max Loehr. "Sur la Determination des M&uies du TdWobjectif." Bulle-
tin de la Society fran^se de Photographie, 1902.

H. RoussoN. "Instructions sur le Photogramm^tre de PExplorateur."
Imprimerie Dubos. Paris, 1900.

12 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

Gaston Tissandier. "La Photographie en Ballon " Gaulhier, Villars et

Fils. Paris, 1886.
A. Vautier-Dufour. "Sur la T^l^photographie." Bull, de la Soc. Vaudoise

des Sdences naturelles, No. 143. Corbaz et Cie. LAusanne, 1902
Ch. von Zi^gler. "Le Perspecteur m^chanique." Fr. Weber. G^nfeve

1900.
E. Wallon. "Traits ^I6iientaire de PObjectif photographique." Gauthier,

Villars et Fils. Paris, 1891.
E. Wallon. "Choix et Usage des Objectifs photographiques." Gauthier,

Villars et Fib, Paris.
Emile Wenz. "Resume historique de PInvention de la Photographie adiienne

par Cerfs volants." Bull, de la Soc. fran9aise de Photographie. Avril,

1902.

II. Photographic Surve3ring m Germany.

Col. Laussedat's photographic surveying methods soon found
admirers and earnest students in Germany and Dr. A. Meyden-
baur became an early exponent of this method. Some writers
even claim that, in 1858, while Meydenbaur was engaged with
obtaining the measurements that were needed for planning
the renovation of the cathedral at Wetzlar, he had, independ-
ently of Col. Lausscdat's work, conceived the idea to utilize
photographs of the building to arrive at the dimensions of inac-
cessible details. -

General von Alster repeatedly recommended in his official
reports that experimental surveys be inaugurated by the Prussian
Government, to famiharize the officers of the General Staff with
the photographic surveying methods, but the war of 1866 pre-
vented action being taken in this direction. Still, Count von
Moltke, Chief of the Prussian General Staff, had soon recog-
nized the value of such methods for military and secret surveys^
and before the next war took place (which was the Franco-
Prussian war of 1870-71), a complete militar}' phototopographic
detachment had been formed and organized. In 1870 this
reconnoitering corps was placed under the command of Capt.
Bemhardi and Lieut. R. Doergens, who furthermore had the

PHOTOGRAPHIC SURVEYING IN GERMANY. 1 3

assistance of the civil photographic experts Hintze, Quidde,
and Schmier.

During the siege of Strassburg this corps made a survey
of a part of the city and the near-by fortifications, plotting the
work in 1:2500 scale, but the early surrender of the city made
it unnecessary to finish the survey. Only a few ranges as obtained
by this survey had been utilized by the army, principally to
concentrate the bombardment to certain districts, sparing others,
especially the noble cathedral building. During the icono-
metric plotting of this map some discrepancies were discovered,
which Dr. Doergens ascribed to defects in the camera lens and
to the imstable character of the instrument, which had been
put together too hurriedly.

No further opportunities were oflFered during the Franco-
Prussian war for other phototopographic surveys and the sur-
veying cameras were subsequently used for obtaining pictures
of historically interesting features, of points of strategic impor-
tance to be utilized in making corrections and additions to the
maps of France which had been distributed among the officers
that were directing the movements of the invading German armies.

Probably the first German publication on the subject of
photographic surveying may be found in Horn's " Photographische
Mittheilungen," April, 1863, being a German translation of Col.
Laussedat's explanations given in his lecture, delivered Jan. 9^
1863, before the "Socift^ photographique " in Paris.

In the June number of the same magazine Dr. Meydenbaur
published his first article on photographic surveying, in which
he makes use of the term " photometrographie ". Subsequently
this was changed to "metrophotographie*' and finally to "photo-
grammetrie" ("Bildmesskunst").

The March number of flom's " Photographische Mitthei-
lungen," 1866, contains an article by Dr. Vogel on Johnson's
photographic apparatus for making topographic surveys, in which
the method for the iconometric determination of horizontal and
vertical angles is described.

14 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

Since 1882 Dr. Meydenbaur has been the Du-ector of the
" Photogrammetric Institute" in Berlin, founded by the Prussian
Government for the collection of photographic data to bring the
German topographic maps up to date and for the preservation
of views of ancient monuments and buildings. Dr. Meydenbaur
is now engaged with the publication of " Das Denkmaler Archiv,"
being a collection of photographic reproductions of buildings of
archeological and architectural interest.

Since 1867 a number of photographic surveys have been made
under the auspices of the Prussian Ministry of War. The first
larger attempt was the topographic survey of Freiburg and vicinity,
scale 1 : 1000, including the architectural survey of the cathedral
of the same city.

From 1869 the theory of photographic surveying has been
included in the lectures on geodesy (Prof. Doergens) at the Royal
Building Academy in Berlin. In 1886, when the new Royal
Technical High School at Charlottenburg, near Berlin, had been
completed, a special chair (filled by Dr. Pietsch) was set aside
for teaching photogrammetry in all its branches, including bal-
loon surveying.

The late Prof. Jordan, as a member of G. Rholf's African
exploring expedition in 1873-74, made a phototopographic sur-
vey of the "Oasis Dachel," including the settlement "Gassr-
Dachel," in the Libyan Desert. (Jordan, "Vermessungskunde,''
Vol. II, Stuttgart, 1893, and "Zeitschrift fuer Vermessungs-
wesen," Part I, Vol. V, 1876.)

Prof. Jordan fully appreciated the value of photography as an
auxiliary to topographic surveys, and he remarks in the article
"Ueber die Verwerthung der Photographie zu geometrischen
Aufnahmen" (Zeitschr. f. Verm., 1876): . . . "Dasz die Photo-
graphie in vielen gewissen Faellen mit ausserordentlichem^ Vor-
theil angewandt werden koennte, z. B. bei schwer zugaenglichen
Gebirgen und auf Entdeckungsreisen, erscheint beim ersten
Blick auf die Sache zweifellos."

The good results obtained by Dr. Meydenbaur in the valleys

PHOTOGRAPHIC SURVEYING IN AUSTRIA. 1 5.

of the Reusz (1873) ^^^ Rhine (1876) considerably increased the
number of advocates for phototopography in Germany.

Dr. Stolze in 1874, while a member of the archaeological
expedition under F. C. Andreas, used a Meydenbaur camera-
theodolite to make a survey of the ruins of Persepolis, of Pasarga-
dae, and of the ancient temple at Djamhit (" Masdjid i Djamftht")
in Shiraz, Persia.

Dr. S. Finsterwalder has made several phototopographic sur-
veys in the Bavarian Alps, including surveys of glaciers, made at
stated time-intervals, to ascertain their variations and movements
in both the horizontal and the vertical sense. During the summer
months of 1888 and 1889 he surveyed the " Vemacht" Glacier in
the valley of the Oetz, in Tyrol, having the assistance of Dr. A.
Bluemke and Dr. H. Hess; in later surveys he was assisted by
Dr. Kerschensteiner.

Dr. C. Koppe has done much literary and practical work in
photogranunetry and phototopography. Recently he has pub-
lished the description of his new phototheodolite, made by F.
Randhagen in Hannover, that may also be used for astronomical
observations (lat. determinations).

Dr. Meydenbaur, Prof. Jordan, Dr. Doergens, Dr. Stolze,
Dr. Schroeder, Dr. Vogel, Dr. Koppe, Dr. Hauck, Prof. Finster-
walder, Prof. Foerster, Dr. Pietsch, Dr. Voigtlaender, and others
have contributed largely toward the popularization of the photo-
topographic methods in Germany.

m. Photographic Surveying in Austria*

. Karl Koristka, having made the acquaintance of Messrs.
Laussedat and Chevallier in Paris in 1867, became interested in
photography applied to surveying, and he probably is the first
who made a phototopographic survey in Austria (survey of the
dty of Prague). Still, the method met with little favor in Austria
owing to the inconvenience of the wet-plate process, until, in 1890,
the authorities of the Military Geographic Institute, in Vienna,

l6 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

ordered a series of experimental photographic surveys to be made
in the vicinity of Vienna, which fully demonstrated the usefulness of
this method for the surveys of certain regions. Among the offi-
cers of the Austrian army we may mention Lieut. L. Mikiewicz,
Maj. Bock, Maj. Pizzighelli, Lieut. Hartl, Capt. Huebl, and others
who have materially aided in the development of photographic
surveying methods in Austria.

Since 1889 many engineers have adopted phototopographic
methods for the surveys of inaccessible regions, or for regions
where the other topographic methods would have been too time-
consuming or too expensive. Among these we may cite V.
Pollack (Eng'r in Chief of the Austrian Government R.R. System),
M. Maurer, F. Hafferl, etc.

Prof. F. Schiffncr, Prof. A. Schell, and Prof. F. Steiner (the
latter teaches thp principles of photographic surveying at the Tech.
High School in Prague) have done much as teachers, writers, and
propagators in improving phototopographic methods and instru-
ments.

LITERATURE (German and Austrian).

Archiv fuer die Offiziere des Kgl. Preuss. Artillerie u. Ing. Corps, 1868.

Deutsche Bauzeitung, 1872.
J. GiRARD. "Laussedafs Arbeiten in Bezug auf die Anwendung der Photo-

graphie zur Aufnahme von Plaenen." Photographisches Archiv, Sept.,

1865.
Dr. A. Meydenbaur. "Ueber die Anwendung der Photographie zu Archi-

tectur und Terrain Aufnahmen." Erbkam's Zeitschr. f. Bauw., 1867.
Dr. A. Meydenbaur. **Das Photographische Aufnehmen zu wissenschaft-

lichen Zwecken, ins Besondere das Messbildverfahren." Unte*s Verl.

Berlin, 1892.
Dr. A. Meydenbaur. "Zum gegenwaertigen Stande des Messbildver-

fahrens." Deutsche Bauz., 1894.
Dr. A. Meydenbaur. "Ueber Behandlung grosser Platten auf Reisen."

Photographisches Wochenblatt, 1888.
Dr. PiETSCH. "Die Photogrammetrie." Zeitschr. f. Vemi., Heft 23 and 24,

1887.
Prof. W. Jordan. "Ueber die Verwerthung der Photographie zu geome-

trischen Aufnahmen." Zeitschr. f. Verm., Heft I, 1876.

PHOTOGRAPHIC SURVEYING IN AUSTRIA. l^

Prof. Stolze. "Photographisches Wochenblatt," 1881, 1882, and 1885.

Prof. Stolze. "Photogrammetrie." "Das Licht im Dienst wissenschaft-
licher Forschung," Dr. S. T. Stein, Bd. II, 1888. Wm. Knapp.

Dr. H. W. VoGEL and Prof. Dr. G. Doergens. "Ueber einen einfachen
photogrammetrischen Apparat." Photogr. Mitt., 1884.

Dr. H. W. VoGEL. "Astronomische Nachrichten," 1888.

Dr. R. Doergens. "Zur Pruefung und Berichtigung des photogramme-
trischen Apparates." Phot. Mitth., 1886.

Dr. R. Doergens. "Ermittelung der Constanten des photogrammetrischen
Apparates." Phot. Mitth., 1886.

Dr. R. Doergens. "Ueber Photogranmietrie und Ueber die Thaetigkeit des
Feld-Photographie Detachments im Kriege 1870-71." K. Schmier.
Weimar, 1897.

Dr. Finsterwalder. "Die Terrainaufnahmen miltels Photogrammetrie."
Bayersches Industrie und Gewerbebl. Muenchen, 1890.

L. MnciEWicz. "Anwendung der Photographie zu militairischen Zwecken.'*
Mitth. ueber Gegenst. d. Artillerie u. Geniewesens, 1876.

G. PizziGHELLi. "Handbuch d. Photographie," Bd. II, 1887. Knapp-
Halle, a./S.

G. PizziGHELLi. "Die Photogrammetrie." Mitth. ueber Gegenst. d.
Artillerie u. Geniewesens, 1884.

Dr. C. KOPPE. "Die Photogrammetrie oder Bildmesskuiist." Weimar^
1889.

Dr. C. KoPPE. "Photogranmietrie imd Internationale Wolkenmessung.'*
Braunschweig, 1896.

Dr. G. Hauck. "Neue Constructionen der Perspective und Photogram-
metrie." Journal fuer reine und angewandte Mathematik, 1883.

Dr. G. Hauck. "Theorie der trilinearen Verwandtschaft ebener Systeme.'^
Journal fuer reine und angewandte Mathematik, 1883.

Dr. G. Hauck. "Mein perspectivischer Apparat." Festschrift der KoenigL
techn. Hodischule zu Berlin, Zur Feier der Einweihung ihres neuen
Gebaudes, am 2ten Nov., 1884, Reichsdruckerei. Berlin, 1884.

Hekm. Ritter. "Perspectograph." Apparat zur mechanischen Herstellung
der Perspective, etc. Maubach u. Co. Frankfurt a./M.

Prof. J. Heller. "Ueber Photogrammetrie." Ber. d. Ver. d. Techniker
in Ober-Oesterreich. Linz, 1890.

Prof. J. Heller. "Ueber neue Erscheinungen auf dem Geziete der Photo-
grammetrie." Ber. des Ver. d. Techniker in Oberoesterreich. Linz,
1891.

Dr. Schroeder. "Archtectur u. Gelandeaufnahme unter Mitwirkung der
Photographie und der einschlagigen Instrumente." Archiv fuer Artil-
krie- und Genie-Offiziere des deutschen Heeres. Juli Heft, 1892.

l8 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

Dr. ScHROEDER. "Die neusten Messbild-Instrumente." Arcihv f. Artil-

lerie- und Genie -OflSziere des deutschen Heeres. October u. November

Hefte, 1892.
V. Pollack. "Ueber Anwendung dor Photogrammetrie im Hochgebirge."

Wochenschrift des Oesterreichischen Ing. u. Archit. Vereins, 1890.
V. Pollack. "Die photograph. Terrain-Aufnahmen mit Beruecksichtigung

d. Arbeit in Steiermark." R. Lechner. Wien, 1891.
V. Pollack. "Photogrammetrie oder Phototopographie." Mitth. d. kgL

kaiserl. Geogr. Gesellschaft. Wien, 1891..
V. Pollack. "Ueber Photogrammetrie." 2^itschr. d. Ver. deutscher In-

genieure, 1893.
V. Pollack. "Ueber photogr. Messkunst, Photogrammetrie und Photo-

topographie." Mitth. d. kgl. kaiserl. Geographischen Gesellschaft.

Wien, 1891.
V. Pollack. "Ueber Photogrammetrie und deren Entwickelung." Monats-

blaetter des wissenschaftlichen Clubs in Wien, No. 5, Bd. XIII,

1891.
V. Pollack. "Die photographischen Terrainaufnahmen." Centralblatt

fuer das gesammte Forstwesen. Wien, 1891.
V. Pollack. "Ueber Fortschritte der Photogrammetrie." Dr. Eder's

Jahrb. fuer Photographie. Wien, 1892.
V. Pollack. "Die Beziehungen der Photogrammetrie zu den topograph,

Neu-Aufnahmen im Bairisch -oesterreichischen Grenzgebirge." Archiv

f. Artillerie- u. Genie-Ofliziere des deutschen Reichsheeres, 1893.
V. Pollack. "Photogrammetrische Arbeiten in Oesterreich." Dr. Eder's

Jahrbuch fuer Photographie, 1894.
V. Pollack. "Ein neuer durchschlagbarer Phototheodolit mit centrischem

Femrohr." Zeitschr. d. oester. Ing. u. Archit. Vereins, 1894.
F. Steiner. "Ueber Photogrammetrie und deren Anwendung." Tech-

nische Blaetter. Prag, 1890 and 1891.
F. Steiner. "Das Problem der fuenf Punkte, eine Aufgabe der Photo-
grammetrie." Wochenschr. d. oester. Ing. u. Archit. Vereins, 1891.
F. Steiner. "Die Photographie im Dienste des Ingenieurs." Ein Lehrbuch

der Photogrammetrie. R. Lechner. Wien, 1891-94.
F. Steiner. "Die Anwendung der Photographie auf dem Gebiete des Bau-

und Ingenieurwesens, mit besonderer Beruecksichtigung der Photogram-
metrie." Technische Blaetter, Heft III u. IV. Prag, 1891.
M. Bock. "Die Photogrammetrie." Mitth. ueber Gegenst d. Artillerie

und Genie Heft I. Wien, 189 1.
R. KoBSA. "Die Photogrammetrie oder Bildmesskunst, und speciell deren

Verwendung im Dienste des Forsteinrichters." Oesterreich. Viertel-

jahrschrift fuer Forstwesen, Heft II, 1892.

PHOTOGRAPHIC SURVEYING IN AUSTRIA. 19

F. Hafferl. "Das Teleobjectiv und seine Verwendbarkeit zu photogram-
metrischen Aufnahmen." Zeitschr. fuer Verm., 2istes Heft, 1892.

F. Wang. "Die Photogrammetrie im Dienste des Forsttechnikers." Mitth.
d. krainischen kuestenlaendischen Forstveriens. Laibach, 1893.

F. Wang. "Photograrametrische Instrumente." Oesterreichische Forst-
zeitung, Heftc i, 2, 3, 1893.

F. Wang. "Die Anwendung der Photogrammetrie im forstlichen Haus-

halte." Oesterreichische Forstzeitung, Hefte 19, 20, und 21, 1892.

E. Dolezal. "Die Anwendung der Photographie in der practischen Mess-

kunst." W. Knapp. Halle a./S., 1896.
Dr. J. W. Eder und E. Valenta. " Photogranmietrie." Dinglers Polytech.

Journal, 1892.
Dr. J. W. Eder. "Photogrammetrische Apparate und Phototheodolite."

Lehrbuch der Photographie, I. Bd., 1892.
O. J. Klotz. "Photogranmietrische Arbeiten in Canada.'' Zeitschr. des

oesterreich. Ing. u. Archit. Vereins. Wien, 1894.

G. Starke. "Phototheodolit von Starke und Kammerer." 2^itschr. d.
oesterreich. Ing. und Archit. Vereins. Wien, 1894.

Von Huebl. "Messbild Photogrammeter." Annaien fuer Gewerbe und
Bauwesen, 1892.

Vox Huebl. "Messtisch Photogrammeter." Lechner's Mitth. aus dem
Gebiete der Photographie und Kartographie." Graben 31. Wien.

IiCFELD. "Ueber Photogranmietrie." Schweizer Bauzeitung, 1893.

Dr. A. MiETHE. "Photographische Mittheilungen." Jahrgang 24.

VoLKMER. "Das Wesen der Photogranunetrie." Wochenschr. d. oester-
reich. Ing. u. Archit. Vereins, Bd.XIV.

G. Fritsch. "Anleitung zu wissensch. Beob. auf Reisen." Dr. G. Neu-
mayer. Berlin, 1888, R. Oppenheimer.

F. ScHiFFNER. "Ueber Photognunmetrie und ihre Anwendung bei Terrain-

aufnahmen.'^ Mitth. aus dem Gebiete des Seewesens, 1887.
F. ScHiFFNER. "Ueber photogrammetrische Aufnahmen mit gewoehnlichen

Apparaten." Photographische Correspondenz, 1889.
F. ScHiPFNER. "Ueber photographische Messkunst." Organ der militair-

wissenschaftlichen Vereme, 1888 und 1889.
F. ScHiFFNER. "Uebfer die photogrammetrische Aufnahme einer Kueste

im Vorbeifahren." Mitth. aus dem Gebiete des Seewesens, 1890.
F SCHIFFNER. "Die photographische Messkunst, Photogrammetrie, Bild-

messkunst imd Phototopographie.'* W. Knapp. Halle a./S., 1892.
F SCHIFFNER. "Photogrammetrische Studien." Photographische Rund-
schau, 1890 und 1891.
F. SCHIFFNER. Tortschntte der Photogrammetrie." Dr. Eder's Jahrb.

fuer Photographie, 189 1.

20 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

F. ScraFFNER. "Die Fortschritte der photographischen Messkunst im

Jahre 1889." Photogr. Rundschau, 1890.
A. R. VON GuTTENBERO. "Die Photogrammetrie im Dienste der Forst-

messung." Oesterreich. Forstzeitung, April 17, 1891.
"Photographische Rundschau," Heft 7, 1891; Heft 6, 9, 11, 1892. Wien.
C. Tronquoy. "Bemerkungen ueber die [planchetU photographique] von

Chevallier." Photographische Correspondenz, 1867.
"Photographische Correspondenz," No. 353, 1890; Seite 380-384, 1892.
Von Huebl. "Die Stereophotogrammetrie." Mitth. d. k. u. k. militair-

geogr. Instituts, XII, 1903.
A. ScHWASSMANN. "Der Stereokomparator." Annalen der Hydrographie

u. maritimen Meteorologie, Juli, 1902.
Prof. Becker. "Ueber Relieffemrohre u. Entfemungsmesser von C. 2^iss.**

Schweizer iZeitschrift fuer Artillerie u. Genie, Seite 365, 1900.
E. Herino. "Ueber die Grenzen der Sehschaerfe." Berichte der mathem.

phys. Classe der koenigl. Sachs. Gessellsch. der Wissensch. Leipzig,

1899.
L. Heine. "Sehschaerfe und Tiefenwahmehmung." Von Graefe's Archiv

fuer Ophthahnologie, No. 51, 1900.
Dr. O. Hecker. "Ueber die Beurtheilung der Raumtiefe und den stereo-

skopischen Entfemungsmesser von C. iZeiss." Jena, Zeitschr. fuer

Verm., No. 30, 1901.
Von Huebl. "Die stereophotogranmietrische Terrainaufnahme." Mitth.

des k. und k. militair-geographischen Institutes. Wien, 1903.
W. Laska. "Ueber eine neue Phototheodolitkonstruction." Zeitschr. fuer

Instrumentenkunde, 1903.
W. Laska. "Ueber ein Problem der photogrammetrischen Kuestenauf-

nahme." Monatshefte fuer Mathem. und Physik, Bd. 12. Wien.
A. Sprung. "Ueber die allgemeinen Formeln der Photogrammetrie.'*

Meteorologische Zeitschrift, 1903.
R. Grimsinski. "Aufnahme von Horizontalkurven durch das Nivellier-

Messtischverfahren." Allgemeine Vermessungsnachrichten, Seite 252

und 253, 1903.
S. Finsterwalder. "Neue Methode zur topographischen Vermessung von

Ballonaufnahmen." Sitzungsberichte der Muenchener Akademie, 1903.
Dr. C. PuLFRiCH. "Ueber einen Versuch zur praktischen Erprobung der

Stereophotogrammetrie fuer die Zwecke der Topographie." Zeitschrift

fuer Instrumentenkunde, Seite 317-334, 1903.
Dr. C. PuLFRiCH. "Ueber neuere Anwendungen der Stereoskopie und ueber

einen hierfuer bestimmten Stereokomparator." Zeitschr. fuer Instru-

mentenk., XXII, 1902, Seite 65-81; 133-141; 178-192; 229-246.
Dr. C. PuLFRiCH. "Ueber die Constructionen von Hoehenkurven und Planen

PHOTOGRAPHIC SURVEYING IN SWEDEN. 21

auf Gnind stereophotogrammetrischser Messungen mit Hilfe des Stereo-
comparators/' Zeitschrift fuer Instrumentenkimde, XXIII, 1903.
Dr. C. PuLFRiCH. "Neure stereoskopische Methoden iind Apparate fuer die

Zwecke der Astronomic." Topographic und Metronomic. Berlin, 1903.
Dr. C. PuLFRiCH. "Ueber eine neue Art der Herstellung topographischer

Karten und ueber einen haerfuer bestimmten Stereoplanigraphen."

Zeitschr. fuer Instnimentenkunde, XXIII, 1903.
Dr. C. PuLFRiCH. ''Herstellung telestereoskopischer Landschaftsaufnahmen

mit Hilfe einer gewoehnlichen Stercokammer." Photographisches

Ccntralblatt, Heft 24, VIH, 1902.
Dr. C. PuLFRiCH. "Ueber eine Pruefungstafel fuer stereoskopisches Sehen."

iZeitschrift fuer Instrumentenkunde, Heft 9, 1901.
Dr. C. PuLFRiCH. "Ueber einige stereoskopische Versuche." Zeitschrift fuer

Instrumentenkunde, Heft 21, 1901.
JuuTJS Mandl. " Ueber die Verwerthung yon photographischen Aufnahmen

aus dem Luftballon." Mitth. ueber Gegenst. des Artillerle- und Genie-

Wesens, XXIX Jahrgang. Wien, 1898.
O. Jesse. "Untcrsuchungen ueber die sogenannten Icuchtendcn Wolken."

Sitzungsberichte der koenigl. Preuss. Akademie d. Wissensch. zu Berlin*

1890-91.
Dr. K. Hexjn. "Die Bcstimmung der Geschwindigkeit nach den Methoden

der Photogrammetrie." Schlomilchs Zeitschrift fuer Mathem. u. Phys.,

Bd. XLIV, 1899.

IV. Photographic Sunrejring in Sweden.

Swedish meteorologists made use of photographs for the
study of cloud formations in 1877, and since then H. Hildebrand
Hildebrandson, Director of the Meteorological Observatory
at Upsala, has used and recommends the photogrammetric
methods for observations to ascertain cloud altitudes and air-
currents. Identical points of the same cloud, photographed
on two simultaneously exposed plates, may be found at leisure
and with a far greater degree of certainty than such points may
be located, in nature, by two observers with transits and tele-
phone connection for a mutual guidance toward the selection
of identical points for observation.

The first attempt to apply phototopographic methods in

22 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

Sweden was probably made by Prof. G. de Geer in 1882, when
he, together with Prof. A. G. Nathorst, surveyed some glacial
fields in Spitzbergen.

Since 1890 the Swedish General StaflF has been actively engaged
with phototopographic surveys, notably under the direction of
Major Lowison. The field work is conducted very much in
the same manner as that executed in Italy under L. P. Paganini,
restricting the use of the camera to the mountain regions above
timber-line, the valleys and wooded areas being surveyed with
the plane table.

Among those who have actively used phototopographic meth-
ods in Sweden we may cite Major H. Kinberg, Prof. Rosen,
Dr. A. Hamberg, Major N. C. Ringertz, J. Westmann, and
others.

LITERATURE.

H. H. HiLDEBRANDSON. "Suf la Classification des Nuages employee h,

rObservatoire m^t^rologique d'Upsala." Photographies de Henri Osti.

Upsala, 1879.
Nils Eckholm et H. L. Hagstrom. "Mesures des Hauteurs et des Mouve-

ments des Nuages." Rapport au Comity mdt^rologique international.

Lisbonne et Upsala, 1885.
H. H. HiLDEBRANDSON et H. L. Hagstrom. "Des principales M^thodes

employees pour observer et Mesuier les Nuages." Upsala, 1893.
Nils Eckholm. "Einige Bemerkungen ueber die Anwendung der Photo-

grammeter zur Messung von Wolkenhohen." Meteorolog. 2^itschrift,

1894, Seite 377.
Nils Eckholm. "A new Instrument for Cloud-measurements." Quarterly

Journal of the Royal Meteorological Society, XIX, 1893.
Fh. Ackerblom. "De TEmploi des Photogrammfetres pour mesurer la

Hauteur des Nuages." Upsala, 1894.
K. P. Olsson. "On the Calculation of Photographic Cloud-measurements."

Quarterly Journal of the Royal Meteorological Soc, Vol. XX, 1894.
E. LuNDAL et J. Westmann. "Mesures photogramm^triques des Nuages li

Upsala, 1896-97. Upsala.
Prof. E. DoLEZAL. "Photogrammetrische Arbeiten in Schweden." Zeitschr.

fuer Verm., Bd. XXXII, Heft 10, 1903.
KONIGL. Krigsvetenskaps-Akademiens Handlihgar och Tidskrift, Nos. 11

and 12. Stockholm, 1901.

PHOTOGRAPHIC SURVEYING IN SWITZERLAND. 23

P. ACKERBLOM. "Ucber die Anwendung der Photogrammetrie zur Messung
von Wolkenhoehen." Meteorologische Zeitschr., 1894.

V. Photographic Surveying in Switzerland.

Since 1889 Prof. Becker, Col. Fahrlander, Prof. Amrein,
and others suggested the use of photographic methods for bring-
ing the general topographic maps of Switzeriand up to a standard
to meet modem requirements. The Topographic Bureau finally
entered upon a series of experimental surveys to ascertain whether
the phototopographic methods had reached that stage of per-
fection that they should replace the plane-table methods here-
tofore in use for alpine topography.

A description of the phototopographic experimental sur-
veys, made under the auspices of the Topographic Bureau of
Switzerland, may be found in:

M. RosENMUND. "Untersuchungen ueber die Anwendung des photogram-
metrischen Verfahrens fuer topographische Aufnahmen." Fritz Haller.
Bern, 1896.

The Topographical Engineer, S. Simon, used the photo-
topographic method extensively in connection with the pre-
liminary surveys of location for the " Jungfrau " railroad and
for improving the terrene representation of some of the General
Staff maps of Switzerland.

The following publications bearing on this subject may
be cited:

S. Simon. "Le Projet de Chemin de Per de la Jungfrau, examine au point
de vue scientifique, technique et financier." F. Schulthebs. Zimch,

1897.

S. Simon. "Photogrammetrische Studien und deren Verwerthung bei den
Vorarbeiten fuer die Jungfrau Bahn." Schweizer Bauzeitung, Hefte
23, 24, u. 25, 1895.

S. Simon. " Photogrammetrische Arbeiten fuer die Jungfrau Bahn." Schwei-
zer Bauzeitung, Hefte 11 u. 12, 1896.

24 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

VI. Photographic Survejring in Italy.

The extensive mountainous regions of Italy are peculiarly
well adapted for the appUcation of photography to their topo-
graphic surveys and we find that phototopography for several
decades past has been practiced in that country with marked
success.

Prof. Porro spent much time, labor, and energy towards
perfecting the methods and instruments to apply photography
to tachymetry and to topography. The results of his investi-
gations, which date back to the year 1853, were published under
the title " Applicatione della Fotografia alia Geodesia " (Sal-
dini, Milano, 1855).

Prof. Porro's instruments, which were supplied with sen-
sitized plates of spherical shape (" Fotografia sferica "), have
been preserved by Salmairaghi, Director of the Polytechnic
Institute in Milan, Prof. Porro having been a member of the
faculty of this institution.

With Porro's death further investigations and experiments
in phototopography ceased in Italy until the year. 1875, when
Michele Manzi, an officer of the Military Geographic Institute
of Italy, utilized some photographs of views, taken in the Abruzzi
Mountains with an ordinary camera, to supplement the topo-
graphic details of his plane-table survey of the Gran Sasso
group. This attempt gave such gratifying resuhs that the
same officer in the following year made a special and more prac-
tical application of photography in the topographic sur\^ey
of Mont Cenis, particularly of the region about the Bart
Glacier.

In 1878 Gen. Ferrero, Chief of the Geodetic Department
of the MiUtary Geographic Institute, pointed out to the Direc-
tory that the resumption of practical tests and experimental
surveys in phototopography were very desirable, in view of the
advances which had been made in the photographic methods

PHOTOGRAPHIC SURVEYING IN ITALY. 25

recently. Phototopography had been suspended in deference
to the claims of several members of the Institute that photo-
graphic data were too unreliable for topographic purposes, par-
ticularly for large scale maps.

In the same year (1878) L. P. Paganini, Engineer Geographer
of the Institute, was instructed to proceed to Apua and resume
phototopographic survey work, with a view toward ascertaining
whether phototopographic surveys would be economical and
more expedient, now that such decided advances had been made
in both the manufacture of photographic lenses and in the photo-
chemical process.

The practical results of Paganini's investigations and experi-
mental surveys during the period from 1878 to 1879 were so
satisfactory that, in 1880, he was directed to begin a systematic
phototopographic survey of the area bounded by the valleys
of the Oreo, the Valsoana, the Cogne, and the Valsavaranche,
comprising an area of about 390 square miles. The survey
of this mountain complex was finished by Paganini in 1885.
In 1884, however, the phototheodolite of the Institute had been
replaced by an improved instrument of superior qualities, made
by Galileo in Florence after plans and specifications submitted
by Paganini and incorporating improvements suggested by the
experience gained in the field.

This phototheodolite, model of 1884, has been fully described
by Paganini in *' La Fototopografia in Italia," Rivista Marit-
tima, Fasc. VT e VII, 1889; also in Rivista di Topografia e
Catasto, Nos. 8, 9, e 10, 1889. A German translation, by
A. Schepp, of L. P. Paganini's " La Fototopografia in Italia "
may be found in the Zeitschrift fuer Vermessung, Nos. 3 and
12, 1 891, and No. 3, 1892. A translated extract from Paga-
nini's article has been published in Appendix No. 3, in the
Superintendent's Report of the U. S. Coast and Geodetic Sur-
vey for 1893.

Paganini's excellent results eflFectively established the effi-
ciency of phototopography for alpine topography and fully

26 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

solved the technical side of the problem. Owing to the untiring
eflForts of the officers of the Military Geographic Institute toward
improving phototopographic methods and instruments, the sur-
veying camera has been adopted as an auxiliary instrument
to the plane table, the combined use of both instruments in
the new topographic survey of Italy having produced the best
results.

The latest improvements to Paganini's camera-theodolite were
first described in a report to the First Geographic Congress in
Italy. A German extract from that report by Fenner may
be found in the Zeitschrift fuer Vermessung, 1893.

The principal departure from the older model (1884) consists
in abolishing the excentrically mounted telescope and converting
the camera itself into a centrally mounted telescope by replacing ,
the ground-glass plate of the camera with an opaque plate having a
Ramsden eyepiece in its center whenever observations with the
telescope are to be made. This new model (1890), having all the
details of a theodolite with vertical circle, may be used, when-
ever the necessity arises, for making angular measurements,
in the same way as with an engineer's transit, simply by exchang-
ing the ground glass for the plate with eyepiece, as just men-
tioned.

This instrument, together with the "photographic azimuth
apparatus " designed for hydrographic surveys, has been described
by Paganini in "Nuovi Appunti di Fototopografia; Applicationi
della Fotogrammetria all' Idrografia, seguiti alia Nota; La
Fototopografia in Italia." Publicata nella Ri vista Marittima,
Roma, E. C. Forzani, 1894.

When the Military Geographic Institute, in 1891, sent some
map specimens and phototopographic instruments to the Ninth
Geographic Congress in Vienna, in illustration of the Italian
phototopographic methods, Col. Robert von Stemeck wrote
to the Institute, in the name of the Committee on Awards,
that the Italian phototopographic exhibit undoubtedly deserved
the first prize. Franz Hafferl, Engineer of Austrian Railways^

PHOTOGRAPHIC SURVEYING IN SPAIN. 2^

wrote : " Votre exposition photogramm^trique est sans comparison
la meilleure. Toutes les autres ne sont que des essais plus ou
moins manqufe de construction d'appareils phototopographiques
et des constructions de cartes d'une petite entendue." Dr. S.
Finsterwalder (Professor of Mathematics and Photogrammetry in
Munich, Bavaria), Vincenz Pollack (Engineer in Chief of the
Austrian railroad system), and Col. Otto Krifka (of the Geographic
Institute in Vienna) also made commendable reference to the
Italian exhibit.

Other publications having reference to the phototopographic
work in Italy, besides those already referred to in the preceding
paragraphs, may be cited as follows:

Giuseppe Bertelli. "Note ed Appunti di Topografia-Fotografia." Rivista

Militare Italiana, Feb., 1884.
Capt. Carlo Marselli. '^La Foto-topografia applicata alia Construzione

della Carta Alpine." . Bolletino del Club Alpino Italiano, XXIV, No. 57

1890.
GiACOMO BuoNOME. ''La Foto-topografia in Africa." Bolletino della

Societa Africana, I e II, 1890.
Prof. Innocenzo Gk>LFAiuELU. "Bolletino della Societa Fotografica Ital-
iana." Firenze, April and May, 1890.
L. Bennati. "La Fotografia nelle sue Applicazione militare." Rivista

d'Artiglieria et Genio, II, 1892.
CoL A. Laussedat. "Iconom^trie et M^trophotographie Notice sur THis-

toire des Applications de la Perspective k la Topographic et k la Carto-

graphie." Paris, Photographe, Sept. et Oct., 1891.
Dr. S. Finsterwalder. "Die Photogrammetrie in den Italienbchen Hoch-

Alpen." Mittheilungen des deutschen und oesterreichischen Alpen-

Vereins, No. i, 1890.
Wochenschrift des Oester. Ing. u. Archit Vereins, Nos. 21 and 23, 1890.

Vn. Photographic Surveying in Spain.

Although an early interest was manifested in photography
applied to surveying in Spain, little has been accomplished in
the practical application of photogrammetrie methods until quite
recently.

28 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

The Madrid Academy of Sciences, in 1862, oflFered a prize for
the best treatise in answer to the query, What is the best process
or method for applying photography to the plotting of maps and
plans ? Of the answers received the memoir, submitted by Capt.
A. Laussedat in 1863, was awarded the prize. This memorandum
was accompained by the plan of the village Buc, near Versailles,
plotted on 1:1000 scale, together with eight photographic \dews .
on which the topography of this plan was based.

In the following year Lieut.-Col. Don Pedro de Zea was com-
missioned to study the French methods and instruments used in
photographic surveying. Lieut.-Col. P. de Zea examined Capt.
Laussedat's phototopographic camera (made by Brunner, Paris),
Capt. Chevallier's "planchette photographique," Sutton's "pano-
ramic camera " (made by Thos. Ross, London), etc., collating
and describing the results of his investigations in:

Don Pedro de Zea. *'Las Applicaciones de la Fotografia al Service militar."
Madrid, 1863.

Don Juan Pie y AUu^, Mining Engineer, published a phamplet
on phototopography, in 1896, in which a specimen surv^ey that he
had made in the province of Almeria, plotted on 1:1000 scale,
was included :

Don Juan Pie y Allxt^. " Fotogrametria 6 Topografia fotografica." Enrique
Teodoro. Madrid, 1896.

A very complete and general work on phototopographic
methods, instruments, and executed surveys, including experi-
mental survey specimens made in Spain, has been published by
Messrs. C. de Iriarte and L. Navarro, in 1899:

CiRiACO DE Iriarte y Leandro Navarro. "Topografia fotografica 6 sea
Applicacion de la Fotografia al Levantamiento de Pianos." Raoul
P^ant. Madrid, 1899.

PHOTOGRAPHIC SURVEYING IN CANADA AND IN ALASKA. 29

Vm. Photographic Survejring in the Dominion of Canada and

in Alaska.

Capt. E. Deviile, Surveyor-General of Dominion Lands, inau-
gurated extensive phototopographic surveys in Canada, which
from their inception, in 1888, were marked with great success.
These surveys were carried out under the auspices of the Canadian
Department of the Interior in the vicinity of the Canadian Pacific
R.R. in the Rocky Mountains. A special triangulation had been
made and a single photographic surveying party of four men
(under J. J. MacArthur) covered an average area of 500 square
miles per annum until 1892. The winter months of each year
were spent in Ottawa with plotting the photographic data col-
lated in the preceding season (under the direction of Capt. Deviile)
on a scale of i : 20000.

At the World's Columbian Exposition in Chicago, 1893, a
phototopographic map of a part of the Rocky Mountain Park,
comprising a dozen sheets of about sixty square miles each,
published on a i .-40000 scale, formed one of the most interesting
exhibits of the government of the Dominion of Canada. The
topography on each sheet was obtained, on an average, from six-
teen stations, giving from seventy to one hundred and twenty
panorama views. Six complete panoramas were taken, on an
average, from stations situated within the Umits of the topography
mapped on each sheet and the development of the terrene was
controlled by about ten additional camera stations falling outside
of the actual sheet margin and furnishing ten additional partial
panoramas.

From fifteen to twenty points per square mile were plotted
iconometrically, and whenever possible such points were checked
by means of vertical and horizontal angles, obser\'ed from the
several camera stations, for locating (instrumentally) a series
of so-called reference points. These fifteen to twenty points
form the control per square mile of topography, all intervening

30 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

details and topographic features being sketched, after a careful
study of the panorama views, in a similar manner as the plane-
tabler would sketch such details, under a careful and critical
study of the surrounding terrene. The published map, scale
1:40000, shows horizontal contours of one hundred feet vertical
interval. The average cost of this survey was about seven
dollars and one half per square mile.

The atmospheric conditions of this sparsely settled region,
between the 51st and the 52d degree northern latitude, are notori-
ously unfavorable for executing the field work of the ordinary
topographic surveying methods, and the periods of reasonably
clear weather at best are of short duration. The field season
lasts about three months — ^July, August, and September — ^and
even during that short period the observers have to contend with
fogs, rain, dense smoke caused by forest fires, and snow-storms
(in the higher altitudes). These conditions being well known,
Capt. Deville's suggestion, to give the phototopographic method
a practical trial for the survey of the Rocky Mountain region,
was endorsed by the Canadian Government. The good results
obtained in the first season showed that the economical and
rapid solution of this difficult problem would not have been
possible without the aid of photography.

The remarkably good results that were obtained in the photo-
topographic survey of the Rocky Mountain regions are in a
great measure due to the ability of the field observers adapting
themselves readily to the new methods, but the credit for the
inception of the work, devising new methods, and a compact and
serviceable topographic surveying camera suiting the prevailing
conditions of the country, and for the general excellence of the
results that were obtained, primarily belongs to Capt. E. Deville,
Surveyor- General of Dominion Lands and author of an excellent
manual on " Photographic Surv-eying," published by the Cana-
dian Government at Ottawa in 1889. This edition, of about
fifty copies, was lithographed in the Sur\^ey's office, having
been prepared for the use of the Dominion land surveyors

PHOTOGRAPHIC SURVEYING IN CANADA AND IN ALASKA. 31

employed under the Department of the Interior for making
the phototopographic surveys.

The Rocky Mountain work was suspended when the ques-
tion arose of making a topographic reconnaissance of S.E. Alaska
for the delimitation of the boundary line between S.E. Alaska
and British Columbia. This topographic reconnaissance work
in Alaska gave the phototopographers of Canada (who for these
new duties were placed under the direction of Dr. W. F. King,
Alaskan Boundary Commissioner to H. M.) another opportu-
nity to demonstrate the superiority of this method above all
other surveying methods for delineating the topography of a
country peculiarly rich in climatic and topographic difficulties.

During the summer months (middle of May to end of August)
of 1893-94 and to a smaller extent in 1895, this method was
used for surveying the topography of S.E. Alaska. Each sea-
son's work was plotted in Ottawa in the following winter on
1 :8oGoo scale with horizontal contours of 250 feet vertical interval.

The number of phototopographers prior to 1893 was com-
paratively small in Canada. Seven of the Dominion land sur-
veyors were given a practical course in phototopography, under
J. J. MacArthur, in the suburbs of Ottawa, to familiarize them
with the methods and instruments devised by Capt. Deville. In
May, 1893, these surveyors were placed in charge of the Cana-
dian phototopographic parties, each chief having assigned him
one assistant (also a D. L. S.), from four to five general helpers,
or packers, and one cook. The survey being jointly made by
both the Canadian and the American Governments, six of the
Canadian parties were joined by one U. S. Coast and Geodetic
Survey officer with an additional packer for each American.

During the summer season of 1893 these parties experienced
an average of but twenty days favorable for carrying on the work
in the mountains, and the Canadian expert phototopographer
(J. J. MacArthur) occupied about seventeen camera stations
during that period. He exposed 108 plates, which controlled
an area of about 11 50 square miles. The other parties, in charge

32 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

of iess experienced observers, averaged from 450 to 500 square
miles each during the first season.

The season of 1894 proved more favorable for the work in
Alaska than the preceding one, both on account of better weather
(averaging about forty days suitable for work in the mountains)
and because the observers were now more experienced in the
rountine and requirements of this class of work. Mr. J. J.
MacArthur covered an area of about 1900 square miles, having
occupied twenty-four mountain peaks and exposed 275 plates
during this season, while the other six parties averaged iioo
square miles each.

These results, however, should not be placed in the same
class with the phototopographic survey of the Rocky Mountain
Park, as the result aimed at in Alaska was only a topographic
reconnaissance, based on a narrow coast triangulation which
also extended inland along the more prominent inlets and rivers.
This triangulation had been made by the U. S. Coast and Geodetic
Survey to control the usual strip of coastal topography and to
form the basis for the hydrographic surveys of the navigable
waters of S.E. Alaska.

The members of the Coast and Geodetic Survey, who had
been attached to Canadian parties in 1893, became familiar
with the practical operations and applications of the photo-
topographic surveymg method, and, in 1894, Dr. T. C. Menden-
hall. Superintendent Coast and Geodetic Survey and American
Boundary Commissioner, had a surveying camera used in con-
jimction with the plane table for the topographic reconnaissance
at the Head of Lynn Canal, Alaska, by which means the area
covered with the plane table alone was doubled by the sub-
sequent iconometric plotting in the office from ninety negatives.

The same surveying camera was used by the Coast and
Geodetic Survey parties in Alaska in 1895 (Portland Canal) and
again in 1897 (Pribilof Islands), under Gen. W. W. DuffieU,
Superintendent U. S. C. and G. S. and American Boundary
Commissioner.

PHOTOGRAPHIC SURVEYING IN CANADA AND IN ALASKA* 33

Photography has also been applied recently to surveys made
for the solution of questions of irrigation in those regions of the
British N.W. Territories where the rainfall is insufficient for
agricultural purposes.

Capt. Deville's first edition of his book on photographic
surveying having been too limited to supply a general demand
he yielded to the pressing demand for an English manual on
this subject by revising and reissuing his book. The valuable
contents of this work, including the elements of descriptive
geometry and perspective, fully justify the expectations that
were connected with its appearance.

LITERATURE (English),

Bridges-Lee. "Phototheodolite," 2d edition. L. Casella, Maker to the

Admiralty, Ordnance, etc London, E.C.
R. Strachy and G. M. Whipple, Supt. "Cloud Photography conducted

under the Meteorological Council at Kew Observatory." Proc. of the

Royal Soc. of London, Vol. 49, 1891.
C. W. Verner. "Notes on Military Topography." Allen. London, 1891^
Stanley. "Photographic Surveying." San. Eng., 1892, p. 71.
Col. A. Laussedat. "Topographical Reconnaissances with the Aid of

Photography." Phot. Times Aknanac, 1895.
B. J. EDWAims. "Color Screens for Use with Isochromatic Plates and

Fihns." Phot. Times Almanac, 1895.
Albert Gleaves. "Some Scientific Applications of Photography." Phot.

Times Ahnanac, 1895.
Otto J. Klotz. "Experimental Application of the Phototopographical

Method of Surveying to the Baird Glacier, Alaska." The University o£

Chicago Press, 1895.

The Canadian phototopographic methods have been de-
scribed in the following publications:

Capt. E. Deville. "Photographic Surveying, including the Elements of
Descriptive Geometry and Perspective." Ottawa, Government Printing
Bureau, 1895.

Otto J. Klotz. "Photogrammetrische Arbeiten in Canada." Zeitschr. des
Oester. Ing. u. Archit. Vereins, 1894, pp. 235-234.

34 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

Col. A. Laussedat. "Sut le Progrfes de TArt de lever les Plans k TAide
de la Photographie, en Europe et en Am^que." Comptes lendus de
TAcad^nie des Sciences, 1893.

Col. A. Laussedat. '^Reconnaissance faite a FAide de la Photographie, pour
la Delimitation de la Fronti^re entre PAlaska et la Colombie Britanique."
Comptes rendus de PAcad^nie des Sciences. Paris, 1894.

Report of the Superintendent U. S. Coast and Geodetic Survey for 1897^
Appendix No. 10, *' Phototopographic Methods and Instruments."
Washington, D. C.

CHAPTER n.

THE ELEMENTS OF PERSPECTIVE (CENTRAL PROJECTION).

Photogrammetry being the inverse of perspective it may
not be out of place here to review, at least in a summary manner,
the principal laws of monocular vision, as they are identical
in a great measure with the laws which form the foundation
of perspective. J. H. Lambert (1728 to 1777) apparently was
the first to lay down rules for finding the point of view of a per-
spective and to determine the dimensions of objects represented
in perspective.

L Visual Seeing.

The eye in order to see an object must receive visual rays
from every illuminated point of the object. It is a well-known
fact that the retina of the eye receives an inverted image of every
sighted object, and yet we all know that objects are seen in
their natural positions, without requiring a mental transposition
of the inverted image into the erect position. The explanation
for this may be found in the so-called " law of visible direction,"
which, according to LeConte, may be stated thus: "The impres-
sion on the retina of the eye produced by a ray of light enter-
ing the eye is referred from the eye along the ray-line back again
into space whence it emanated, and therefore back to its source
or proper place."

Every luminous impact which the retina receives by a light
ray passing through the nodal point of the lens into the eye
is immediately and intuitively referred outward, along the same

36 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

path which the entering ray traversed, to the true place which
the luminous point occupies in space. In other words, objects
sighted as such in space are always the results of " outward
projections " from the images on the retina through the nodal
point of the eye as center.

n. Central Projection.

If we project from a fixed center — say from the nodal point
of the eye — the " visible " parts of an object upon a plane inter-
posed between the eye and the object, the outUnes of such pro-
jection will produce the same impression on the retina of the
eye as the outlines of the natural object, provided, of course,
that one eye only was used and at the same time that the rays
which emanated from the different points of the pictured object
could be made of the same kind (of the same intensity and color)
as those coming from the corresponding points of the object
itself. The view of such a perspective would then produce sen-
sibly the same impression on the eye of the observer as the object
in nature.

Of the different perspectives capable of being represented
on a plane surface we are interested here mainly in the so-called
monocular or focal and linear or mathematical perspectives,
both outgrowths of descriptive geometry and consisting in the
application of the rules of projection in general and those of
orthogonal projection in particular.

m. Photographic Perspectives.

The iconometrical problem to be solved in phototopography
may be stated in the following general terms. From a given
perspective (central projection) of a body, projected from a
fixed center (point of view, nodal point) upon a (vertical) pic-
ture plane, we are to construct the horizontal orthogonal pro-
jection of that body.

PHOTOGRAPHIC PERSPECTIVES. 37

With reference to Fig. i, Plate I, we may say the perspective
a, 6, Cy dy e in the vertical picture plane F7 is the central pro-
jection of the object i4, B, C, D, E in space from the nodal
point O as center or point of view.

Any one object will produce in a given picture plane but one
perspective image from the same point as center. A point B
of the object is pictured but once, in b, b being that point in the
picture plane VV where the visual or projecting ray of the point B
penetrates the plane VV.

Of the numerous methods by means of which perspective
\iews may be constructed we shall refer only to those which
have reference to iconometric plotting. To elucidate the close
connection between the three elements that control or define
the central projection or perspective, viz., object, picture plane,
and center of projection, with reference to phototopography
we may premise, with reference to Fig. 2, Plate I:

A — ^The picture plane VV (photographic plate) is sup-
posed to be vertical.
B — ^Through the center of projection O (eye-point) a

horizontal plane HH is placed (" horizon plane").
C — ^A vertical plane is laid through the center O, inter-
secting the picture plane at right angles in the line w;
it is the so-called " principal plane."
D — ^A plane GG (*' ground plane ") is placed parallel
with the horizon plane HH, but falls below it; the
distance OO between the two planes is equal to the
elevation of the point of view O (in the horizon plane)
above the datum plane (to which all elevations of
the survey are referred). The ground or datum
plane in iconometric plotting is identical with the
plan and it is represented by the surface of the paper
upon which the topographic map is being plotted.
The line of intersection gg. Fig. i, Plate I, of the ground plane
GG with the picture plane VV is known as the " ground line "
of the perspective.

38 PHOTOTOPOGKAPHIC BIETHODS AND INSTRUMENTS.

The line of intersection AA, Fig. i, Plate I, of the horizon
plane HH with the picture plane F7 is termed the " horizon
line " of the perspective.

The line of intersection w of the plane (" principal plane ")
passing through O and intersecting the vertical picture plane VV
at right angles, Fig. 2, Plate I, is called the " principal line "
of the perspective.

The intersection CX of the two lines hh and w, Fig. 2, Plate I,
is the " principal point " of the perspective. It marks the point
of penetration in the picture plane of the " principal ray " 00\
The principal ray is drawn from the center O (point of view
or nodal point) horizontally to intersect the picture plane VV
at right angles.

The point 0\ where the vertical through the station O pierces
the ground plane GG is termed the " foot of the station."

The length of the principal ray OCy, equal to the vertical
distance of the point of view O from the picture plane, is termed
the "distance line."

When the point of view coincides with the second nodal point
of a camera-lens this same line, the distance line, is known as
the "focal length" of the camera.

The perspective view a of a luminous point, ^4, Fig. 2, PL I,
in the vertical picture plane Wis identical with the point of pene- ,
tration of the visual ray 0-4, passing from the luminous point A
to the center of projection O (point of view or nodal point).

If we have several parallel vertical picture planes FF, V'V'y
V"V"^ . . . , Fig. 3, PI. II, the impression produced on the retina
of the eye at O will remain unchanged, no matter which plane
FF of the series may be retained in its position while the others
are removed.

All planes V'V\ V"V"y . . . , placed parallel to the picture
plane VV are termed "front planes" and any line drawn in a
front plane will be parallel to the picture plane and is called a
"front line." Front planes may be placed either before or behind
the picture plane.

PHOTOGRAPHIC PERSPECTIVES. 39

The perspective view ab of a line -45 is found in the vertical
picture plane VV, Fig. 4, PL II, by joining the perspectives a and b
of its end-points. The perspective ab of a line AB coincides
with the trace produced in the vertical picture plane FV by a plane
(so-called "visual plane") passing through O and AB; it is the
intersection of these two planes.

The perspective a, b, c, dy eoia. curve A, B^ C, Dy E is found
by locating the perspectives of a series of its points, a, 6, c, d, «,
Fig. I, PL I, in the vertical plane VV and drawing a continuous
curve through these points. The perspective of a curve may
also be obtained by locating the perspectives of a series of tangen-
tial lines enveloping the curve. The perspective of a curve
a, by Cy dy e is the intersection with the picture plane of that conical
visual plane which contains the curve Ay By Cy Dy E ^s trace
and which has its apex in O.

To draw the perspectives of the superficial planes of bodies,
the figures inclosing the same (f.i., the perspective of the pentagon
Ay By Cy Dy Ey Fig. 4, PL II) are drawn in perspective by con-
structing the central projections of their perimeters.

The perspectives of parallel lines when produced will inter-
sect each other in one point, the so-called "vanishing point."

The perspectives of all horizontal lines {AB and A'B'y Fig. 5,
PL III) have their vanishing point (2?) on the horizon line hh
in the picture plane VV.

Lines perpendicular to the picture plane have the principal
point of the perspective as vanishing point (in the picture plane).

Horizontal lines intersecting the picture plane under an angle
of 45® vanish in the so-called "distance points" on the horizon
line, one on either side of the principal pomt. Their distances
from the principal pomt are equal to the distance line of the per-
spective.

The so-called upper and lower distance points are the vanishing
points for lines falling within the principal plane or that are
parallel with it and which intersect the picture plane under an
angle of 45®. The distances of these two points from the principal

40 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

point are likewise equal to the distance line of the perspec-
tive.

Lines parallel with the picture plane, lines in front planes^
have no vanishing points in the picture plane. Their perspectives
are lines parallel to the original lines.

Vertical lines (are parallel to the picture plane in our case)
have no vanishing points and their perspectives are parallel with
the principal line w, Fig. 5, PL III.

Horizontal lines when parallel with the picture plane have
perspectives that are parallel with the horizon line.

The scale of a front plane is the proportion between the per-
spective and the original. It is expressed by the ratio between
the distances from the station (point of view) to the picture plane,
the distance line, and that to. the figure's front plane (the plane
containing the original figure).

The relationship between object (prism ABCD-A'B'Ciy),
picture plane (VV), and ground plane (GG) may be shown more
clearly with reference to Fig. 5, PI. Ill:

O is the station, eye-point, point of view, nodal point, etc.
A vertical line passing through O will intersect the ground plane
in Oi. The point Oi is the orthogonal (vertical) projection in
horizontal plan of the station O and it is called the " foot of the
station" O.

The perspective ai of a point Ai', situated in the ground plane
GGj is obtained by joining the foot of the station Oi with the point
i4i', erecting a perpendicular to the ground plane in the point of
intersection a/ of OiAi' with gg and joining O with Ai'. The
intersection of the ray 0-4 1' with the vertical aia/ just mentioned
will be the perspective of the point Ai' of the ground plane GG,

AAi being a vertical line in space, its perspective aai will be
parallel with the line w, and, if we draw the ray OAj its inter-
section a with the line drawn parallel to w through ai, previously
found, will be the perspective of A.

To find the perspective of a line AB, the perspectives a and b
of its terminal points A and B may be located in 77 and joined

PHOTOGRAPHIC PERSPECTIVES. 4I

by a straight line ab. Frequently it will be more convenient,
however, to use the intersection T of the line AB with the picture
plane VV together with the vanishing point D of the line ab to
locate the perspective ab of the line AB. This vanishing point D
is the intersection with the picture plane VV of a Ime drawn
through the station O and parallel with the line AB. If AB is
horizontal, the line OD will fall within the horizon plane and
intersect the horizon line hh at D.

The line TT^, which is the trace in the picture plane VV of
the plane ABA'B\ is termed the "vanishing line "of the plane
ABA'B\

CHAPTER m.
PINHOLE PHOTOGRAPHY.

The photographic camera produces perspectives upon the
photographic plate through the chemical action of the light
rays upon the sensitized film, and to establish the conditions
that are to be fulfilled, in order to regard a photograph as a true
perspective, we will first consider the so-called " pinhole pic-
tures," which are produced by a camera of the simplest form^

The pinhole camera consists of a box made entirely light-
tight with the exception of a minute hole O, Fig. 6, Plate IV,
in the front wall of the box. The rear side of the box is remov-
able and may be replaced by either a photographic plate-holder
or a ground-glass plate. With such a " camera obscura " photo-
graphs may be obtained without a lens or optical apparatus,
simply by means of the small round aperture O in the thin front
wall of the box.

I. Diameter of the Pinhole.

When exercising some care, the pinhole may be made by
burning it into a thin blackened cardboard with a needle heated
to red heat. The following table gives the diameter in inches
that may thus be burnt into the cardboard with needles of difi'er-
ent sizes:

Commercial number of se\nng-needle 3 6 8 q

Diameter of burnt hole in inches 1/26 1/34 1/44 1/49

LENGTH OF EXPOSURE FOR PINHOLE CAMERAS.

The best results, however, have been obtained with a round
hole carefully drilled into a sheet of copper or brass 0.2 mm.
thick. The border of the hole should be perfectly smooth, with-
out " burr," and it should be beveled that the hole forms a
truncated cone, the larger circle or base of the cone to face the
sensitized plate in the camera-box.

n. Length of Exposure.

The following table, published by F. C. Lambert, gives the
corresponding exposures, in minutes, for pinhole-camera expos-
ures, if, with the same plate-brand, identical illumination, same
subject, and a lens working at //16, the correct exposure would
have been one second.

Distance of
Pinhole from

Diameter of Pinhole, in Inches.

the Sensitized

Plate Siirface, in

Inches:

x/So

1/44

I/3S

x/2%

8 •

—

This table plainly indicates that there is little danger of over-
exposing a plate in the pinhole camera, particularly as these
exposures are not strictly limited to the time given in the table;
they depend greatly on the general character of the plate, on the
developer, and on the general conditions of illumination during
the exposure, thus giving the operator a wide range regarding the
time limit of the exposure.

44 PHOTOTOPOGRAPHIC METHODS AND INSTRUBiENTS.

m. Focal Lengths of Pmhole Cameras.

The depth of. focus is practically unlimited, as shown in the
preceding table of F. C. Lambert. Still, there will alwa)rs be a
certain distance between image plane and pinhole that will give
the best result for a given aperture, and Capt. Colson recom-
mends the following focal distances for a set of apertures of
four different sizes:

Diameter of pinhole in millimeters 0.3 0.4 0.5 0.6

The best definition is at a focal length, in centimeters ...11 20 30 44

Using the focal length corresponding to the size of aperture,
as given in the above table, the time of exposure for a plate in the
pinhole camera, compared with the exposure required when using
a lens under identical conditions and with a medium stop, may
be generally accepted to be:

25 50 100 200 times longer for a diameter of hole of:

0.3 0.4 0.5 0.6 mm.

The size of a pictured object, when photographed in a pin-
hole camera, is proportional to the ratio between the distance
of the object from the camera and the distance from the pin-
hole to the sensitized film surface.

IV. Determination of the Values of the Pinhole-camera Constants.

It will be a simple matter to determine the values of the con-
stants of a pinhole camera that are required to be known for
making iconometric constructions.

If the angles of the box are exactly 90°, if the aperture is in
the point of intersection of the diagonals of the camera front,
and if means are provided for setting the camera level (for exposing
the plate in vertical plane), the two lines joining the opposite

VALUES OF THE PINHOLE- CABIER A CONSTANTS. 45

middles of the four sides which compose the rear frame of the
box will represent the horizon line (HH) and the principal
line (VV) of the photographic perspective. The intersection (P)
of these two lines will be the principal point a^d the distance (OP)
between the aperture and the sensitive film surface will be the
constant focal length or the distance line of the photographic
perspective a, 6, c, Fig. 6, Plate IV.

By referring to Fig. 6, Plate IV, it will readily be seen that
the rays of an object A, By Cy after passing through the aper-
ture O, produce an inverted image a, i, c on the photographic
plate. The image obtained in a pinhole camera originates in
the same way as a perspective is drawn, with the exception
that the picture plane V'V is not interposed between the eye-
point O and the original A, B, C, but is here placed behind the
eye-point, at a distance PO equal to OP^, producing an inverted
and reduced image a, 6, c of the original Ay By C.

By introducing the " negative " with the image a, 6, c between
the eye-point O and the original .4, 5, C at VV, Fig. 6, Plate IV,
and at a distance from O equal OF^OP (in- and reverted),
it would become a " positive." F being in the prolongation
of the distance line OP and V'V intersecting the line OP* at
right angles, the line hh of the " positive " will be horizontal
and w vertical. The point a will again be in the point of inter-
section of the light-ray 0-4, the point b in the intersection of
the light-ray 05, and the point c in the intersection of the light-
ray DC with the plane of the positive V'V. A positive copy
of a negative will be as true a perspective of the original as the
negative. Negatives, however, may be used for obtaining any
measurements that may be required from the perspective for
the iconometric plotting. Measurements are often preferably
made on the negatives, as the production of the positives without
distortion requires considerable care and experience, the amount
of distortion depending gready on the character of the material
on which the positives are made.

The data given in this chapter may prove useful when a

46 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

pinhole camera is selected for phototopographic or photogram-
metric experimental studies in case of an emergency, or when
the cost of the apparatus must be considered, the pinhole camera
being reconunendable chiefly on account of its cheapness and
simplicity.

CHAPTER IV.

THE FUNDAMENTAL PRINCIPLES OF ICONOMETRIC MAP-
PLOTTING ("ICONOMETRY").

Under " iconometry " we understand the measuring of
dimensions of objects from their perspective views (" Bildmess-
kunst "). It refers to the plotting of terrene forms directly on
the plotting-sheet from the photographs of the landscape.

If a photographic perspective of an object, the focal length
(" distance line *'), the second nodal point (" principal point ")
of the camera-lens, and the horizon line of the perspective are
given — ^if the point of view and the central projection of an object
are given — ^these data will be insufficient for the determination
of the object with reference to position and size.

If, however, two such perspectives of the same object, obtained
from two suitably located stations, be given, the dimensions of
the object and its position with reference to the two stations may
be determined iconometrically, very much in a manner analogous
to that in which a point is located (by intersection or by the so-
called radial method) on the plane-table sheet by being observed
upon from two known plane-table stations.

L Orienting the Picture— Traces on the Plotting-sheet

The positions of two camera stations A and -4', their linear
horizontal distance AA^ and two photographs, exposed in ver-
tical plane, one from each station, may be given. Each picture
may, furthermore, contain the image / of the same object T
and the image a of the other camera station, Fig. 7, Plate IV.

48 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

After the base line AA' has been laid down in reduced scale
iliili', Fig. 7, Plate IV, and the pictures MN and M'N' are
brought into the same relative positions with reference to the
line A\A\\ which they had with reference to the base line AA^
in the field at the time of their exposure, the position T of the
point pictured as t and t on the respective pictures MN and
M'N' may be located (with reference to the line AiA\yhy
drawing the radials A\i and A\f!^ when their point of intersec-
tion will fix the relative position of T with reference to A\ and A\.

The position of T on the map, plotted to scale with refer-
ence to the reduced base line or with reference to the plottisd
stations A\ and A\y would be found by projecting the point
of intersection T into the plotting or ground plane.

A topographic map being the orthogonal projection of the
terrene forms into horizontal plan, the horizontal projections
into the plotting-plane of the rays A\U A\a\\ A\'(y and A\*a \x^
used to locate the plotted positions of pictured points / and a
and the horizontal projections of the picture planes (which -now
become " picture traces ") are utilized in this connection, instead
of actually using the pictures in the iconometric plotting as was
indicated in the diagram of Fig. 7, Plate IV.

in order, therefore, to plot the horizontal projection T\ of
a pictured point / with reference to the plotted base line A\A\y
it will become necessary to ascertain the correct positions df
the picture traces with reference to A\ and A\ — it will become
necessary to "orient" the picture traces hh and A' A', Fig." 8,
Plate V.

This orientation of the picture traces forms a very important
part in iconometric plotting, as the subsequent fixing of loca-
tions of pictured points is accomplished mainly by bringing
the horizontal projections of their radials (lines of horizontal
directions drawn from the different stations to identical terrene
points) to intersect. Any error in the orientation of the pict\u?e
trace produces corresponding errors in the plotted positions of
pictured points.

ORIENTING THE PICTURE — TRACES ON THE PLOTTING -SHEET. 49

A. Iconometric Plotting when using a Surveying Camera only.

A base line measured in the field may have been plotted to
scale, AiAi'y Fig. 8, Plate V, and two pictures, MN and M'N'^
Fig. 9, Plate V, may have been obtained from the camera sta-
tions A and A' respectively by means of a surveying camera.
The focal lengths of the pictures =/ and f respectively, the
positions of the principal points P and P' and the horizon lines
HH and H'H' may be known. It is desired to locate T\ with
reference to the plotted base line A\Ai.

We have ^4 iPi = /; A I'Pi' = Z' ; the length of the base =AiA /,
and the abscissae /iP=/iPi, h'F =/i'Pi', Pai' =Plal^ Fai =Pi'ai,
Figs. 8 and 9, Plate V.

The distances Aiai'^ and Ai'au Fig. 8, may be found graph-
ically by constructing the right- angle triangles AiPiai and
Ai'Pi'au or they may be computed from

lai' =\/ (^iPi)^-f(Pia7)^,
^i'ai=\/(4i'PiT-f(Pi'ai)2.

These distances are laid oflF upon AiAi from A\ and from ^4/
respectively a semicircle is described over each length, Aiai'
and Ai'au and two circles are drawn about Ai and Ai with
/ and /' respectively as radii. The intersections of these two
pairs of circles will locate the horizontal projections Pi and P/
of the principal points P and P' on the two picture traces hh
and A'A', the latter being represented by the tangents Pia/
and Pi'ai.

B. Plotting the Picture-trace when using a Camera or
Phototheodolite.

In this case the angles a and a'. Fig. 8, Plate V, may be
measured directly in the field and plotted on the base line AiAi\
a Bt Ai and a! at -4/. We lay off the distances

Axai'Wp'+{Fa\')^
lUMi A I'ai =\/(n2-f-(Pi'ai)2

so PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

(found by construction or computation) and describe circles
about A I and Ai with / and f respectively as radii. The
tangents drawn from ai' and ai to these circles will locate Pi
and Pi respectively when Pih should equal tiP=x, measured
on the picture MN, and Pi'h' ^P'h' ^xf on M'N'.

When using a phototheodolite a well-defined point T may
be bisected with the principal lines YV and VVy Fig. 9, Plate V,
from the two stations A and i4i, in which case these angles of
orientation are laid oflF upon the base line at A\ and at A\ re-
spectively, and the distances / and /' are laid oflF on the lines
A\T\ and A\T\ respectively (=^iPi and =-4i'Pi'), when the
perpendiculars to A\P\ in P\ and to A\P\ in Pi' will represent
the picture traces hh and h'h' in correct orientation with refer-
ence to -4i, i4i', and T\,

When the pictures of several triangulation points B, C, and D
and the base line are given, the orientation of the picture traces
hh and h'h' upon the plotting-sheet may be accomplished as
follows:

The radials A\B\^ A\C\^ A\D\ . . . , as well as the radials
Ai'B\^ AiCu AiD\ . . . , are drawn upon the iconometric plot-
ting-sheet, the points Bu Ci, Di . . . being already plotted on
the same. The points 6j, Ci, P, di, and ai' are then transferred
from the horizon line OOi of the photographic perspective MN,
Fig. II, Plate VI, upon the perfectly straight edge of a strip
of paper, which now is placed upon the radials converging to Au
as a center, Fig. 10, Plate V, and moved about until

61 falls upon the radial line AiBu

ci '' '' '* *' '' AiCu

di '' '' ** '' *' AiDu

ai' '' '' '' base " AxAi'.

The line AiPi should then be perpendicular to the straight
edge hh of the paper strip, Fig. 10, Plate V, and the line hh,
drawn along the paper strip's edge on the plotting-sheet will

DETERMINATION OF PRINCIPAL. AND HORIZON LINES. 5 1

represent the oriented picture trace of MN\ AiPi will be the
distance line and Pi the horizontal projection of the principal
point P.

The same having been done regarding the point Ai% its
picture J/W and the paper strip OiOi' (Fig. ii, Plate VI),
both picture traces hh and A'A' will have been oriented. The
plotted positions of any other pictured points that may be iden-
tified on both pictures MN and M'N' may be similarly located
by plotting their abscissae (measured on the horizon lines OOi
and (yOi) upon the picture traces hh and h'W (Fig. lo, Plate V)
on the proper sides of the principal points Pi and Pi'.

Lines drawn from the station points i4ii4i' through such
corresponding points, transferred to their respective picture
traces, will locate the relative positions of such points on the
plotting-sheet by their points of intersection.

n. Arithmetical Determination of the Principal and of the
Horizon Line on the Photographic Perspectives.

In the preceding paragraphs it has been assumed that the
photographic perspectives were already provided with the prin-
cipal and the horizon lines. Such, in point of fact, would be
the case with an adjusted surveying camera or phototheodolite.
If the instrument is out of adjustment or if an ordinary camera
be used (one provided with a device for maintaining the image
plane in a vertical position during the exposure of the plate),
the correct positions of the principal and horizon lines, as well
as the length of the distance line, must be ascertained. In photo-
topographic work this may be accomplished in various ways.

A. Determination oj the Principal Point and Distance Line of
the Photographic Perspective,

A plumb line suspended in front of the camera m such a
way that the line w. Fig. 12, Plate VT, may be photographed

52 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

upon the negative will serve to establish the direction of the
principal line on the trial plate. This negative may, further-
more, contain the images a, &, c . . . of three or more points Ay
B, C, . . . of known positions and elevations. A line hh is drawn
at right angles to the pictured plumb line w on the photographic
perspective and a strip of paper is placed with its straight edge
along this line. The images a, b, c ... of the known points

A, B, C. . . are projected upon the paper straight edge, held
in position at hh, by drawing parallels to w through these pic-
tured points.

After the radials from the plotted station 5i, Fig. 12, Plate VI,.
have been drawn through the plotted points Ai, -Bi, Ci . . .
the paper strip is adjusted upon those radials in such manner
that the image projections ai, 61, ci . . . (previously marked
on the strip) will fall upon their corresponding radials; a Une
drawn along the edge of the paper strip while in this position
will represent the oriented picture trace, as indicated by the
line hi hi.

If we now draw a perpendicular line (SiPi) to hihi from the
plotted station 5i, the point Pi will be the horizontal projection
of the principal point P and 5iPi=/ will be the distance line
for the perspective MN.

Should the positions of the points A, B, C. . . with refer-
ence to the station S be not known, it will become necessary
to observe the horizontal angles A SB, BSC, CSD . . . instru-
mentally from the station S and plot them in their proper order
upon a sheet of paper {AiSiBu BiSiCi . . . ) and adjust the
paper strip hh upon these radials in the same manner as just
described.

B. Determination 0} the Position of the Horizon Line on the

Perspective.

When the elevations AA\ BB', CC . . . (Fig. 13, Plate VII)
of the points A, B, C . . . above the horizon plane SOO' of the

DETESMINATION OF PRINCIPAL AMD HOSIZON LIMES. 5$

Station S are known, the position of the horizon line 0(y on
the perspective MN may be found by computing the oidinates
oaf, W, af ... from the equations:

aa':AA'=Sa':SA',
W.BB'''SV:SB',

whence

, Sa'xAA'

o^ — jX' — y^

,^ SVXBB'

The distances 5a', SV^ S(f . . . are taken from the plotting-
sheet. The horizontal distances 5^4', SB'y SC . . . and the
diflFerences in elevations AA\ BB'y CO ... are known.

For example, the difference in elevation between A and
i4'=ioo m., the distance of A' from the station 5 = 1000 m.,.
and the distance 5a', measured on the plotting-sheet, =0.05 m.>
then we will have

, 0.05X100

aa'^y^ =0.005 m.

-^ 1000 ^

The horizon line OCy on the negative will be 5 mm. ver-
tically below the pictured point a (measured in a direction parallel
with the pictured plumb line w). A line OCy drawn through a'
at right angles with the pictured plumb line w will locate the
horizon line. The computed ordinates W=yi, cc'=y2...of
the other pictured points 6, c . . . will serve to check the position
of the horizon Une 0(y] it should be tangent to the arcs described
with aafy bVy af . . . about a, by c . . , respectively as centers.

54 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

in. Graphic Method for Detennixiing the Positions of the Prin-
cipal and Horizon Lines on the Perspectives.

The following method for orientmg the picture trace, pub-
lished by Prof. F. SchifFner, in 1887, and mentioned by Prof.
Steiner, leads to the same result graphically as the preceding
one does arithmetically.

The horizontal projections Au 5i, Ci, and Si of. three points
Ay By Cy and station 5, Fig. 14, Plate VII, may be given. From
Su as center, radials are drawn through Au Bu and Ci. Through
a point a on the radial SiAi a parallel to SiCi is drawn and
the distance a'V — taken from the negative MN, not shown
in the figure — ^is laid off from a=abi upon this parallel line,
while the distance Vcf is laid off upon the same line from hi
-bi'ci'.

Parallels to the radial SiAi are then drawn through the
points 61' and c/ and produced to intersect with the radials
SiBi and SiCi. The line h'h' connecting these two points
of intersection will be parallel with the direction of the picture
trace.

The same distances a'V and b'c' — taken from the negative —
are laid off upon this line h'h' from 02=^2^2 and from 62=^2^2
respectively. The parallels to the radial SiAu drawn through
these points ^2 and ^2> are brought to intersections with the radials
SiBi and SiCu when the line M, passing through these inter-
sections V and (/y will represent the picture trace, oriented with
reference to Su Au Bu and Cu

The distance SiPi of Si from hh represents the distance line
(focal length) of the picture MNy and the point Pi will be the
horizontal projection of the principal point of the perspective.

After having transferred Pi (with reference to a', Vy and (f)
to the perspective MN by means of a strip of paper, a parallel
to the pictured plumb line w drawn through the point Pi will
locate the principal line upon the negative.

THE "five-point PROBLEM/* 55

nL The " Pivc-pbint Problem " (by Prof. F. Steiner), or Locating
the Plotted Position of the Camera Station by Means of the
Perspective when Five Triangulation Points are Pictured
on the Same Photographic Perspective.

In the methods considered until now it had been assumed
that the position of the camera station Si on the plotting-sheet
was known with reference to the plotted triangulation points
Ai, Bi, Ci . . . .

In case the panorama pictures have been taken from a camera
station Si of unknown position and a series of known points
are pictured upon the panorama views, both the position of the
camera station may be found (with reference to the positions
of the surrounding points of known positions) and the picture
trace may be oriented by means of Prof. F. Steiner's " five-point
problem," if one of the panorama views contains the pictures
of five or more points of known positions.

A. DetenninaHon of the Principal Point and Distance Line.

A panorama view MN may contain the images a, b, c, d, e
of the triangulation points Ay B, C, Dy £, already plotted on
the working-plan, and also the picture of a suspended plumb
line or other vertical (or horizontal) line sufficiently long to be
used for drawing parallel lines to the principal (or horizon)
line.

The points a, b, c, d, and e of the negative MN are projected
upon the straight edge of a strip of paper =ai, 61, cu du and ci.
Radials are now drawn from one — Au Fig. 15, Plate VIII —
of the five plotted points as center to the other four points, Bu
C\y Du and £1. The paper strip is then placed over the radiab
AiBu AiDu and AiEi, that fti falls upon AiBu di upon AiDu
and ei upon AiEi, when the strip will have the position ai,
bi, Cu du ei. The line drawn through Ai and ai (the latter
having been transferred to the sheet by means of the paper strip)

56 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

will be tangent in A to the ellipse £i (which passes through Aiy
J5i, Z?i, and £i and through the plotted station S\).

The paper strip is now placed over the radials AiBu A\Cu
and AiDu that b^ falls upon -4iBi, C\ upon A\C\j and d\ upon
AiDu when the strip will have the position indicated by a2^
62* ^2, d2, e2, and the line Aia2 will be the tangent in i4i to the
ellipse £2 (passing through the points Ai, J5i, Ci, Z>i, and the
plotted station point 5i).

The plotted position of the station point Si with reference
to the five plotted points Ai, Bi, Ci, Z>i, and £1 will be at the
fourth point of intersection Si of the two ellipses £1 and £2-

After drawing the radials SiAu SiBi, SiCi, SiDi, and 5i£i
the paper strip is placed over these radials in such manner that ai
falls upon SiAu h upon SiBu . . . and ei upon 5i£i, in the
position indicated by a, fc, c, d, e^HH, when HH will be the
plotted picture trace.

The perpendicular upon HH passing through Si=SiPi
represents the distance line and Pi is the principal point of the
negative projected into the horizontal plan, which, in order
to locate the principal line, may now be transferred to the per-
spective by means of the paper strip in the manner already
described.

B. Simplified Construction for Locating the Plotted Position of
the Camera Station by Means of the " Five-point Problem"

The method just described being rather complicated, Prof.
Schiflfner recommends the following construction. Fig. 16, PlatelX,
in which the drawing of the two ellipses £1 and £2 is avoided:

The plotted positions of the same five points A, B, C, D
and £, together with a negative containing the images a, 6, c, rf,
and By of these points may be given. It is desired to find the
fourth point of intersection Si of the two ellipses £1 and £2
without actually drawing their perimeters.

THE "five-point PROBLEM. ' 57

The two tangents 63^1 and b4Bi to the ellipses Ei and £2
in Bi are located in precisely the same manner as the two tan-
gents aiAi and a2Ai for the point Ai were found in Fig. 15,
Plate VIII. The intersections Ri and -R2 of the tangent pairs
^1-4 1, bzBu and aa^i, b^Bi, Fig. 16, Plate IX (belonging respec-
tively to the ellipses £1 and £2)9 are situated on a line QX, form-
ing one side of the polar triangle QXTy common to hoth ellipses.
This line RiR2=QX intersects the diagonal AiDi in X and
the quadrilateral side BiDi in the point Q. The lines drawn
through Q from Ai and through X from JBi will intersect each
other in the fourth point of intersection Si of the two ellipses. ^

This method may also appear rather complicated in view
of the mainy lines that have to be drawn before the picture trac6
EH and the position of the camera station may be plotted.

C ApplicaHon 0} the " Five-point Problem" to the Special Case,
where the Five Points range themselves itUo a Triangle, on the
Working-sheet,

The application of the five-point problem becomes very
much simplified when the five points i4, 5, C, Z>, and E form
a triangle of which two sides i4iCi and Ci£i, Fig. 17, Plate X,
contain three points each.

If we place the strip of paper upon the radials, drawn from 4i>
that ei falls upon A\Eu di upon i4ii?i, and ci upon AiCi, it will
have the position indicated by ^2, &2> ^2> ^2* «2> and the first ellipse
will resolve into the lines CiiSi and Aia2. If we now place the
paper strip ai, &i, ci, du ei upon the radials drawn from £1 to Au
to Bi ^d to Ci, that ai falls upon EiA^ &i upon -EiSi, and Ci
upon EiCij it will assume the position ai, 61, ci, di, eu and the
second ellipse will resolve into .the lines AiCi and £1^1.

The intersection 5i, of the two lines Aia2 and £1^1, locates
the position of the plotted station point Si with reference to the
five given points Ai, Biy Cu Du Ei. By placing the paper
strip upon the radials SiAi, SiBu SiCi, SiDu and 5i£i in

58 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

such manner that ai falls upon SiAibi, upon SiBi, Ci upon
SiCi . . . , its edge HH will locate the picture trace, Pi' will be
the horizontal projection of the principal point P, and SiPi will
be the distance line.

D. To find the ElevaHon of the Camera Horizon for a Station
that has been located by means of the ^^Five-point Problem.^^

To ascertain the elevation of the station 5, plotted after one
of the preceding methods, it will be necessary to know the ele-
vations of at least two of the five given points. The elevation
of the station horizon SOC/, Fig. 13, Plate VII, above the datum
or ground plane S'OiOi, may be designated by X, H and Hi
may be the elevations of A and B respectively, both supposed
to be known. The ordinates of the pictured points a and b
are aa'^y and bV =^y,^

From the relation S'ai'.S'Ai'^aa' :AA'

or Sa':SA'^y:{H-X)

S(f
we find y^SA'^'^"'^^'

SV
and yi'=^(Hi-X).

As the diflFerence between y and yi may be found by direct
measurements made on the negative, y—yi^m will be known
and the value for X may be computed from the equation

since the measures for Safy SA'^ SV, and SB' may be obtained
from the plotting-sheet, measured in the scale of the map.

THE "THKEE-FOINT PROBLEM." 59

The above equation may be written in the general form:

H-X H-X ^ 1 Sa' , 1 SV

"* n F"' «'°5r ^^ 7-5^'

hence

ffi'it' p—H' p— Hi ' ft
n—p

By substitution of this value in the equations

H-X ^ Ht-X

the numerical values for the ordinates y and yi (governing the
position of the horizon line) may be found.

V, The " Three-point Problem."

If the triangulation points are not sufficiently dose together
that five or more points may be pictured on one photographic
perspective, and if stations are occupied with the camera that
are not directly connected with the trigonometric system, it will
become necessary to employ other means than those hereto-
fore considered for locating the position of such detached camera
stations with reference to the triangulation sjrstem.

To connect detached camera stations with the triangulation
by observations made at the camera station, at least three tri-
angulation points should be visible from such station. When
the camera party is in advance of the triangulation party many
camera stations will be located by the triangulation party by
observing upon a signal left at the camera station, if such signal
be visible from two or more triangulation stations (the camera
station will be a " concluded point " of the triangulation system).

6o PHOTOTOPOGRAFmC METHODS AND INSTRUMENTS.

The determination of the position of a detached camera
station by observing upon three fixed and known points (pro-
vided with signals) is generally known as the " three-point prob-
lem " (station-plotting, station-pointing, etc.), or " Pothenot's
method," although Snellius was probably the first to use this
method (in his trigonometric surveys in the Netherlands in the
second decade of the seventeenth century).

A, Mechanical Solution of the " Three-point Problem*'
(using a Three-arm Protractor or Station-pointer).

The simplest solution of the three-point problem is purely
mechanical in application. The two observed angles M and N
are laid oflF upon a three-arm protractor (" station-pointer '*)
or upon a sheet of tracing-paper, and the three arms or lines ^lili,
SiBu and 5iCi, Fig. i8, Plate X, are placed over the three fixed
and plotted points Au Bu and Ci in such manner that the three
lines of direction 5i-4i, SiBu SiCi pass through their respective
points Au Bu and Ci, the point S being transferred to the
working-sheet while holding the two horizontal angles M and N
in unchanged position.

B. Graphic Solution of the " Three-point Problem.** .

^ I. Using the So-called "Two-aRCLE Method."

Theoretically the best graphic method is probably that by
which the position of the fourth, or station, point is located, at
the intersection of two circles, one passing through Ai and Bi
and having over AiBu as chord, the angles of circumference
^AiSiBi =J/, Fig. 1 8, Plate X, the second circle passing through
Bi and Ci and having over the chord BiCi the angles of cir-
cumference eqiial to BiSiCf^N.

From the plotted triangle side AiBi we lay off at ili and JBi
the angles BiAiCi and AiBiCu each equal to

' = 9o®-i4i5i-Bi=»9o-Af,

and about the point Cu thus obtained, we describe a circle
i4i5i5i with the radius =Cii4i=Ci5i. The observed angle
AiSiBi—M will then be an angle of circumference over AiBiy
and the point Si will be located somewhere on the arc over AiBi.
By means of the angle BiSiC^N another circle BiCSi is
described over the triangle side JBiC, in a similar manner, about
the point C2 as center, having C2Bi=C^ as radius. The
observed second angle BiSiC=N will be an angle of circum-
ference over the chord BiC and the point Si will be on the arc
over BiC hence its true position is at the (second) point of inter-
section Si of the two circles.

2. Using the Method of Bohnenberger and Bessel.

The following method, by Bohnenberger and Bessel, is readily
applied and simple in construction. If we describe a circle
through two of the given points, through Ai and Bi, Fig. 19,
Plate XI, and through the station 5i, the angles designated by M
and those designated by JV in the figure will be respectively
equal, being angles of circimiference over the same arcs AiDi
and DiCi respectively.^

Hence if we lay off the observed horizontal angle N on AiCi
at ill, and the other observed horizontal angle M on the line
AiCi at Cu the point of intersection Z?i of their convergent
sides CiDi and AiDi will fall upon the line connecting the third
plotted triangulation point Bi with the station point Si.

After having thus determined the direction of the line BiDi
or JBiSi the position of the point sought may be foimd as follows:

At any point x on the produced line BiDi the observed angles
M and JV are laid off to either side of BiDu in the sense in which

62 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

they were observed at S. Lines AiSi and Ci5i, drawn through
A I and Ci, parallel to xy and xz respectively, will locate the
plotted position of the station point Si (upon DiBi) with refer-
ence to the three plotted points Ai, Bi, and Ci.

This solution is recommended only when BiDi is sufficiently
long (in Fig. 19, Plate XI, it evidently is too short) to assure a
correct prolongation toward Si.

The picture trace HH, containing the horizontal projections
of the pictured points a, ft, c, may now be oriented in the known
manner by adjusting the paper strip, having the three points ai,
biy and ci marked on its edge, over the radials 5i-4i, 5i-Bi, and
SiCi to bring ai on 5i-4i, bi on SiBi, and ci on SiCi.

VI. The Orientation of Picture Traces, Based on Instrumental
Measurements Hade in the Field.

When no points of the area to be mapped phototopographic-
ally are known, the elements (horizon line, principal point, and
distance line) of the photographic perspectives can no longer
be determined from the photographs alone. Instrumental obser-
vations will have to be made at the camera stations in the
field to supply the data needed for their determination. This
method, among others, having been adopted by Capt. Deville,
Will be described in the chapter giving the description of the
Canadian surveying-camera.

VII. Relations between Two Perspectives of the Same Object,
Viewed from Different Stations.

(Pro}. Guide Hauck's Method.)

A more general application of photogrammetric methods

dates from the publication of Prof. G. Hauck's investigations

and results regarding the relationship between trilinear systems

of different planes (Guido Hauck, "Theorie der trilinearen

TWO PERSPECTIVES OF THE SAME OBJECT. 6j

Verwahdtschaft ebener Systeme," Journal fuer reine und ange-
wandte Mathematik, L. Kronecker und WeierstiJass, Bd. 95,
1883). In this publication Prof. Hauck discusses the relation-
ship between the projections of the same object upon three differ-
ent planes. -The practical value of his theoretical deductions
was fully established and tested practically by the students of
the Royal Technical High School of Berlin who attended Prof.
Hauck's lectures and exercises connected with the course in
descriptive geometry in 1882.

In the discussion of the relation between three perspectives
of the same object Prof. Hauck refers to some properties of
decided value in iconometric plotting. The principal law (as
deduced by Prof. Hauck) with reference to photbtopography
may be stated as follows:

If an object be projected from three different points as cen-
ters upon three different planes that may have any position in
space, one of these projections (perspectives) can be evolved
from the other two by means of graphic construction. Or,
expressed in terms more suited to our case, if an object has
been photographed on three plates exposed from different
stations, any one of these photographic perspectives may be
evolved graphically from the remaining two. A topographic
map (the orthogonal projection of the terrene) may be regarded
as a central projection or perspective in a horizontal plane,
having its center of projection (point of view) at infinite dis-
tance, and we may state Prof. Hauck's law as follows :

From two photographs MN and J/W of the same terrene,
taken from different stations S and 5', the orthogonal horizontal
projection of the terrene may be obtained graphically by means
of rays emanating from the so-called " kernel points " (" Kem-
punkte ") as centers.

The line of intersection of the two photographs (the two
planes of projection or perspective planes) MN and M'N\
Fig. 20, Plate XI, will be the "perspective axis."

To better illustrate the connection between two different

64 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

photographs we will first refer to the simple case of two vertical
perspective planes or photographs MN and M'N'^ Fig. 20,
Plate XI.

A. " Kernel Paints " and " Kernel Planes:'

Let S and 5' represent the two camera stations (centers of
projection or points of view for the two vertical photographic
perspectives MN and M'N'), let 5^ be the picture in MN of 5',
and s be the picture of S in M'N', then these two pictured sta-
tions s and j' will be so-called " kernel points " (" Kempunkte *').

The two picture planes MN and J/W intersect each other
in the line IQ, the so-called " perspective axis."

Planes passing through the line 55' (base line) will contain
the " kernel points " 5 and s^; they are termed " kernel planes "
<" Kemebenen '').

A kernel plane M2N2f laid through any point A, pictured
as a and a', will intersect the first picture plane MN in the line as^
and the second picture plane Jf'iV' in the line sa\ These lines
of intersection {as' and a's) will intersect the " perspective axis "
IQ in the same point Q ; they will contain the pictures a and a'
of the point A, and they will pass through the picture 5 and s'
of the two camera stations 5 and 5'.

The lines S'A, SA, SS', as", and a's fall within the " kernel
plane " M2N2. All lines as' for all points pictured in MN
will pass through the pictured station point s' (image of the second
camera station 5')> and all lines as for the picture plane Jf W
^11 pass through the pictured point s of the camera station 5.
Furthermore, all pairs of lines (as' and a's) joining the per-
:spectives (a and a') of identical points (A) with their corre-
sponding pictured station points (" kernel points " s' and s)
will intersect the " perspective axis " (IQ) of the two pcture
planes (MN and M'N') in identical points (Q).

From two photographs of the same object which also con-
tain the pictures of the two reciprocal stations peculiar advan-

TWO PERSPECTIVES OF THE SAME OBJECT. 6$

tages may be gained for the iconometric plotting, inasmuch as
such pictured stations ^ and s will be " kernel points."

The perspective axis of the picture planes may also play an
important part in iconometric plotting, not only for pictures
exposed in vertical planes, but even more so for inclined picture
planes.

If two photographs MN and Af'iV' are given (in Fig. 21,
Plate XII, their traces are represented by the lines HH and WH')
representing the same object, viewed from the two stations S
and 5' without containing the pictures of the stations, the posi-
tions of the pictures 5 and s' of the corresponding camera sta-
tions S and S' may be located upon the picture planes (out-
side of the actual field of the photograph) by construction.

The horizontal projections ^1 and ^i' of the " kernel points ^^
5 and s' are identical with the points of intersection of the plotted
base line SS' and the picture traces HH and H'H'^ Fig. 21,
Plate XII. Hence, if we revolve the picture planes MN and
M'N' about their ground lines, until they coincide with the
ground plane, the line IQ, common to both picture planes (the
" perspective axis "), will be represented by the two lines i(/),
and the " kernel points " 5 and / of the revolved planes MN
and JfiV will fall upon the lines S\{S\) and S\{S\) respectively
(these lines are perpendiculars upon the picture traces in the
horizontal projections S\ and s\ of the kernel points ^ and s').

To find the lengths Si{S\) and Si' {Si') (the ordinates of
the " kernel points " in the picture planes above the ground
lines) we erect perpendiculars to the base line in S and 5' with
lengths equal to the elevations of the camera stations above
the ground plane =5(5) and S'{S') respectively.

The line {S){S') — the vertical plane passing through the base
line 5 has been revolved about the horizontal projection of the
base line into the groimd plane to coincide with the latter —
will intersect the lines Si{Si') and s\{S\i) — they are perpen-
diculars to SS' in the " kernel points " ^ and s — and the lengths
S\{S\') and 5i'(5ii') will be equal to the ordinates of the " ker-

66 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

nel points " s and s' above the ground lines of MN and

The " kernel points " s and ^, Fig. 20, Plate XI, may be
located after this manner in the picture planes of any two photo-
graphs, provided such picture planes are not parallel (or even
nearly so) with the base line 55'.

jB. Use of the ^^Perspective Aods^^ {Line of Intersection) of two
Picture Planes that show identical Objects viewed from
different Stations,

If a series of characteristic points of the terrene, pictured
in a vertical plane MN, Fig. 22, Plate XII, are connected with
the " kernel point " s by straight lines, these will, when pro-
duced, intersect the perspeqtive axis IQ, and if the images of
the corresponding identical points in the vertical picture plane
M'N' are joined with the " kernel point " 5', and if these lines
are likewise produced to intersect the " perspective axis *' /fl,
the points of intersection of IQ with the first group of lines (belong-
ing to MN) will be identical with the points of intersection of
IQ with the second group of lines (belonging to M'N').

If we now provide the " perspective axis " with a scale of
equal parts (having the zero or origin of graduation in the ground
plane), lines drawn through the " kernel points " and through
corresponding images of identical points in both picture planes
will intersect identical points of this scale.

The length O'O, Fig. 22, Plate XII, intercepted on the scale
of the " perspective axis " by the two horizon lines of the picture
planes MN and M'N' represents the diflference in elevation
of the two camera stations 5 and 5'. The scale IQ may be
drawn on both pictures to show on both lines i(/), Fig. 21, Plate
XII, after the pictures have been separated. Frequently the
picture itself will not be sufficiently extended to contain the
line IQy in which case such a scale may still be used by placing
it upon a line XX'\ in MiV, and upon 22", in M'N', some dis-

TWO PERSPECTIVES OF THE SAME OBJECT. 67

tance from but parallel with the perspective axis ifi, Fig. 22,
Plate XII, provided the following relation remain satisfied :

For any other point 5, photographed as b and f in the pic-
ture planes MN and M'N' respectively, the following propor-
tional equation should be fulfilled:

The triangles sx^ocf, s^Q and ^'202', s'^Q being, respectively,
similar, Xf^ must be equal to 202' (as ^Q is conunon to both
triangles s^Q and ^/?fi), which means the spaces on the scales
XX" and 22" are to be identical in numerical value. The two
scales (or either of them) may, if more convenient, be placed
beyond ^ or s!y f. i. at ttf', in which case

sp-.sto^sP'.sxo^sfP'.s'zQ.

It should be noted that the scale is now to be read from f toward
io. It may be stated generally that the scales shoidd be placed
parallel with the ** perspective axis " IQ and at distances from the
** kernel points " directly proportional to the distances of the latter
from the " perspective axis " of the picture planes, their correct
position being found from the horizontal projection or from
the groimd plane. To avoid obscurity and obliteration of details
m the field of the photograph it will generally be more expe-
dient to draw these scales outside of the picture proper.

To find the proper position of the second scale on the second
picture, after the position of the scale on the first picture has
been decided upon, we again refer to Fig. 21, Plate XII, where
HH and H'H' are the two picture traces, S and 5' are the
horizontal projections of the camera stations, P and P' are the
traces of the principal lines // and /'/' (Fig. 22), or the horizontal

68 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

projections of the principal points, and, finally, h the selected
position for the first scale.

To find the corresponding position A' of the second scale
we draw a line hh parallel to SS^ through h, when

hence

= distance of the second scale from
the " kernel point " s' in the second picture.

The conditions and relations described in the foregoing
paragraphs may often prove of value in iconometric plotting;
f. i., if we consider the case of a straight line i, Fig. 23, Plate XIII,
the image of which appears in picture MN as /, but in the second
picture 3f'iV' only a short piece /' is seen. It may be desirable
to locate in the picture plane MN the reciprocal position of a
point X, shown on the line / in MN, but falling outside of the
picture limit of AfW on the prolongation of /'.
To find the position of oc' we proceed as follows:
The pictured point x of the line /, pictured in MN, is con
nected with the kernel point (s') and the line (s')x is produced
to its intersection (x) with It. After transferring the point (x)
to the line il of the second picture plane M'N', to {{x)), and
connecting the latter with the " kernel pofait " (5), the intersec-
tion of {{x)){s) with /' produced will represent the point sought,
xf, on the prolongation of the line /'.

Vm. To Plot a Figure, Situated in a Horizontal Plane, on the
Ground Plan by Means of its Perspective.

Excepting the shore lines of lakes and coasts and the out-
lines of marshes, figures in horizontal planes are not frequently
met with in topographic surveys, and the simplest way to map
these would be to expose photographic plates in a horizontal

TO PLOT A FIGUSE BY MEANS OF ITS PERSPECTIVE. 69

position from a captive balloon at points of known positions
and at identical or known elevations.

The mapping of such figures, when photographed on ver-
tically exposed plates, from stations above the figure's plane
is also an easy matter. It may even be done with but a single
perspective view of such figure (obtained on a vertically exposed
plate from a station of known position), provided we also know
the difference in elevation between the camera station and the
horizontal plane containing the figure, and provided we know
the positions of the principal point and horizon line together
with the length of the distance line (focal length) of the photo-
graphic perspective.

We have, with reference to Fig. 24, Plate XIII, i?i?= horizon
plane of the camera station 5, 00'= horizon line of the photo-
graphic perspective MN, GG= ground plane or horizontal plane
coinciding with the surface plane of the lake ABCDy SSo = h
= difference in elevation between the camera station S and the
water level of the lake.

With a given perspective abed of the lake A BCD in the ver-
tical picture plane MN, known focal length, given position
of the principal point P and known difference in elevation, A,
between the water surface of the lake and the camera station,
the projection of the lake-outline (AiBiCiDi) in horizontal plan
may be drawn.

The ground line OqOo' (line of intersection of ground plane
GG with the vertical picture plane MN) is drawn through Po
(horizontal projection of P) parallel with the horizon line 00',
PPo being equal to h (measured in the plotting-scale). If we
now project the pictured points a, b, c, d upon OoOo' = ao> ^o>
Co, do, the radials from the foot 5o of the station S drawn through
the points ao, bo, cq, do, will pass through the corresponding
points of the lake shore-line Ai, Bu C\, Di that are to be plotted.

Referring to the vertical plane passing through the camera
station S and through the pictured point a (it intersects the ground
plane in SqAq or in SqAi) we find from the similar triangles

70 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

55o^i and aooAi the horizontal distance SqAi from the camera
station to the point sought, either graphically or arithmetically.

Imagining the vertical plane SSqAi to be revolved about
5o^i until it coincides with the ground plane GG, the points S
and a will assume the positions (5) and (fl), Fig. 24, Plate XIII,
and the line (5) (a) will pass through Au hence Ai may be
located in the ground plane as the intersection of (S){a) with
SoOq. The same may be done for the other points Bu Ci, and
Di by revolving the vertical planes SSqBij SSqCu and SSoDi
about Sobo) SqCq^ and Sodo into the ground plane GG to locate
the positions of 5i, Ci, and Di.

To avoid the drawing of so many auxiliary lines on the work-
ing- or plotting-sheet, these constructions are preferably made
on a separate sheet of paper, and the following method may be
adopted :

The vertical planes passing through 5o^, Sobo, SqCq, and
Sodo may be supposed to be revolved about SSo> as common
axis of rotation, until they all coincide with the principal plane
SS0P01 Fig. 25, Plate XIV, the surface of the paper representing
the principal plane, when i2'-ff = trace of the horizon plane
in the principal plane, AfiV= trace of the picture plane in
the principal plane, GG = trace of the ground plane in the
principal plane, 55o = A=difIerence in elevation between the
station S and the ground plane GG, measured in the plotting-
scale, 5P=5oPo = true length of the focal distance of the pho-
tograph MN.

The radials SqUo, Sobot SqCo, and Sodo are laid off upon the
line GG from 5o=5o(^), So{bo)y So(co)i and 5o(do)> and the
verticals (ao)(a), {bo){b)y (co)(c)t and (do)(d) are made equal to
the ordinates aoo, bbo, ccq, and ddo, respectively, measured on
the picture.

Radials drawn through (a), (6), (c), and (d) from S will
cut off on the line GG the horizontal distances 5o(-4), So(B),
So(C), and So{D). These distances, laid off on the radials
So(^o, Soboj SoCoi and Sodo, on the plotting-sheet will locate, in

TO PLOT A FIGURE BY^MEANS OF ITS PERSPECTIVE. 71

the scale of the map, the plotted positions of the characteristic
points ill, Si, Ci, and Z?i of the lake, with reference to the ground
line OoOo', which is identical on the plotting-sheet with the
picture trace.

We may reach the same results by utilizing the orthogonal
projections of the points a, 6, c, and d and those of Ai, Bi, C\,
and Di into the principal plane instead of revolving the ver-
tical planes separately into the principal plane, as done above.

With reference to Fig. 26, Plate XIV, we would then have:

PP== principal plane, MAT = picture plane, iJif^ horizon
plane, containing the camera station 5, GG= ground plane or
surface plane of the lake A BCD.

If we draw the radials 5oao> ^o^oj •S'oCo, and S{^o from Sq
(the orthogonal projection of S in GG) through the orthogonal
projections of the pictured points a, 6, c, d on the ground line
OqOo', the points sought will fall upon those radials. After
projecting the points a, 6, c, and rf, in the picture plane 'MNy
upon the principal line (=a, /?, y, and d) the radials 5a, 5/?,
5^, and Sd (drawn in the principal plane PP) will locate the
points ao, /Jo> T01 and ^o> respectively, upon the line 5oPo (in
the ground plane), and these represent the orthogonal projec-
tions of the points i4, 5, C, and D in GG upon SqPq. Hence
the points i4, 5, C, and D may be found by erecting perpen-
diculars upon SqPq in ao, ^qj ro> and 5o, respectively, and their
points of intersection with the radials Sq^Loj S^b^y 5oCo, and 5o(io>
respectively, will be the positions of the plotted points A, B, C,
and D.

Also this construction is preferably made upon a separate
sheet of paper. Fig. 27, Plate XV, where the radials So^o, 5o6o>
SqCo, and Sodo are drawn through their corresponding points
on the plotted picture trace or ground line OqOo', but the rest
of the construction is made on the separate sheet of paper, con-
sidering the surface of the latter to coincide with the principal
plane (Fig. 28, Plate XV, where the designations are the same
as in Fig. 25, Plate XIV).

72 PHOTOTOPOGRAPHIC METHODS 'AND INSTRUMENTS.

The points d, /?, a, and y on the line PPq (principal line)
represent the projections into the principal plane of the pictured
points a, 6, c, and d, their positions being found by transferring
the ordinates ddo, bbo, aao, and ccq of the pictured points d, b^
a, and c to PPq from Po> Po^=^o> -Po/?=Mo> Poa = aao, and

The radials from S through d, /?, a, and ;r- locate the points
^o> i^oj «o> and yq on the line GG or on 5oPo> and by transferring
the distances 5o^o> SqPo, Soao, and Soyo, Fig. 28, Plate XV, to
the principal line 5oPo> Fig- 27, Plate XV, and drawing lines
through do, /?o, olq, and yo parallel with OqOq, their intersec-
tions with the corresponding radials Sodot Soboy SoOoy and SqCq
will locate the plotted positions Di, Bi, Ai, and Ci of the points
Dy By A, and C of the shore line of the lake.

IX. To Draw the Horizontal Projection of a Plane Figure ABCD
on the Ground Plan by Means of the So-called " Method of
Squares," if its Perspective in Vertical Plane, abed, and the
Elements of the Perspective are given.

If we imagine the figure covered with a net of squares in
such manner that one set of sides is parallel with, while the other
is perpendicular to, the ground line, such net may be used to
draw the outline of the figure upon the ground plan. It will
only remain necessary to cover the pictured figure abed with
the perspective of the net that has been selected for the ground
plan. The lines representing the squares in perspective must
have the proper relation with reference to both, the principal
ray and the horizon Une, to conform with the net in the
ground plan.

The simplest disposition of the lines forming this auxiliary
net is the one mentioned above, with one set of sides parallel
with, and the other perpendicular to, the horizon line; still, any
other disposition of the net lines or sides may be made: they

THE "method of SQUARES." 73

may fonn equal-sized squares or not and their directions may
include any angle.

In Fig. 29, Plate XVI, in illustration of this method, the
lines of the perspective, corresponding to those sides of the
rectangular figures that had been drawn at right angles to the
ground line OqOo, will vanish in the principal point P, while
those drawn parallel with the ground line OqOo' will be parallel
with the horizon line 0(y.

Selecting the lines of this rectangular system so that one
line of each system passes through each one of the characteristic
points a, 6, c, and d of the pictured lake, the perspective of this
net will appear as shown by the fine lines in Fig. 29, Plate XVI,
where OoOo' represents the ground line of the picture plane
MN.

If we again plot in the principal plane SSqPoP, Fig. 30,
Plate XVI, and retain the same designations as in Fig. 25, Plate
XIV, the points do, /?o> «o> ^md 7-0 will represent, in the ground
plane GG^ the mtersections of the horizontal projection of the
principal ray SP^SqPq with those net lines that had been drawn
parallel with the ground line through D, B, A, and C

After plotting the picture trace OqOo', of the perspective
MN, Fig. 29, Plate XVI, in the ground plan by means of the
radials 5oao, Sobo, etc.. Fig. 31, Plate XVII, the distances Sodo,
S0P09 etc., taken from Fig. 30, Plate XVI, and laid oflF upon
S0P09 Fig- 31 > Plate XVII, will locate the intersections of SqPq
with those net lines (parallel with OqOq') in the ground plan
that correspond to the lines dd, bfi, etc., of the perspective MN,
Fig. 29, Plate XVI.

If we now transfer the points oq', Pq, V> do', and Co' from
Fig. 29, Plate XVI, to the edge of a paper strip and place the
latter upon the picture trace OqOo', Fig. 31, Plate XVII, that
the points Pq of both will coincide, then the lines ao'Ai, V-Bi,
etc., drawn parallel with SqPq will represent those net lines
that are perpendicular to the ground line OqOo'i and the plotted
positions Ai, Bi, C\, and D\ of the points A, B, C, and D are

74 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

treated on the ground plan as the intersections of corresponding
net lines of both systems as indicated iii Fig. 31, Plate XVII.

The points Ai, B\, Cu and Di will, of course, also be bisected
by the radials SoOq, Sobo, SqCq, and Sodoy which fact may make
some other disposition of the net lines more desirable for a figure
of a different shape.

When the figure is bounded by a sinuous perimeter, the
squares of the net should be selected sufficiendy small to enable
the draughtsman to draw the perimeter sections falling within
each square sufficiently accurate to obtain a correct reduction
representing the general course of the figure's outline.

X. The " Vanishing Scale."

We had seen — Fig. 31, Plate XVII — that the radials drawn
from the so-called foot Sq of the station S represent directions
to the points i4i, Bx, Cu and Z?i in the ground plane. If we
now could determine from the perspective the distances {SqAu
SqBi, etc.) from the foot So of the station to the points to be
plotted their location in the ground plane would become an easy
matter.

The distances SqAi, SqBu etc., may be determined from
the perspective by means of the so-called vanishing scale, which
may be constructed as follows, with reference to Fig. 32,
Plate XVII, where -lfiV= trace of picture plane, ilil = trace
of horizon plane, and GG= trace of ground plane, all in the
principal plane, and where 55o= elevation of the station 5
above the ground plane GG, or above the foot 5o of the station.

A scale of equal parts is laid off upon GG to both sides of Pq
and radials are then drawn from 5 through the graduation-
marks. Their intersections with MN form the so-called van-
ishing scale which may serve to locate the distances from the
foot So of the station S to points that are to be plotted in the
groxmd plane from the picture.

THE "vanishing SCALE." 75

The picture trace OqOo', Fig. 33, Plate XVIII, may have
been plotted and the radials SoOm Sobo, etc., may have been
drawn on the working-sheet. It is desired to locate the posi-
tion i4i of a point A in the ground plane that is pictured as a
in MN, Fig. 34, Plate XVIII, by means of the vanishing scale.

Take the ordinate aao from the photographic perspective MN
(the vertical distance of a above the ground line OqOo') and
lay it off upon the vanishing scale PoPy Fig. 32, Plate XVII,
from Po> equal to PqX.

The line ax in the picture plane MN, Fig. 34, Plate XVIII,
drawn parallel with the horizon line 00' and passing through a,
is the perspective of the line AiX, drawn parallel with the ground
line and passing through Ai, Fig. 33, Plate XVIII, in the ground
plane. Hence, if we lay off SqX, Fig. 32, Plate XVII, upon
SqPo9 from 5o, Fig. 33, Plate XVIII, the point Ai in the ground
plane will be situated upon the line XAi, drawn parallel with
the ground line OoOq through X. The plotted position Ai
of the point A will be at the intersection of the radial Soao with
this line XAi.

CHAPTER V.
PHOTOGRAPHS ON INCLINED PLATES.

Until now we have regarded phototopographic plates exposed
in vertical planes, and although the general use of inclined
plates is not recommended for phototopographic purposes on
accoxmt of the complications that will arise in the generally
simple constructions underlying the iconometric plotting from
vertically exposed plates, and because the relations that exist
between the elements of the perspective and the orthogonal
projection into horizontal plan will not- be so readily recognized.
Occasions may arise, however, where the selection of the availa-
ble or accessible stations will be so circumscribed as to make
exposures on inclined plates a necessity (to insure a good con-
trol of the inaccessible terrene forms). Photographs may also
have been obtained from balloons or with an ordinary camera
not supplied with devices for adjusting the plate into vertical
plane, or photographs originally taken for illustrative purposes
may perchance find use for iconometric plotting.

With reference to Fig. 35, Plate XIX, we have PP= prin-
cipal plane, il£r= horizontal plane passing through the nodal
point of the camera-lens at station 5, GG= ground plane, MN=
picture plane, 0'P= trace of the picture plane MN in the hori-
zon plane HHy Oo'Po= ground line of the picture plane, 5o=foot
of the station 5, P'Po= principal line of the picture plane,
7^= principal point of the perspective AfiV, 55o= vertical of
the station 5. It pierces the ground plane in the foot of the
station and passes through the picture plane MN above (or

below) the horizon line at s. The point s is the vanishing point

PHOTOGRAPHS ON INCLINED PLATES. ^^

for the perspectives of all vertical lines that may be pictured
in MN. FSP = FsS = a = angle of inclination of the plate MN,
5P= perpendicular through S to the horizon line Cyp, 54= line
of direction from 5 to a point i4, pictured as a in MN.

If we revolve SP in the vertical plane PP about P until
SP falls within the picture plane, the point S will fall into (5)
and the line Sa will fall into {S)a.

The vertical plane, passing through 55o and containing
the line 5-4, will intersect the ground plane in 5oao. If we
revolve the line SoPo within the vertical plane PP about Pq
until 5oPo falls into the picture plane MN, the point 5o will
fall into (5o) and the trace 5oflo will have assumed the position

(5o)«o.

The intersection i4o of the trace S^/Iq with the line of direc-
tion Sa would locate the plotted position in GG of the pictured
pomt a.

The line sa intersects the ground line in oq, and 5oao will
be the radial in the ground plane from the foot Sq of the station 5
that passes through the plotted position (in GG) of Aq. To
find i4o on 5o^ we first locate in the picture plane the inter-
section {A) of the revolved lines {S)a and (5o)ao. This point
(id), revolved within the vertical plane ao5o5, will locate Aq
upon 5oflo-

To locate the position of i4o in GG in the manner just indi-
cated we should know the position of the line O'Py as well as
the points 5 and P. These are known, or may readily be found,
if the position of the principal point P', the length of the dis-
tance line SP'y and the value of the angle of inclination a are
known.

When a photographic plate is purposely exposed in aa inclined
position in a surveying camera, it will generally be done in such
manner that the principal line //' still coincides with the inter-
section of the picture plane MN and the principal plane PP^
Fig. 35. Plate XIX.

When the angle of inclination a is an angle of elevation (depres-

78 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

sion) the horizon line O'P will fall below (above) the line repre-
senting the horizon line of the plate when exposed vertically.

The angles of inclination for inclined plates should be
observed directly in the field, and if the constant focal length
of the camera =/ is known, the line SP may be found as the
hypothenuse of the right-angle triangle having the angle = a
and adjoining side = /=5P'.

A, To Plot the Picture Trace of an Inclined Plate.

To plot the picture trace the horizontal angle included between
the optical axis of the inclined camera and the horizontal direc-
tion to some known point should be known or measured.

Should the length 5'5i', Fig. 36, Plate XX, and the posi-
tion of the line connecting two camera stations be known and
also the position of a third point i4, visible from both stations^
no instrumental measurement of a horizontal angle a need
be made, provided the plates containing the pictures a of the
third point A are oriented in such manner that the picture a
of that third point be bisected by the vertical thread, by the
principal line /f of the perspective.

We have with reference to Fig. 36, Plate XX: 5'= plotted
position of the station 5, 5'5i'= plotted length and direction of
the base line, 55 = elevation of the station 5 (laid off in the reduced
plotting-^cale), Fig. 37, Plate XXI. The horizontal angle a
(at 5', Fig. 36, Plate XX), included between the plotted base
line 5'5i' and the principal plane (or the horizontal projection
5'Po of the optical axis SP') may have been observed in the field.

We revolve the line S'S about 5'Po, Fig. 36, Plate XX and
Fig. 37, Plate XXI, into the plotting-plane, when it will assume
the position S^S), and erect at (5) a line {S){P) perpendicular
to S'{S). The angle of inclination of the plate MN^y is laid
off from (5) upon {S){P). We make {S){F) equal to the
constant focal length of the camera = /, when the line (/)(/'),
drawn perpendicular to (5)(P') through (P), will represent

PHOTOGRAPHS ON INCLINED PLATES. 79

the principal line //' of the perspective MNy Fig. 37, Plate XXI,.
revolved about S'Pq into the plotting-plane.

The point of intersection {s) of (5)5' with (/)(/') represents
the vanishing point for all vertical lines that may be shown in
picture MN.

The intersection Pq of the perpendicular line (/)(/') with
the horizontal projection of the optical axis S'Pq will be the
trace of the inclined principal line //' in the ground plane (draw-
ing plan). The line P^gy perpendicular in Pq to 5'Po> is the
ground line or the trace of the inclined picture plane MN in
the drawing plan GG. -

B. Plotting the Lines 0} Direction to Points pictured on an
Inclined Photographic Plate.

The inclined picture plane MN is revolved about Pog inta
the drawing or ground plane, Fig. 37, Plate XXI, when it will
appear as {M)(N)y the principal point P falling upon 5'Po =
(/)(/') in (P) and (P)Po is equal to PPq.

To plot the direction from 5' to a point A, Fig. 36, Plate XX^
pictured in MN as a, we first locate the orthogonal projection a^
of the pictured point a in the ground plane (plotting-plane) •
We project the image point a, Fig. 37, Plate XXI, upon //' or
upon PPo=a, and describe a circle about Pq with Poa=Po(a)
to locate the position (a) of the projected point on the principal
line (/)(/'), revolved into the ground plane. (The positions
of the pictured points a in Figs. 36 and 37 do not correspond;
both should be on the same side of //' in the picture planes.
iM)iN)-,)

The perpendicular to 5'Po, Fig. 37, Plate XXI, in ao and the
vertical that passes through a intersect each other in Oq. The
point ao, Fig. 36, Plate XX, is located on the plotting-sheet as the
intersection of (flo)«o (perpendicular to 5'Po through (ao) -^)
and {a)ao (parallel with 5'Po or with (/)(/') through (a) -).

S'oq, Fig. 36, Plate XX, will be the horizontal projection

8o PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

(in the plan) of the line of direction (or radial) from 5' to the
point i4 to be plotted.

C. Determination of the Altitudes of Points pictured on an
Inclined Plate.

It is desired to find the elevation H of the point Ay pictured
in MN as a, above the ground plane GG. With reference to
Fig. 36, Plate XX, the elevation aao=aao, in Fig. 37, Plate XXI,
corresponds to (a)ao-

If Z>= horizontal distance of the plotted point A from the
station 5' (taken from the plot ting-sheet), h=aao = aao=^(a)ao9
il= elevation of A above GG, and d=5'ao (Fig. 36), taken from
the plotting-sheet, then the elevation H of the point A may be
found, either graphically from a diagram. Fig. 39, Plate XXIII,
or it may be computed from the relation

D. Applications of Pro}. Guido Hauck^s Method.

The constructions described for locating the horizontal direc-
tions to points photographed on inclined plates may be greatly
simplified by applying Prof. Hauck's method, utilizing the prop-
•erties of the " kernel points" of two photographs obtained from
different stations, but covering the same terrene.

In Fig. 38, Plate XXII, 5 and 5' may represent the two camera
stations, 5o and 5o' are the foot pomts of S and 5' respectively,
MN and M'N' may represent the inclined picture planes, both
containing the images a and a', respectively, of a point A and the
pictures s' and s of the stations S' and 5. The orthogonal pro-
jections of the pictured points a and a' in the ground plane are
€Ko and ao. A^'is the orthogonal projection of A into the ground
plane GG. We had seen that -?, s', and tt are "kernel points "

PHOTOGRAPHS ON INCLINED PLATES. 8 1

for the picture plane MN and 2', Sy and ;/ are the " kernel
points " for M'N\

The line connecting a and ^ in MN and the line a^s in MW
intersect each other in the same point Q of the line of inter-
section of the two pictures planes (MN and M'N'), and they
also intersect the ground lines gg^ mn and 7^ respectively.

All lines in MN connecting s' with pictured points and those
in M'N' connecting 5 with the images in M'N' of the same points
will intersect each other in points Q of the line of intersection
(" perspective axis ") of the picture planes.

The points 2 and 2' (the intersections of the verticals passing
through the camera stations 5 and 5' with the inclined picture-
planes MN and M'N') are the vanishing points for the pictures
of all verticals shown in the negatives. Whenever the pictures
contain images of vertical lines, the intersections of their pictures
would locate 2 and 2' on MN and M'N' respectively; still, when
the picture plane is inclined in such a way that the principal
line of the same would coincide with that of the vertically ex-
{x>sed plate (if the former were revolved about a line as axis
passing through the second nodal point and being parallel with
the horizon line 0(y, or HH')^ the kernel point 2 may more
readily be located upon jf, as previously shown for 5 in Fig. 37^
Plate XXI.

The horizontal direction SqAq {SqAq) intersects the ground
line gg^ of MN {M'N') in Oq (respectively in a^'). Fig. 38, Plate
XXIL In order to locate the position ol Aq with reference
to a on MN (to a' on M'N') we connect a and 2 (also a' with
2')t which line locates Oq (and Oq') upon the ground line gg^ of
the picture plane MN (and M'N' respectively).

The interesction -4 of the lines 5oao and 5'ao' will now give
the plotted position "in the ground plane GG of the point A.

CHAPTER VI.

PHOTOTOPOGRAPHIC SURVEYING METHODS.

From the preceding chapters we find that in order to utilize

for iconometric purposes the data contained in a photographic

perspective we should know:

First. The three constants or elements of the perspective,

which are the focal length, together with the horizon and

the principal line, or the focal length and the principal

point, together with either the horizon line or the principal

line of the perspective.

Second. The position of the picture plane with reference to

fixed points of the terrene, which means the elements for

the orientation of the picture trace in the plotting-plane.

To plot the position of any geodetic point in both the horizontal

and in the vertical sense, we should know, or be able to ascertain,

First. The horizontal angles included between the principal

plane and the lines of direction from two or more stations

to the geodetic point.

Second. The angle of elevation (or depression) which is the

vertical angle included between the horizon plane and the

line of direction to the geodetic point.

If the constants or the elements of the perspective are known,

the geodetic elements (the horizontal and vertical angles) needed

for plotting the position of the geodetic point may be ascertained

cither graphically or arithmetically.

Phototopographic methods being generally applied with a view

toward obtaining a graphic record of the measurements in the

PHOTOTOPOGRAPHIC SURVEYING METHODS. 83

form of cartographic representation of the terrene, we shall give
in these pages principally graphic solutions of the more important
problems met with in phototopography.

I. Analytical or Arithmetical Phototopographic Methods.

A. Method 0} Prof, Jordan.

In Chapter I, section III, mention has been made of Prof.
Jordan^s map of the oasis " Dachel " and village " Gassr-Dachel,*'
based on Remel^'s photographs. Care was exercised to expose
the plates in vertical plane, and horizontal directions to at least
three points of each photograph were measured instrumentally
to obtain the required data for the orientation of the pictures.
Vertical angles to at least two such points for every picture were
also observed to give the means for locating the horizon lines
of the pictures, thus enabling the draughtsman to deduce the
elevations of other points pictured on the photographs. With
reference to Fig. 40, Plate XXIII, we have :

00'= horizon line of photographic perspective MN;
//'= principal line;
P = principal point ;

5= second nodal point (focus) of camera lens;
5P=/= focal length of picture JlfJV= principal ray;
a, by and c = images of three points Ay B, and C;
cTi, a2i and as^horizontal angles a'5P, l/SPy and c^SP;

5iV= direction of the meridian passing through the
station S;
<l>i ^2> and ^3=azimuthal angles NSa', NSVy and NSc^ re-
spectively;
Hi, H29 and -^3 = elevations of the points Ay By and C above the
plane of reference or ground plane.

The photographic plate MN having been exposed in vertical
plane, it will be evident that for the three points a, b, and c
respectively the abscissae xi, X2t and Xs should be

84 PHOTOTOPOGRAPflIC METHODS AND INSTRUliENTS.

Xi=^f ta.nau
X2=f tan a2,
iV3=/tana3,

tu ^ \ f sin (a2-ai)

^2-^i-/(tana2-tana0«/-^^3^^^^^^

N T sin (a8-«2)

and i^a— JC2=/(tana3--tana2) = / •

'^ ^ ' cosa3Cosa2

The values for (iV2— ^i) and (^3—^2) may be scaled oflF directly

on the negative, MN, and the values for (a2-ai) and (ag— ^2)

may be taken from the field records of the observed horizontal

cos 0:3
angles, when the value for may be computed by means

of the formula

^2—^1 cosa3 sin (ft2~Q^i)
XZ—X2 cos «! sin (as — a2)'

•- , . - cos 0^3 ■

If we substitute tan r for , and as

' cos «!

I +tan r , ^

we may write

cosa3

^ / , V cosai cosai+cosag
tan (45 + r) = = —

cos as cos ai -cos as

cosai

«! +^3 ai -as
cos cos

sm sm —

2 2

E5-°"('t^')-("-t^')=

hence tan =cot (45 +n cot i 1.

PHOTOTOPOGRAPHIC SURVEYING METHODS. 8$

From this equation ai+az may be computed.
By inspection we find from Fig. 40, Plate XXIII,

as --a2 = ^3 — ^2 = «2>

By adding these two equations we obtain

ai—as = <j>2 — <f>3'

Knowing (ai H-as) and (ai—az) we can readily find ai and a^;

also a2=ai + €i

or =a3""«2«

We had found

, sin (a2-«i) 1 sin ^
'cosaicosa2 cosaicosa2'

t (^2—^1) cos ai cos a2

hence * = : ,

sm ei

, sin (a3-a2) / sin £2

and :»3— ^2=/ ^ =*/ >

'cos as cos a2 cos as cos a2

. (rc3 - X2) cos as cos a2

whence /= : .

' sm 62

Thus two elements of the perspective MNy the focal length /,
and the principal line ff (given by the abscissae x^ ^2? and x^)^
oiay be foimd.

With the aid of the observed vertical angles ^, the third ele-
ment, the horizon line OCX, may now be located on the photo-
graph.

86 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

The vertical angle fis^cSc' having been observed at S to
the point C we find

cc'=y3=5c'-tanfi3= — - — tanfig,
•^ ^ cos as ^

and for the point a the vertical distance to the horizon line would
be

aai =yi =5a'-tan /?i = — — tan fii.

cos CCi

The horizon line 00' will be the common tangent to two

circles, one described with the radius = tan^i about a

cosai ^

and the other with a radius = tan/?3 about c.

cos as ^

At least two vertical angles having been observed for each

exposed plate, the horizon line 00' may thus be located and

marked upon the negative, when the principal point P may

also be located on 00' by means of the principal line //', the

latter being tangent to the three circles described about a, 6,

and c with the radii X\y X2, and x^ respectively.

B. Method of Dr. Le Bon,

Dr. G. Le Bon (who used his instrument chiefly for the plot-
ting of ancient buildings and monuments in India) provided
the ground-glass plate of his camera with a net of squares, each
square having i cm. sides, one set of the latter being drawn
parallel with the horizon, while the second set of lines is paral-
lel with the principal line of the perspective. The lines repre-
senting the horizon and principal lines are again subdivided into
millimeters.

This arrangement enables the operator to obtain the measure-
ments of objects directly by inspection of the image on the gradu-
ated ground-glass plate.

PHOTOTOPOCJRAPHIC SURVEYING BiETHODS. 87

To determine the dimensions of the front of a building Dr. Le
Bon measures a certain length directly upon the same and then
takes a picture by exposing a photographic plate in vertical
plane and parallel with the base of the front (fajade) of the
building.

For example, to find

First. The distance D of an object, the height H of which
is not known, Fig. 41, Plate XXIII:

Two stations S and S' are occupied on a base line B (which
is measured directly in the field) laid off in a direction perpen-
dicular to the base of the object.

If the height of the image measured on the ground glass
at the first. station is h, at the second station h'j and if the focal
length for both exposures be the same and=/, then

and for the second station S'

{D+B):H=f:h'.

h and A' being known — they may be measured directly on the
negative or on the ground-glass plate — ^we find, after dividing
the second equation by the first.

D+B

D '■

h-h'

~ h' '

BN
whence i? = i

h-W

88 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

Second. The height il of an object is to be found when

the fractional length H' has been obtained by direct

measurement (Fig. 42, Plate XXIV).

On the image of the object — on the graduated ground-glass

plate — the lengths for the heights h and hf may be read oflF directly,

and as J3"' is also known, we find H from the equation

C. Method of L. P. Paganini (Italian Method),

This method has been extensively used for the new topo-
graphic survey of the kingdom of Italy and for the Colonial
possessions in East Africa (" Eritrea").

COMPUTATION OF THE ELEMENTS OF THE PHOTOGRAPHIC

PERSPECTIVE WHICH ARE NEEDED FOR THE

ICONOMETRIC PLOTTING.

I. Determination of the Focal Length of the Photographic
Perspective.

(a) When the Reference Point is Bisected by the Principal Line of the

Perspective.

A triangulation point 5, Fig. 121, Plate LX, may be visible
from the camera station V. The camera is directed toward S
in such manner that the image 5 of the distant peak S is bisected
by the vertical thread //'.

F= camera station or the point of view of the perspective

MN;
Pi = principal point of the photograph;
VPi = focal length or distance line of the perspective, denoted

by/;

PHOTOTOPOGRAPHIC SURVEYING BiETHODS. 89

5'= Orthogonal projection of 5 in the horizontal plane which
passes through F;
F5i'= horizontal distance from V to 5, designated by D\
55'= apparent difference in elevation between V and 5, des-
ignated by L.
After having carefully measured the ordinate Ps=y on the
negative, we can determine the focal length from the equation

'"" L •

Example No. I. — ^The station V may be occupied over the
centre of the triangidation point Reale Accampamento and the
bisected point 5 be the signal upon Cian del Lei. The camera
having been leveled and adjusted over Reale Accampamento is
turned in azimuth until the signal Cian del Lei is bisected by the
vertical thread //' and the first plate is then exposed (Fig. 122,
Plate LXI).

The focal distance, read off on the scale a of the lens tube
= 244.5 Dim., and the values for Z> and L, taken from the records
of the new trigonometrical survey of Italy, are

i)«distance from Reale Accampamento (signal) to

Punta Cian del Lei (signal) =3270.7 m.

Elevation of station mark at Punta Cian del Lei =2811.72 m.
Elevation of camera horizon at Reale Accampamento = 2191.80 m.

Difference of true elevation = 619.92 m.

The ordinate y, carefully measured on the negative (from the
principal point P to the image s of the point Cian del Lei) gave
46.25 mm.

Computation of £ :

True difference in elevation =619.92 m.

Correction for curvature and refraction = —0.72 m.

Z= apparent difference in elevation =619.20 m.

90 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

Computation of /:

logZ?= log 3270.7 =3-5146407

log y = log 0.04625 = 8.6651 117

colog L = colog 619.20 = 7.2081691

iog /=9-38792i5
/= 244.30
Scale- reading for / was =244.50

Difference = 0.20 mm*

(b) The Image of the Reference Point Falls to either Side of the Principal
Line of the Photographic Perspectiye.

If the image 5 of the reference point 5 be to either side of
the vertical thread jf of the perspective MN, Fig. 123, Plate
LX, the principal ray VP making an angle e with the horizontal
direction VS' to the reference point 5, then the value VP^j may
be found as follows:

d= horizontal distance Vsf (Fig. 123);

y and :»= coordinates of the image 5\

X>= horizontal distance between the camera station V and the

reference point 5, and
L= apparent difference in elevation between V (or s^) and s.

From the similar triangles VSS' and Vssf we find
D~d'

, Dy
hence a «= -j-.

PHOTOTOPOGRAPHIC SURVEYING METHODS. QI

From the horizontal triangle sfPy we find

COS e
hence }= -jocose.

Example No. II, — In the panorama (series of ten photographic
perspectives) obtained September 21, 1884, vertically above the
trigonometrical point (of the new Italian geodetic triangulation
system), near Reale Accampamento of Valsavaranche, there is
one plate (P^, Fig. 122, Plate LXI) which contains the image
of the triangulation station Punta Ruja (signal).

The horizontal angle o>, between the optical axis of the camera
for this plate and the horizontal direction to Ruja (signal), is =

S<^ 49' 27".75.

The horizontal distance: Reale Accampamento — Ruja = 2)=-
5804.2 m. and the elevation of Ruja =31 73.5 m. are taken from
the triangulation data.

By careful measurement y is found to be =41 .45 mm.

It is desired to find the focal length, /, for this perspective,
which may be obtained, approximately, by reading the gradua-
tion on the objective tube =244. 50 mm.

Computation of L:

Elevation of station mark at Ruja =3173.5 m.
Elevation of the camera horizon (00') = 2191.8 m.

True difference in elevation = 981.7 m.
Correction for refraction and curvature = — 2.3 m.

Apparent difference in elevation = 979.4 =L

92 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

Computation of /:

log D=log 5804.2 m. =3-7637424

log y = log 0.04145 m. =8.6175245

log cos w=log cos 5® 49' 27".75 = 9.9977522

colog L = colog 979.4 = 7-0090399

log/ = 2.3880590
/= 244.38 mm.
Scale reading = 244.50 mm.

Difference = o. 1 2 mm.

Had we measured the abscissa x instead of the ordinate y
the focal length for the negative could have been computed by
the formula

f—x cot OJ.

Example No. III. — The measured value for x may have been
found to be = 24.90 mm.
Computation of /:

log ^=log 24.90 =1-3961993

log cot w^log cot 5° 49' 27^.75 =0.9913737

log /= 2.3875730
/= 244. 103 mm.

2. ORIENTAnON OF THE PICTURE TRACES.

There exists a close connection between the phototopographic
stations and the new triangulation of Italy. A generous dis-
position of the trigonometric points had been made with the
special purpose in view that they were to serve as the foundation
for the subsequent topographical survey. These points have

PHOTOTOPOGRAPHIC SURVEYING METHODS. 93

been carefully selected, their positions have been precisely com-
puted, and their locations have been permanently marked in the
field, irrespective of the character of the surrounding topography
or of the order of triangulation to which the point may belong.
This large number of triangulation points not only facilitates
the application of the phototopographic sur\'eying method and
assures the accurate determination of the panorama stations
(in the horizontal and vertical sense), but it also greatly simplifies
the subsequent iconometric plotting, as the greater part of the
perspective contains one, two, or more pictured triangulation
points, notwithstanding the instrument commands a field of view
of but 42° horizontally. Two adjoining plates have a common
margin of an angular width of 3°, reducing the effective field of
view of one plate to 36*^ (Fig. 124, Plate LXII).

Thus the picture traces are easily oriented for the icono-
metric work, the salient topographic features (deduced from
the perspectives) may, be frequently checked, and such nega-
tives (containing the images of triangulation points) may also
serve to verify the focal length / of the panorama pictures, check
the position of the principal point P, and they give the means
for testing the location of the horizon line 00^ on the pictures.

The perspective MJNT, Fig. 125, Plate LXII, may contain
the images of two trigonometrical points 5 and 5'.

In the preceding pages it has been shown that the horizontal
distances, d and rf', from the camera station V to the pictured
points S and S' may be found from the relations

Dy D'V

d^-f and d'^-jf-.

In the triangle VS\Si we know the lengths of two sides, d and
J', and also the value of the included angle, 5iF5i', which may
be either measured directly at the camera station or, when the
latter is also a triangulation station, the value for the angle may
be taken from the triangulation records.

94 FHOTOTOPOGRAFHIC METHODS AND INSTRUMENTS.

The Other two angles, f and d, of this triangle may now be
found as follows:

r-» d'-d V

tan =:FTjCot-,

2 d''\-d 2

r-l-aH-V=i8o% hence =oo*^

^o.

If we replace i(r+^) by M and Kr*"*) by iVT, we find after
addmg the equations

m+9)+(r-9)]'M+N^r,

and by subtracting the equations

The principal ray VP should be vertical to the horizon line OC
and both triangles VPSi and VPSi^ should be right-angle triangles.
Hence the focal length / should be

/=(f-sin y
and f=d^ -sin d.

To ascertain whether the pictured intersection of the cross-
wires P coincides with the principal point of view P upon the
perspective, the measured lengths of the abscissae x and a/
(Fig. 125, Plate LXII) should be the same as the computed
values

jc=/cot r,
^=/cot^.

The angles of orientation w and a/ are:

a>=9o°-r>
c«/=9o°-a.

PHOTOTOPOGRAPmC SURVEYING METHODS. 95

3. Determination of the Elevations of Pictured Terrene Points.

The vertical angles of elevation a and a' of the two refer-
ence points 5 and 5' may be computed from the equations:

tana =-^,

tana'=-^.

These angles are either taken from the triangulation records
or they may be observed directly from the camera station, and
to check the position of the horizon line OC the ordinates
y and y, measured on the perspective, are compared with those
computed by means of the equations

y=' tana,

-^ cos CJ

y = 7 tana'.

•^ cos 0/

Example No. IV. — In the panorama obtained Sept. 19, 1884^
from Punta Percia (this peak is on the divide separating the
valleys of the Rhfemes and the Valsavaranche) two trigonomet-
rical stations, Punta Rouletta and Gran Punta di Nomenon, of
the new Italian geodetic survey appear upon the same plate (see
Fig. 126, Plate LXIII).

The following values are given for the computation:

Elevation of Punta Rouletta =3384.10 m. ( Taken from the

i catalogue of tri-
Elevation of Punta di Nomenon =3488.42 m.[ angulation points

{Elevation of P.
Percia + height
of instrument

Distance: Percia-Rouletta=« I> =3250

Distance: Percia-Nomenon=»Z>' =9720

' Measured upon
the projection of
the iconometric
working - sheet,
scale j^jfijfjf

96 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

The horizontal angle V (Fig. 126, Plate LXIII) at Punta
Percia, included by the horizontal directions to Rouletta (signal)
and Nomenon (signal), =28^02' 30".

A careful measurement of the coordinates of the pictured
points P. Rouletta and P. Nomenon, on the negative with
a millimeter scale provided with a microscope and vernier, en-
abling the computer to read to 0.05 mm. (the vernier is divided
to read to 1/20 of the graduation unit), produced the following
values:

The coordinates of Punta Rouletta,

:v=46.o5 mm.; ^ = 13.75 mm.
The coordinates of Punta di Nomenon,

^ = 75.40 mm.; / — 7.30 mm.

It is desired to find:

(i) The focal distance for this negative = /, the preliminary
value, read off on the scale attached to the objective cylinder, is
found to be 244.50 mm.

(2) The correct position of the principal point (P), which
will be fixed by the determination of the abscissae x* and txf.

(3) The position of the line of horizon 00*^ which will be
located by ascertaining the values for y and /.

Computation to determine the apparent differences in elevation
between the camera horizon (P. Percia) and the two pictured points
P. Rouletta and Punta di Nomenon.

Altitude of P. Rouletta =3384.10 m.

Altitude of camera horizon =3202-30 m.

True difference in elevation = 181 .80 m.
Correction for curvature and refraction = — 0.71 m.

Apparent difference in elevation =L = 181 .09 m.
Altitude of Punta di Nomenon =3488.42 m.

Altitude of camera horizon = 3202 . 30 m.

PHOTOTOPOGRAPHIC SURVEYING METHODS. 97

True diflference in elevation = 286.12 m.
Correction for curvature and refraction— — 6.35 m.

Apparent difference in elevation =L' = 279 . 77

Computation of d-= —y.

log I>- 35 "8834

log y« 8. 1383027

colog 1=7.7421055

log (2» 9.39229 16
d^ 246.77 mm.

D'
Computation of df^-j,'/^

log P' -3.9876663

log /= 7.8633229

colog L'= 7-5531989

log (f'» 9.404 188 1
J'«= 253.62 mnu

rf+d'= 500.39
d'-d^ 6.85

Computation of the angles y and b\

r-9 d'-d V
tan = iTTl'Cot — ;

F=28<>o2'3o"; - = 14^01' 15";

y+a-i8o-F = i5iO 57' 30"; ^ = 75° S8' 45"=^^.
log (rf'-d) = log 0.00685 =7.8356906

log cot -=logcoti4*^ 01' 15" =0.6025567
colog (d'-i-J)= colog 0.50039 =0.3006914

log tan ^=8.7389387

^=3° 08' i6".i=iV.
2 •^

Hence JIf +JV=r=79° 07' oi".i

Jf _JV=5=72° 5o*> 28".9

98 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS,

(a) Computation of the Focal Length ( — /).

log d=9. 3922916 log d' =9.4041881

log sin 7'=9.992ii8o log sin ^=9. 9802269

log /=9-3844096 log /=9.3844iso

/ =0.242331 m. / ==0.242334 m.

Mean ralue for /« 242. 332 mm.

iP) Computation of the Abscissa (x and x') for Platting Lines of Hori*
zontal Directions to Pictured Points of the Terrene and for Checking
the Position of the Principal Point,

x=^f'\zxio) x^ = f't3.na/

a>=90°-r = io*=* 52' s8".9 a/=gcP-d==if o()' 3i''a
log / = 9 - 38441 23 (mean log) log / = 9 . 38441 23 (mean log)
log tan a>=9. 2838945 log tan a>' =9.4896222

log :x; = 8 . 6683068 log ^x/ = 8 . 8740345

jk:=46.59 mm. :jc' = 74.82 mm.

^ meas. on plate =46.05 nmi. xf meas. on plate = 7 5. 40 mm.
difference = J:v = o. 54 mm. Ao(f =0.58 mm.

Mean difference =0.56 nmi.

From this difference we infer that the principal point P of
the photographic perspective should be transj)osed toward the
pictured point of Punta di Nomenon by 0.6 mm.

</) Computation of the Ordinates (y and y') of Pictured Terrene Points
of Known Elevations to Check the Position of the Horizon Line {00^
on the Negative.

y = tan a, /= — ^tan a'.

-^ cos 6; ^ cos (J

PHOTOTOPOGRAPHIC SURVEYING METHODS. 99

Where

a « angle of elevation of Punta Rouletta - 3« 1 1' 30" f ^^^^^ ^F^f ^« V?^"
a'- angle of elevation of Punta di Nomenon- 1<» 38' 30" j p^nTa Percia

log/- 9-3844123

log tan a« 8.7463444

colog cos (o^ 0.0078820

logy- 8. 1386387
y— 13.761 mm.
y measured on plate— 13.75 mm.

Difference— 0.0 1 mm.

log/- 9.38441 23

log tan a'— 8.4572812

colog cos &>'— 0.0197731

log /- 7.8614666
/— 7.269 mm.
y measured— 7.30 mm.

Difference— 0.03 mm.

The correction for y is so small that it may be disregarded;
the length measured on the plate for / should be reduced by
0.03 mm. and the corrected horizon line would fall 7.27 mm.
below the pictured point P. di Nomenon (Fig. 126, Plate LXIII).

(9) Orienting a Panorama.

The angles of orientation of the perspective (o) and a/) regard-
mg the two pictured triangulation stations had been found to be

(o^io"" 52' 58".9
and ci/«i7° 09' 31''.!

Owing to the fact that the distance D and ZX in the pre-
ceding example are large in comparison with the ordinates
y and /, it may be preferable first to determine / by means of
the abscissae and then to compute the values for the ordinates
(y and /) based upon this value of / and the observed angles
of orientation (o and 0/.

If we construct the decagon (see Fig. 124, Plate LXII) rep-
resenting the horizontal projection of the ten negatives obtained
at the station Punta Percia by means of the elements, obtained
by obswvations only, we will find the direction to the principal
point P of the perspective containing the pictures of Rouletta
and Nomenon to be =350° 00' 00".

lOO PHOTOTOPOGRAPHIC METHODS AND INSTRUliENTS.

Direction to Punta Nomenon (signal) =332° 42' 00'.
Direction to Punta Rouletta (signal) = 0° 44' 30".
From these lines of direction we find the following values for
the angles of orientation:

Direction to Rouletta « 360® 44' 30"
Direction to point P ^ 3So^ 00' 00"

«= 10® 44' 30"

Direction to point P -= 350® 00'
Direction to Cima

Nomenon =332® 42'

Which differ from «'«- 17** 18' 00"

' the preceding values w= 10" 52' s8".9
by J*.;= ± 8' 28".9 «'- 17' 09' 3i".i

This small angle J(o (at V) corresponds to an abscissa Jx,
which in turn represents the error in position of the pictured
point P in the horizontal sense. (In the preceding example,
panorama from station Percia, Sept. 19, 1884, Jx=o.$6 mm.)

From the right-angle triangle with angle at F=Ja; and the
two sides Jx and / (Fig. 126, Plate LXIII) we find the value
for Jx for our negative from the equation

Jx^'f tan J(o.
log / = log 242 .332 = 2 . 38441 23
log tail 0° 08' 28".9 == 7 . 3922081

log Jjc= 9. 7766204
Jx^ To. 598 mm.,

representing the error in the position of the principal point of
the perspective, which, however, has little influence upon the
precision of the graphical operations (iconometric plotting)
which are to be executed in order to transpose the topographic
relief upon the chart from the photographic perspectives as
long as the error in question (Jx) does not exceed 2 mm. for
the entire panorama of ten plates, each controlling an angle
of 36° horizontally.

4. Checking the VERxicALrrv of an Exposed Plate.
For mountain work, where the differences in elevation between
the several terrene points (which are pictured on the negative)
and the camera horizon are relatively great, it is important to

PHOTOTOPOGRAPHIC SURVEYING METHODS. lOI

know whether the negative had been exposed while in vertical
plan and whether the photographic perspective has a correct
horizon Ime OCy^ since the ordinates of the various terrene
points in question would become too long when the plate is
(inclined) not vertical. Therefore to assure oneself whether
the picture plane (ground-glass or negative plate) of the photo-
theodolite is vertical and also whether the optical axis of the
camera is horizontal (the trace of this axis upon the perspective
plane b represented by the intersection P of the pictured cross-
threads OCy and jf) bring the plane containing the optical axis
and the " horizontal wire " OO' representing the horizon line
into horizontal plan and direct the instnmient to a well-defined
distant point situated high above the camera horizon, and
at the same time the vertical thread should be visible against
the groimd glass, so that the distant point may be bisected by
it. If we now measure the ordinate of the elevated distant
point on the ground-glass plate and repeat this measurement ,
after revolving the camera in azimuth i8o°, clamping the hori-
zontal circle, and " transiting " the camera (revolving it about
its horizontal axis of revolution i8o°), the two lengths thus
obtained for the ordinate of the point in question should be the
same, otherwise the conditions of the apparatus are not satis-
factory and the instrument should be adjusted until the two
measurements give the same results.

With reference to Fig. 127, Plate LXIII, we have

FP= optical axis of the camera made horizontal and the vertical
thread bisecting the distant point 5;
MP A wangle of inclination of the picture plane against the verti-
cal plane;
P^= length of ordinate of pictured point 5, measured on ver-
tical thread from P.

The plane of the perspective NM not being vertical to the
optical axis, it will assume the position N'M^ for the indirect
position of the camera (as described above) and the ordinate Po^

I02 FHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

measured on the ground-glass plate will now be shorter than
when measured {=Po) before. Had the picture plane been
vertical originally it would have coincided with AB for both the
direct and reversed observation; the ordinate measured in both
positions would have been = P5.

The instrument must be adjusted to make

The error in position of the principal point P of the perspective,
considered in the last numerical example, will appear immediately
if one wishes to determine the value of the focal length =/ by
means of the abscissae measured on the ground-glass plate (or on
the negative).

We had found by measurement

:» =46.05 nun. and xf « 75.40 mm.

and the observed correpsonding angles of orientation were

ft; = 10® 44' 30" and ft/ = 17® 18' 00".

The twofold determination of / may be derived from the
relations

X x'

/-.ITT. »»<» /-

tan a> ^ tan of'

log je= 1 . 6632296
colog tan o>a- o . 72 19207

log /= 2.3851503
/= 242 . 745 mm.

log x'* 1. 8773713
colog tan Qt'^ o . 5065903

log /= 2.3839616
/— 242. 082 mm.

Mean value for /= 242.41 mm., differing but slightly from the value /« 242.33
mm., previously obtained. Difference » 0.08 mm.

Example No. V. — Giving the means for ascertaining the
attainable degree of accuracy of the Italian phototopographic

PHOTOTOPOGRAPHIC SURVEYING METHODS. lOJ

method the following computation is of greater interest in a general
way, as the panorama station was selected over a trigonometrical
point of the Italian geodetic triangulation system, thus admitting
a direct comparison between the elements of the perspective
and the exact values of these same elements deduced from the
data of the triangulation work.

In the panorama views, obtained on Sept. 21, 1884 (see Ex-
ample No. 11), vertically above the trigonometrical point known
as Reale Accampamento there is one plate (P*) which con-
tains the pictures of two triangulation points, Punta Ruja
and Gran Cima di Nomenon (the same points as previously
mentioned). From the geodetic computations we take the follow-
ing data:

Elevation of Pimta Ruja (signal-mark) *= 3173-5 di.

Elevation of P. di Nomenon (signal- mark) =3488.4 m.

Elevation of camera horizon (Reale Accampa-
mento) =2191.8 m.

Distance: R. Accampamento-Ruja=Z? =5804.2 m.

Distance: R. Accampamento-Nomenon=Zy =5029.6 m

Horizontal angle: 7=Ruja-Accampamento-

Nomenon =13*^ 51' 04". 50

It is desired to find (see Fig. 128, Plate LXIV):

(i) The focal length, /, approximately found by reading the
scale attached to the objective tube = 244.50 mm.

(2) The position of the principal point of view P which is
located by the abscissae x and xf.

The coordinates obtained by carefully executed measurements
on the negative (the same as in the preceding) are for

Punta Ruja: x =24.80 mm.; y =41.45 mm.

Gran P. di Nomenon: a/ =34.05 mm.; / =63.50 mm.

ICH PHOTOTOPOGRAFHIC METHODS AND INSTRUMENTS.

CompukUion of the apparerU difference in elevation:

Elevation of P. Ruja « 31 73-5 m.
Elevation of camera ho-
rizon -o 2191^01.

True difference in levels 981.7 m.
Correction for curvature

and refraction — 2.3

Apparent difference of

elevation » 979.4 m.= Z.

Computation of d^-yy,

log £>- 3. 7637424

logy« 8. 6175245

cologZ.« 7.0090399

Elevation of Nomon — 34884 m.
Elevation of camera ho-
rizon -»2i9i^m.

True difference in level =» 1296.6 m.
Correction for curvature

and refraction — 1.7 m.

Apparent difference of

Lppi
elc

levation

1294.9 m.»L*

log rf- 9. 3903068
^» 245 . 644 mm.

Computation of d'=~ •/.

log £>'« 3. 7015334

log /= 8. 8027737

cologZ.'- 6. 8877638

log <f'« 9. 3920709
J'»246.644mm.

j-f.^'s 4.92.29 nmi.
d'—d^ 1. 00 mm.

Computation of the angles f and d:

^ r-d d'^d ^ V

tan -5 = . , .. cot — ;

2 d+d' 2

V- 13° 51' 04".5o; — &> 55' 32".2S.

r+*-i8o-F-i66« 08' 55 '.50

r±£.

830 04' 27".7S

log (d'—d)'^ 7.0000000
V

log cot

-0.9155404

colog (J+ <iO— 0.3077790

log tan ^--—« 8.2233194

tli- o® 57' 29".io-iV

Af+iV- 84*^ 01' s6".9 -r
- - - ' " Y'.7 -a

Jlf-iV-82<» 06' 58".7
Compuiation of f:

/— d sin Y-
log <f 1.9.3903068
log sin ;^- 9. 9976402

log/- 9. 3879470
/—0.244313 m.

f'^d' aind,
lopf<r- 9. 3920709
log sin J- 9. 9958757

log/ -9 3879466
/= 0.244313 m.

PHOTOTOPOGRAPHIC SURVEYING METHODS. IO5

Mean value for j^ 244.31 mm. is the same as obtained in numerical example
No. I.

Computation of the abscissa x and x' (to check the position of the principal
point P):

x^f itji to.

o>«9o«-r=5*' 58' 03"- 1

log/=- 9 3879468
log tan ta= 9.0192462

logaf= 8.4071930

af«25.S4mm.

Measured x= 24 . 80 mm.

Diff.= 0.74 mm.

a/-/ tan w'.
a;'-9o°-d=.7«> 53' oi".3

log/= 9 3879468
logtan<(;'= 9.1413601

loga:'= 8.5293069

*'= 33 83 mm.

Measured :v's 34 . 05 mm.

Diff. » 0.22 mm.

The mean difiference» 0.48 mm. « error in the position of the principal point P
of the perspective P* (Accampamento panorama). The true point P* is 0.5 mm.
to the rig^t of P^ Fig. 122, Plate LXI, and the vertical line //' (the principal line)
on the plate, Fig. 128, Plate LXIV, should be moved towards Punta di Nomenon
(image) 0.5 mm.

The preceding five numerical examples may well serve to
elucidate the relations between the elements of the photographic
perspectives and their corresponding parts of the terrene, as well
as to give the means fcu" forming a correct idea of the degree
of accuracy, attainable in the Italian photographic surveymg
method.

In practical work it would be too time consuming to make
such computations (with the necessary minute and careful graph-
ical measurements) for every negative, or even for every set of
panorama views.

If the phototheodolite has been carefully planned and is well
constructed, the optical axis should always remain perpendicular
to the image plane, hence be horizontal when the latter is ver-
tical. The value / for any (or for all) panorama views, obtained
with the same objective and with the same constant focal length
(obtained under the same reading of the scale attached to the
objective tube), may be computed from the formula

^"tan 18°*

I06 FHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

In the panorama set of Accampamento Reale station we
had one plate, P', Fig. 122, Plate LXI, containing the image of
the signal at Punta Cian del Lei, bisected by the principal
line, VFi and another plate, P*, containing the images of two
other points, Punta Nomenon and Pimta Ruja.

The horizontal shiftings in azimuth

P'VP^, P^VP^ P37i>*...,

representing the horizontal swings in azimuth of the camera for
each successive exposure, are all alike, each being =36*^, and the
horizontal angles

FVm, mVP^, P^Vm'...

will each be =18°.

The horizontal angle, included between the principal lines
VP' and VP^ (between the horizontal direction to the reference
point P. Cian del Lei and the principal line of perspective
No. 5) will be=4X36° = i44° (Fig. 122, Plate LXI).

We find in the trigonometrical records, or from direct ob-
servation at the station Accampamento Reale:

Horizontal angle: Cian del Lei-Accampa-

mento-Nomenon ™I3S*^ S8' 23".25

Horizontal angle: Nomenon-Accampamento-

Ruja = 13*=* 51' o4".5

Hence the horizontal angles of orientation for the fifth plate:

^ a/ = (P5-F -Nomenon) = 8<^ 01' 36^.75

-2^0) =(Nomenon— F— Ruja)-a/= 5° 49' 27^.75

IS"" 51' 04".5o

In the computation for the abscissae x and xf imder Example
No. V we had foimd

^'-90°-* = 7^ 53' oi".3
and 6; = 90° - r = 5"" 58' 03''.!.

PHOTOTOPOGRAPmC StfRVEYING METHODS. I07

The dififerences between these values for (u is =08' 3S".3S and
for a/=oS' 3S''.4S, or the difference in the mean value of w and
It/ is expressed by

Ja;=T8' 3S".4o,

being about the same error in azimuth of the principal point P
that had been found for the panorama obtained from the station
Punta Percia with the same instrument (considered imder
Example No. IV, where we obtained Jo; =±8' 28".9). This
error, J<o=±y 3S".4o, corresponds to a horizontal linear dis-
placement, Jx, of the principal point P of the photographic per-
spective:

J:»«=T 0.610 mm*

5. Application of Franz Hafferl's Method for Finding the Focal-
LENGiH Value of a PnoTOGRAPmc Perspective from the Ab-
sasSiE OF TWO Pictured Terrene Points.

When the horizontal distances, D and ZX, are great com-
pared with the differences in elevation between the pictured
points imder consideration and the camera station, the ordi-
nates, y and /, will be rather short and the accurate measure-
ment of their lengths will be difficult.

In such case it may become advisable to determine the value
of the focal length / by means of the abscissae, x and x', of the
pictured terrene points. To do this L. P. Paganini uses the
following method, suggested by Franz Hafferl of Vienna.

We have with reference to Fig. 129, Plate LXIV:

00^= horizon line of photographic perspective;

Vs and Vs' = horizontal directions from the camera station V to the

pictured points s and s';
yP= perpendicular to the horizon line 00'.

It is desired to find the length value for /.

I08 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

Describe a circle through the three points V, 5, and 5', the
center of which may be at C. The angle 5C5' is double the
angle sVs' and the perpendicular CM to the line ss' will divide
this line and also the center angle ^Cs' into two equal parts;
hence

4sCM^:^s'CM^:^sVs'.

If if be the radius of the circle described through the three
points 5, s', and F, we will have, from the triangle ^JlfC, the
following relation :

^ ^ SM x-\'0(f I ^^^ X'\'0(f

sin K 2 sin K' 2

After having drawn the diameter mn parallel to ss' we find
j^VP^VA-^AP.

VA being vertical to mn it will be the middle proportional to mA
and An\

mA:AV^AV:Any

mA'An=^{AVf.

We can replace mA by {mC-AC)^R ; AC-=^SM-SP\

and as 2l»=nC+i4C=i2H— ^,

wewiUhave AV^^\l^R-'~^{R+^^^\

and finally .IP^CM =5M coty=^^cot-F.

Example No. VI. — Determination of the value / for a plate
by means of the abscissae of two pictured points.

PHOTOTOPOGRAPHIC SURVEYING METHODS. IO9

From the data derived from the computation in Example
No- IV we find

7=28^ 02' 30"

X = 46.05 mm. and ^ = 7S-4o mm.

ivH-:r' = 121.45 mm. ^-^=29.35 mm.

x-^-yf vf — X

= 60.725 nun. = 14.675 mm.

Computation of i?=

nm.

X-\-0(f

sinF"

log-

= 1

•783367s

cologsin 7=0.3277972

log if = 2.1 1 1 1647
if = i29.i7i mm.

Computation of 74 = \] ( if + ^j \R ^] ;

yf-x
R^ ==143.846 mm.

yf —X
R =114.496 mm.

log [R+'-^J =2.1578978

log (r ^j =2.0587903

log fI^ =4.2166881
log VA =2.1083440
VA =128.335 mm.

no FHOTOTOFOGRAPHIC METHODS AND INSTRUMENTS.

Computation of PA = cot V:

x-\-xf
log-^ 1.7833675

log cot 7=0.2735641

log P-4 =2.0569316
Pi4 =114.007 mm.

Hence /= Fi4 H-Pi4 = 242.342 mm.,

which compares very closely with

/= 242.332 mm.,

obtained under numerical Example No. IV.

In practical work, like the great Italian topographic sur-
vey, it woidd take too much time and labor to determine the
focal length (/), after the method just shown, for each perspec-
tive, or even for each panorama set. If there is no reason for
doubt that the optical axis of the camera intersects the picture
plane at right angles (as it does for the Italian phototheodolite
with a sufficient degree of precision) it will be more simple to
determine the value / for an entire panorama set (and also for
all subsequent panoramas that may be executed with the same
objective, and with the same focal length, which may be veri-
fied at each exposure) by simply checking the scale-reading on
the objective tube, which should remain the san ^or all pictures.
Under this supposition the focal length is cor— ^ xi in the fol-
lowing manner:

Since the azimuthal swings of the camera after each exposure,

P'FP2, P^YP^, PWP^...,'

are all alike, and each being equal to 36^ (Fig. 122, Plate LXI),
the angles

FVm, mVP^, P^Vm' . . .

PHOTOTOPOGRAPHIC SURVEYING METHODS. Ill

will each be = i8^. If x^ denotes the maximum length of the
abscissae of the plates then

x^^^Fm^mP^^P^m'^ . . . =/tan i8^
x^

hence /=

tan i8°*

In the preceding (page 93) it has been stated that two adjoining
negatives of a panorama set have a vertical marginal strip of the
pictured terrene in common, and the width of this strip may
be expressed in arc by the angle pVq (Fig. 122, Plate LXI).

If the negatives are suflSciently clear (showing a good defi-
nition) it will be an easy matter to locate a point m, either on
the negative or on the photographic print, that may be identi-
fied on both overlapping strips pq (Fig. 122, Plate LXI) of two
adjoining plates P' and P^, which will be on or near the horizon
line 00% and distant from the principal line }f of plate P' by
mF, and distant from the principal line of plate P^ by mP^, mP^
being =WjP2.

If we now select such points m', m", m'" . . . , that can be
readily identified upon two adjoining perspectives,

P2andP3, P3andP4, P^andP^...,

we will obtain ten values for m for the entire set, and the focal
length, /, for the panorama may be determined by means of
the preceding formula,

x^
'""taniS^'

where re» is the arithmetical mean of the ten greatest abscissae

Fm, mP^, P^m'

Example No. VII. — By means of ten negatives of a pano-
rama station, obtained with Paganini's phototheodolite, described
in " La Fototopografia in Italia," the following values were
found for the distances oc^:

112 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

jf"* for P^ —P^ =» 77.10 mm.

JC» " P2 -P3 =77.15

jc» " pa -P* -77.00 '

«" " P* -Ps =77.40
«"» *' P« -P« =77.40

-:»'» = 77.194 mm.

»mean value.

«"» " P« -P7 =77.20

«"• " P« -Pio = 77.40

' *

jc" " P»o-P»- = 76.9o "

log 77.194 = 1.8875835

colog tan 1 8® =0.4882240

log/=2.3758o75
/ =237.6 mm.

The above values were obtained by using the n^ative plates
and reading the measurements scaled off (by means of dividers)
on the graduated rulers of the graphical instruments (" icono-
meters ") of the Royal Military Geographical Institute.

Using the positives (albumen prints) of the same panorama
the following results were obtained :

x"^ for P^ —P^ =76.25 mm.

Xm " p2 _p3 =76.20 "
jfW «« p3 _p4 =76.10 "

«>» *' P» -P»o=76.7o *'

jC« " piO_pi =76.00 "

x"»=76.25 mm.— mean value.

log 76.2,
colog tan 18

5 = 1.8822398
"=0.4882240

log/ =2.3704628

/=?34.67 mm. ==234.7 mm.
^ The negatives gave 5c^ = 7 7. 1 9 mm.
The positives gave 5P'» = 76.25 mm.

Diff . = 0.94 mm.

The evident contraction of the greatest abscissa — ^amoimting
to very nearly one millimeter on the prints — ^is due to the shrink-

PHOTOTOPOGRAPHIC SURVEYING METHODS.

age in the 24X18 cm. albumen paper. Whenever " positives ''
(prints) are used in the iconometric map construction, this shrink-
age should be ascertained and taken into account. Of course,
the elements of the " contracted " photographic perspectives are
now substituted for those of the glass negatives.

6. Supplement.
I. FoRics Showing Arrangements of Field Records for Panoraiia Views.

Stetion

on Punta Bivula (trigon. pt.)» on the ridge between the valleys
of the Valsavaranche and Rhymes.

Date,

September 18, 1884.

Orientation of the
Panorama.

Perspectives
belonging

to the
Panorama.

Directions
to the Prin-
cipal Points
of View.

Pocal
Distance.

Remarks.

Punta Gran Paradise,
78® 2f 00"

Punta della Grivola,
X23« 47' 00"

pi
P*
P»

P*
P'
P«
P*
pio

yS"* 27'
114 27

244.5 mm.

150
186

27
27
27

Time of exposure :

with small-

est d i a-

hragm.

258 27

294 27

330 27

6 27

42 27

.<4

10'

lO"

12"

9»

IO»

9«

IO»
ID"
TO"

phraj
No. 7

Fine w e a -
ther

Directions and Vertical Angles of the
Trigonometrical Points.

CompuUtion of Elevation of Station and
Elevation of Lane of Horizon.

Station on partly removed signal.

Elevation of instru-
ment = 2 . 30 m.

Geodetic point, ele-
vation = 3413. 69 m.

Elevation of lines of
horizon of the
panorama = 3415 .99= 3416 m.

(The adjoining page of the record book may be utilized for topographic
sketches from station, for detailed remarks, names of roads, etc.)

114

PHOTOTOPOGRAPHIC liETHODS AND INSTRUMENTS.

Station <

on Punta Percia, on n
ranche and the Rhymes

dge between the valleys of the Valsava-

Date,

September 19, 1884.

Perspectives

Directions

Orientation of the

belonging
to the

to the Prin-

Focal

Remarks.

Panorama.

cipal Points

Distance.

Panorama.

of View.

Time of exposure :

Punta deir Erbetet,

170

° 00'

244.5 n^-

6»

' Shorter e x -

282^ 04' 00"

206

posure than
before on ac-

P»

242

8»

278

9'.

count of the

314

its

io»

great reflec-

P«

350

^§

tion from sur-

9«

rou ndin g
glacier.

p»

Diaphragm
No. 7

98
184

00
00

C/3

'°a| Fine weather

Directions and Vertical Angles to the Sur-

Computation of Elevation of Station and

rounding: Trigonometrical Points.

Elevation of Line of Horizon.

Cima di Breuil 220*^ 54' 00"

Elevation of Invergnan— 3607 .72 m.

Elevation i 33 00

Diff . of elev. + coir. « 406 . 15

Punta deir Erbetet 282 04 10

3201.57

Elevation 3 36 30

Elevation of Nomenon =3488.42

Diff. of elev. -1- corr. = 284 . 94

Cima di Nomenon 222 42 00

Elevation i 38 30

3202.48

Elevation of Toss ■=» 3302 . 24 .

Cima di Rouletta 44 30

Diff. of elev. -1- corr. =» 99 • 84

Elevation 3 11 30

3202.40

Punta deir Invergnan 80 07 00

Elevation of Breuil « 3454 . 62

Elevation 3 42 00

Diff. of elev. + corr. « 25 2 . 64

Cima di Toss 34 ii 30

3201.98

Elevation 30 30

Elevation of Rouletta « ^ ^84 . 10

Diff.

of elev. + cor

« 182.28

Elev. of line of horizon^

3201 . 82
3202 . 30

FHOTOTOPOGKAPHIC SURVEYING METHODS.

II. Form Used for Recording the Elevations of Secondary Points of the

Panorama Views.

Names or

Numbers of

Points.

Stations
whence They
were Derived.

Elevations of
Stations.

Difference of
Elevations-

Elevation of
Point.

Remarks.

D. General Arithmetical Method for Finding the Plotted Positions
of Terrene Points when Pictured on Vertically Exposed
Picture Planes.

With reference to Fig. 43, Plate XXIV, we have

5 and 5' = the two camera stations;
MN and JlfW=two photographic perspectives obtained from
5 and 5' respectively;
a and a' =two pictures of a point A ;

/=5P=5'P' = constant focal length for both pictures or plates;
Z) == 5(^4 = horizontal distance from 5 to -4 ;
2y=5o'-4o= horizontal distance from S' to A;
d=5oao;
d'^SoW;

.B=5o5o'= horizontal distance between the two stations S and
5', the elevation of A above the horizon plane of the
station S=H and above the horizon plane of the
station S'^H\

Finally, the horizontal angles included between B and the
principal planes that pass through the two stations 5 and S' =
ao and ao' respectively.

If we refer the pictured points to the principal point P of
the photographic perspective by means of the rectangular sys-

Il6 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

tern of coordinates formed by the principal and horizon lines
(/f and 00') the coordinates of a on MN will be

and those of a' on M'N' will be

If the camera is in perfect adjustment, if the base line B
has been measured in the field, and if the angles ao and ao' have
been observed, we know the values of

-B, ao, ao', /i ^, ^, y, and /

(the coordinates are measued on the negatives MN and M'N')
and we can now compute:

(i) The horizontal angle y (or f) included between the prin-
cipal ray SP (or S'P') and the horizontal direction Sa'
(or S'ai') to any point A from the equation

X ( txf\

tan 7- = -T- (or tdLnf=-rJ.

(2) The vertical angle ^ (or ^) included between the plane
of horizon for the station 5 (or 5') and the line of direction
Sa (or S'a') to any point A from the equation

tani9=J (or tan^=^),

and as

d^VfT^ (or d'=>^/pT(p^)f
we may write

PHOTOTOPOGRAPHIC SURVEYING METHODS. II7

Of the triangle SoAqSq' we know the side SqSo'^B and the
angles /-, ao, f, and ao'; hence

B sin (f.+ao') sin(f+ao')

D sin[i8o°-(r+ao+r'+ao')] sin (r+ao+r'+aoO'

whence

B.sin(7^+aoO

D^SoAq^-z

sin(^+ao + r'+«oO

A TV e/j g sin (r+ap) \

r ^='^«^^='sin(r+ao+r'+aoOr

We can now compute from

tan ^9=-^,

the difference in elevation between A and 5 (or 5'),
H^D tsinfi (or iI'=Zytan^).

£. General Arithmetical Method for Finding the Plotted Positions
of Terrene Points when Pictured on Inclined Picture Planes.

For inclined picture planes we will have to take the angles
of incUnation of the plates into consideration. Under angle of
inclination of a plate we understand that angle which is included
between the optical axis of the inclined camera and the horizon
plane of the' camera station (second nodal point).

Referring to Fig. 38, Plate XXII, and Fig. 44> Plate XXV,
we have

a=horizontal angle included between the principal plane and
the vertical plane passing through the station 5 and the
point A, pictured as a;

^= angle of elevation of the point A ;

;^ = angle of inclination of the photographic plate MN;

11$ rwowwTOGiupmc methods and instruments.

/ - o.^r,v. knKnt of r = i8o® - r ;
^\> -^on^^n line when MN is vertical {00' is permanently
tuArkcii on the camera);
r-r^*^^H^^ P^"^^ ^^^ ^^^ vertically exposed plate;
t\ _ Y -onlinate of a, Fig. 44, Plate XXV;
^. -A -iibscissa of a, very nearly =a'P', Fig. 44, Plate XXV;
J-vani^ing point ("kernel point") for all vertical lines
pictured on MN,

Fnun inspection of Fig. 44, Plate XXV, we find directly

tan,?-

act'
'Sa'

■mi! xp
'Sa'~Sa'~

Pp-Pn

ycos ^—fsin f
Vx2 + (5;r+OTr')2

y cos ;'+/sin r

y cos /•

-/sinr

V'jc2 + (/C0£

■ r+ysinr)*'

and

tana-

'Stc''

Sx+pn i

cosr+ysin r

•

For the vertically exposed plates we had found

y X

tan/9= . and tan a =-7-.

\/jc2+/2 /

The preceding formulas for the inclined plates will assume
the form of the latter if the angle of inclination y is reduced
to O, as sin z' will then become equal to O and cos y equal to i.

After the values for a and ^ (or a' and ^) have been com-
puted the value for SqAq^D (or 5oMo=Zy) and for AA'^H
(or AA"=^H') may be obtained as follows:

Referring to Fig. 38, Plate XXII, we find

D sin (£'-«')

hence !? = -

B sm[i8o°-(a + e + e'-a')r
5. sin (£'-«')

sin (a + e + fi'-a')'

PHOTOTOPOGRAPHIC SURVEYING METHODS. II9

we obtain

and from -jr^Xacap

TT ^ .^ ^ y cos r— / sin r

wx^ 4- (/ COS 7* +y sin y'f

If an ordinary surveying camera with constant focal length is
used, and it should become desirable to expose a plate in an
inclined plane, the complement d of the angle of inclination y
of the optical axis may be more readily (but only approximately)
determined than y by carefully measuring the distances AD,
Fig. 45, Plate XXV (in the direction of the line of a suspended
pliunb-bob), and DB, AB, being parallel with the photographic
plate.

F. General Analytical Determination of the Elements of a
Photographic Perspective,

When in addition to the photographs other data — obtained
by the necessary instrumental measurements — are given for a
graphical determination of the focal lengths of the pictures,
their horizon lines and their principal lines, then these elements
may also be determined analytically.

A picture MN, containing the images a, b, and c of three
known points A, 5, and C, may be given and the position of
the camera station (whence this picture was obtained) may be
known with reference to the three plotted points A', 5', and
C, Fig. 46, Plate XXVI.

To orient the picture trace (or the ground line) ggf with
reference to the plotted station 5' and plotted points A% B\ and
C the latter are preferably referred to a rectangular system of
coordmates {S'Y and 5'X, Fig. 46, Plate XXVI) having die
plotted station 5' as the origin. To simplify matters one of
the axes of the system may be laid through one of the plotted

1 20 FHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

points. In Fig. 46, Plate XXVI, the axis of abscissae 5'X passes
through the point cf.

The coordinates of the points 4', B\ and C, measured on
the plotting-sheet, may be

XiFi, X2F2, and Xs respectively.

The coordinates of the orthogonal projections (on the pic-
ture trace g^) of the corresponding points pictured on the photo-
graph UN and located upon the radials S'A\ S'W, and S'C^
may be designated by

x\y\, x\\y\\^ and x\\i respectively.

The horizontal distances measured on the photographic
plate between a and 6, between h and c, and between a and c
(the same as those measured on the picture trace between a' and Vy
between V and dy between a' and d) may be designated by

ml, w", and w"i respectively.

From an inspection of Fig. 46, Plate XXVI, it will be evi-
dent that

(i) yi::^i = Fi:Xi;

(2) y\\\x\\^Y 2\X2\

(3) yi:yii=win:wii;

(4) (^iii-^i) : (^11-^1)=^" :wl;

(5) (^iii-^i)2 + (yi)2 = (mm)2.

From these five equations the five imknown quantities of
^ii y\y ^n> yii> and xui — the coordinates of the points to be
located — may be computed.

From the area of the triangle S'a'd^

y\'X\\\ ^m"!

GRAPHICAL ICONOMETRICAL PLOTTING METHODS. 121

we find the focal length

nan •

The angle of orientation /•, included between the principal
ray 5'P' and the base line 5'C', may be derived from the equation

/ yi

cosr=— ^ or =-177.
^m win

The principal point P' may be located upon g^ by laying
off on the picture trace ggf from (/ the length,

P^c^^xiiismf'

The differences in elevation between the station S and the
three points Ay B, and C being known it will be an easy matter
to draw the horizon line upon the photograph.

n. Graphical Iconometrical Plotting Methods.

A. Col. A. Laussedat's Method (French Method).

Col. Laussedat's methods of constructing topographic maps
from (photographic) perspective views of the terrene having
been widely published, they form the groundwork for all sub-
sequent work in this field. They are chiefly of a graphical char-
acter and in harmony with the laws of perspective. Col. Laus-
sedat considers two general cases in reconnoitering expeditions
where phototopographic methods may be applied with advantage:
First. The observer may remain sufficiendy long in one

locality to make a survey on a large scale, say 1:20000

and even larger for special purposes.

122 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

Second. The explorer moves rapidly from place to place,
gathering only the most necessary data on his itinerary
to enable him to plot the topography of the traversed
country as a " running survey" on a small scale — say,
1:50000 or smaller — ^preserving and representing only the
principal topographic features met with on the track
survey.

In the first-mentioned case the explorer will measure one
or more base lines with as great an accuracy as the means and
time at his disposal will admit. He will cover the area to be
mapped with a system of triangles connected with the base
lines, and inasmuch as the triangulation stations will also be
occupied with the surveying camera the scheme should be laid
out with due reference to the subsequent iconometric plotting
of the topographic features.

When applying the ordinary surveying methods the tri-
angulation scheme would probably be laid out with a view toward
covering as large a territory as possible with each triangle, occupy-
ing the smallest possible number of intervisible points.

With the application of photography, however, the conditions
become somewhat changed. Every topographic feature that
is to be plotted iconometrically should be seen from two or
more camera stations, and as each camera station is to be con-
nected with the triangulation system, either directly or indirectly,
the number of triangulation points should be a relatively large
one. Often it will not be desirable that the highest peaks trigo-
nometrically laid down on the map should be occupied with
the camera, especially when fogs prevail in the higher altitudes,
and when other camera stations would answer the requirements
just as well.

Regarding the second case, where the explorer follows a
certain route without making side excursions and never stopping
longer in one place than is absolutely necessary for his observa-
tions, the phototopographic method becomes even more valuable
than in the first case, particularly when traversing open and

GRAPHICAL ICONOMETRICAL PLOTTING METHODS. 1 23

broken country. For this kind of topographic reconnaissance it
may well be said that the photographic method surpasses all
other surveying methods regarding the amount of data which
may be collected in the field in a limited time period.

All topographic operations and instruments serve to measure
distances and vertical and horizontal angles. A photographic
perspective of which the elements are known will give all the
data needed to determine the vertical and horizontal angles of
lines of direction drawn from the point of view to all points
pictured on the photograph.

The points A and B shown on the plate MNy Fig. 47, Plate
XXVI, may represent the pictures of two mountain peaks. The
points marked a and b will be their projections upon the horizon
line HH'. The angle aSb=a will be the horizontal angle of the
lines of direction 5-4 and SB if 5 is the point of view on the
distance line SP.

The vertical angles p and j- may be shown in horizontal plan
by revolving the vertical planes passing through 5-4 and SB
about the lines Sa and 56, respectively, until they coincide with
the horizon plane Hi?', when

a{A)^aA,
(A)aS^AaS^go'',
i8=^5a=(i4)5a=09).

The vertical angles /9 and y may now be measured in horizontal
plan as (fi) and (7-).

To indicate in a general way Laussedat's method of icono-
metric plotting and to show how the plotted features of the terrene
may be obtained from the photographs we will refer to Figs. 48
and 49, Plate XXVII, where A, B, and C represent three camera
stations (plotted in horizontal plan. Fig. 48), whence three per-
spectives I, II, and III, Fig. 49, of the same knoll D may have
been obtained. The traces of these three pictures on the plotting-
sheet may be Hj^H^f H^Hb, and HcHc- All three photo-
graphs having been taken with the same instrument of constant

124 PHOTOTOPOGRAFHIC METHODS AND INSTRUMENTS.

focal length, the distance lines Pa^} PbB^ and PcC will be
equally long.

I. Orientation of the Pictuse Traces on the Plotting-sheet.

The three stations A, 5, and C are plotted either as parts of
the triangulation S)rstem or by measuring the base line AB on
the ground and observing the horizontal angles CAB and CBA,
when the sides AC and BC may be found graphically or by
computation and the triangle ABC be plotted upon the working-
plan.

Horizontal angles or directions to D having also been observed
from Af Bf and C, its position with reference to Ay By and C
may also be plotted.

To orient or plot the three picture traces we must know the
horizontal angles a^, (X-b^ and acy which are generally observed
for each picture by means of the horizontal circle attached to
Laussedat's phototheodolite.

These angles are plotted from Ay By and C on the lines -42?,
BDy and CD with reference to the position of D on the photo-
graphs, whether to right or left of the principal line VV. The
constant focal length =/ of the three negatives I, II, and III is
now laid oflF on the radials AP^y BPb^ and CPc Perpendiculars
erected in Paj Pbj and Pc to the lines A Pa, BPb, and CPc re-
spectively, will represent the picture traces HaHa^ HsHsf
and HcHc* The abscissae P^d^i, Pb^bj and Pcda measured
on the negatives I, II, and III, should be made equal to the
distances Pa^a* Pb^b^ and Pcdc on the picture traces.

The point D is termed a " reference point," and every picture
that is to be used for iconometric plotting should contain the
image of at least one such reference point of known position in
both the horizontal and vertical sense.

2. Locating Points on the Plotting-sheet that have been Identified
ON Several Photographs.

After the picture traces have been oriented any (other) point
T of the terrene shown on two or more pictures may readily

GRAPHICAL ICONOMETRICAL PLOTTING METHODS. 12$

be plotted without requiring additional instrumental measure-
ments in the field.

To locate the plotted position of the point T, Fig. 48, Plate
XXVII, shown as Ia and 'c on two pictures I and III, Fig. 49,
Plate XXVII, the abscissae P^Ia and Pch are laid oflF on the
picture traces HaH/ and HcHc, respectively, from Pa and Po
Fig. 48, and on the side of P corresponding with the position of
the image / with reference to the principal line VVy Fig. 49, Plate
XXVII.

Lines drawn from A and C, Fig. 48, through Ia and tc will
represent horizontal directions to T from the stations A and C;
their intersection at T will locate the position of the tree in
horizontal plan.

3. The Iconometric Determination of Elevations op Pictured
Terrene Points.

The horizon line HH' of a perspective view, Fig. 47, Plate
XXVI, being the intersection of the horizon plane with the ver-
tical picture plane, will intersect points in the picture which in
nature have the same elevation as the optical axis SP of the
camera. All pictured points falling above the horizon line are
higher and all points falling below the horizon line are lower
in elevation than the point of view S,

The distances Sa and 5i4, Fig. 50, Plate XXVII, are measured
on the plotting-sheet and the ordinate (-4 a, Fig. 47, Plate XXVI)
of the pictured point a (its distance from the horizon line) is
taken from the negative. Perpendiculars to 5-4 are then erected
in a and A (on the plotting-sheet. Fig. 50, Plate XXVII), and
the one in a is- made equal to the ordinate (-4a, Fig. 47, Plate
XXVI) of the pictured point =a(a). Fig. 50, Plate XXVII.

If we now draw the line 5(a) to its intersection with the per-
pendicular to 5-4 in -4, the triangle Sa{a) and 5(^)^4 will be
similar and the angle -45(-4) will represent the vertical angle
of the visual ray from 5 to -4, revolved about SA into the plane

126 PHOTOTOPOGRAPmC METHODS AND INSTRUMENTS.

of the horizon. From the similar triangles Sa{a) and SA(A)
we derive the proportional equation

A(A):SA--a(a):Sa,
whence

a(d)'SA

A(Ay^

a(a) is measured on the negative; 5-4 and 5a are taken from
the plotting-sheet. -4(^4) measured on the plotting-scale will
give the difference in elevation between 5 and A.

In practical work the elevations of the camera stations are
known and by adding the height of the instrument including the
value for A (A) to the elevation of the camera station the absolute
elevation of the geodetic point A is found, which, however, is still
to be corrected for curvature and refraction.

A second value for the elevation of the geodetic point A is
foimd in the same manner from another negative containing
an image of A and obtained from another station. The mean
of several such determinations is adopted for the final value for
the height of A.

4. Drawing the Plan Including Horizontal Contours.

After some little practice, points pictured on different nega-
tives but representing identical geodetic points will readily be
identified by the observer and he will select characteristic points
to reproduce the watercourses, water-sheds, roads, shore lines,
etc., on the plotting-sheet.

After these principal guide lines are well located on the chart
the buildings, outlines of woods, marshes, etc., are plotted, includ-
ing everything that is to be shown on the finished map.

Enough points must be plotted iconometrically to form a
good control for a correct delineation of the relief. Should the
number of points determined on the plan be sufficient only to

GRAPHICAL ICONOMETRICAL PLOTTING METHODS. 1 27

give an adequate control for the delineation in the horizontal
sense, additional points should be plotted from the photographs
to obtain an equally good control of the terrene in the vertical
sense.

The planimetric work completed, elevations of as many of
the plotted points as seem necessary are determined and inscribed
on the chart Horizontal equidistant contours may then be
drawn by interpolation to harmonize with the elevations suffixed
to the points of control on the chart, conforming the courses of
the contours between the determined points to the configuration
of the terrene as it is shown on the panorama views.

It cannot be denied that a certain amount of study and
practical application are requisite to enable the iconometric
draughtsman to interpret forms correctly when shown in per-
spective. Yet it should also be admitted that such transla-
tion or conversion of the configuration of the terrene into hori-
zontal projection may be accomplished far more accurately
at one's leisure in the office by means of geometrically correct
perspectives than, could be accomplished by sketching in the
field.

When topographic features as seen from one direction are
sketched by the plane-tabler, their forms will often be found to
have been misconceived when they are again seen from another
point of view. Of course, forms sketched on the plane-table
sheet may then be corrected, in a measure at least, but many
details are sketched that will not be seen again from other sta-
tions, and even those that are again observed from other stations
may not be modified to conform with their true shapes unless
the original station whence they were first seen and sketched
could again be occupied to verify the suggested changes, and,
generally speaking, topographers regard a second occupation
of a station with little favor, considering it too great a loss of
time, retarding progress and considerably increasing the cost
of the work.

In iconometric plotting, however, it would be an easy matter

128 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

to refer back to the panorama views obtained from some other
station, and the plotting of topographic details should not be
attempted without having first made a careful study of (and a
close comparison between) the various pictures representing
identical areas but seen from different points of view.

B. Dr. A. Meydenbaur's Method (German Method).

The pantoscopic lens (made by E. Bush in Rathenow, Prussia)
of Dr. Meydenbaur's surveying camera commands an angle of
about ICO®. By excluding the external rays of the effective
field of these lenses by means of diaphragms (within the camera)
pictures are obtained subtending a horizontal angle of but 60®
(irrespective of the 5 nun. wide margins with which two adjoining
plates lap over each other) requiring six plates for a complete
panorama.

After the camera has been adjusted over a station the pano-
rama is photographed by exposing six plates in succession, each
successive turn of the camera in azimuth covering an angle of 60°,
two adjoining plates lapping over each other by a margin of 3°
in arc. Fig. 51, Plate XXVIII. These common margins, con-
taining identical sections of the panorama view, may well serve
to find the value for the focal length of the negatives.

From the panorama set of six plates exposed from one sta-
tion objects or geodetic points may be selected on the middle
lines of the common margins of adjoining plates that must be
equidistant from the principal lines of adjoining plates.

I. Determination of the Focal-length Value for the Photographic

Perspective.

After having selected a series of such reciprocal points, using
a magnifier of low power if needed, on all six plates, we shall
have twelve determinations (represented by the length /) of
the positions of the principal line and the greatest discrepancy
between any two values should not exceed 0.2 mm. if the instru-
ment is well adjusted. The sum 2/ of two such distances

GRAPHICAL ICONOMETRICAL PLOTTING METHODS. 1 29

represents the effective length of one picture, or the length
of one side of a regular hexagon, with an inscribed circle of
the radius equal to the constant focal length =/ of the negatives.
The value for the focal length may be found graphically or it
may be computed from the formula

tan 30°

When " positive prints " are to be used in the iconometric
map construction this focal length often will have to be changed
to correspond with changes that may have taken place in the
dimensions of the prints compared with their negatives. The
total linear changes in a print, measured in the direction of the
principal and horizon lines, may readily be found by comparing
the distances between the " teeth " (metal plates permanently
marking the principal and horizon lines in the image plane of
the camera) on the negative with those included between their
contact prints on the positive.

With reference to Fig. 52, Plate XXVIII, we have:

aft = original length of horizon (or principal) line between the
teeth of the camera or between their imprints on the
negative;

a^l/ = \ht corresponding length measured on the positive;

CO =/= constant focal length of the camera or negative.

The focal length (fO of the contracted (or expanded) posi-
tive may be foimd graphically by drawing the triangle oiO,
placing the line a'V (measured on the positive) parallel with ab'
and moving it (maintaining its direction parallel to ab) towards
(or from) O until a' falls upon aO and V upon bO.

dO will be the focal length to be applied when considering
the horizontal angles deduced from the positive. Had ah beea
measured in the direction of the principal line, <fO would be
the focal length for the positive to be considered when deducing
vertical angles from the point. The focal lengths cfO should be

130 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

ascertained for every print that is to be used in the iconometric
map construction.

The topographic map proper is constructed iconometrically
from the negatives and positives in a manner very similar to
that described under Col. Laussedat's method.

Referring to Fig. 53, Plate XXIX, we have:

I and 11= negatives of plates exposed at stations / and // re-
spectively. I shows the image of a signal at station
//, and negative II shows the image of a signal at
station /;
/ //=base line measured between the two camera stations /
and //.

Both negatives show the image / of the same tower T.

2. Orientation of the Picture Traces on the Plotting-sheet.

After the base line / //, Fig. 53, Plat« XXIX, has been plotted
in reduced scale we describe circles about / and // with the radius
equal to the constant focal length of the negatives,

cO^U

and produce the line / // beyond both station points, make

Illo^O^h (Plate I)
and II 1 0^0 h (Plate II),

describe arcs from II q as center with IIqC—x\i (Plate I) and
from Iq as center with Iqc^x^^ (Plate II) as radius.

cllo will be the trace of picture I and cIq will be the picture
trace of II oriented at station // with reference to the base line
III (Plate XXX).

Fig. 54, Plate XXX, illustrates a more simplified way of orient-
ing the picture traces.

After the base line / // has been plotted the horizontal angles
«! and all (azimuthal deflections of the optical axis from the

GRAPHICAL ICONOMETRICAL PLOTTING METHODS. 13I

base line / // for the negatives I and II) are plotted at / and //
with reference to their positions in regard to the principal planes
at the stations / and // as shown in the negatives I and II (whether
the station's image falls to the right or to the left of the principal
line).

The constant focal length =/ of the negatives is laid off on
the principal Une=f/0 for negative I and = c"0 for negative II.
The images of the stations are projected upon the horizon lines,
^11 upon HiHi (Plate I) and Si upon HuHu (Plate II), when

4f OS II =a^ = horizontal angle included between the principal

plane and base line / //, and
r"05i = a" = corresponding horizontal angle for station //.

These angles, a' and a", are transferred from the negatives I
and II to their corresponding ends of the base line / //, as indi-
cated in Fig. 54, Plate XXX. Now lay off the focal length /
from the base stations / and // upon the sides of the angles of
and al'^Icf zxA^IIcf' respectively, and erect perpendiculars
WW and W'W' to Id and lid in d and d' respectively. They
will represent the traces in horizontal plan of the vertical picture
planes /. and // in correct position and orientation with reference
to the base line ///. The remaining two sets of five plates
each of the panoramas at the stations / and // are easily oriented
and plotted; the next plate in order at station //, for instance,
would have the principal ray (optical axis) in the direction
(a" +60°), the third (a" +120^), etc., Fig. 55, Plate XXXI.
Every plotted camera station will be surrounded by a regular
hexagon the sides of which represent the picture traces of the
six negatives forming the panorama set for the station.

3. Locating Points, Identified on Several Photogsafhs, on the
Plotting-sheet.

The horizontal locations of all points identified on two or
more plates are plotted by locating the intersections T of the
lines of horizontal directions It, lit'. Hit" .... Fig. 54,

132 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

Plate XXX, in the same manner as has been described for CoL
Laussedat's method.

4. The Iconometric Determination op Elevations op Pictured
Terrene Points.

The elevations of points iconometrically plotted are foimd
in the Same way as described for Col. Laussedat's method. If

the scale of the map is ^ we will have, Fig. 54, Plate XXX, the

elevation of station / above II=^H=^I{Si).

The values of y.=5i(5i), /// and Si 11=1'' are found by
direct measurement with a small ivory beveled scale divided
into 0.5 mm., of which o.i mm. may well be estimated after some
little practice. .

C. CapL E. DeviUe's Method {Canadian Method),

This so-called Canadian method has been in use under the
auspices of the Canadian Department of the Interior since i888-
Capt. Deville, Surveyor-General of Dominion Lands, has given
a detailed account of his methods in "Photographic Surveying,'^
published at the Government Printing Bureau in Ottawa in 1895,
and the following paragraphs have been largely taken from
Deville's book.

I. General Reicarks on the Field-work.

The area to be surveyed is covered with a triangulation net,
preferably before the phototopographic work is begim, and a
secondary or tertiary triangulation, if needed, is carried along
with the phototopographic work to locate the camera stations,
in both the horizontal and vertical sense, with reference to the
primary triangulation stations already established.

GRAPHICAL ICONOMETRICAL PLOTTING METHODS. 13J

The surveyor makes a plot of the entire triangulation covering
his territory in the field, and he locates on the same all the stations
that he may occupy to enable him to recognize the weak points
in his scheme and to plan his operations with a thorough under-
standing and to secure a good assurance of success. The instru-
mental work in the field is done merely to locate the camera
stations and .certain reference points (used for the subsequent
orientation of the picture traces), all topographic details being
deduced from the pictures.

The camera stations are located either by angles taken from
the station to surrounding triangulation points (resecting, three-
pomt problem), or by angles observed from the latter to the
camera station (intersecting, concluding), or by both methods
combined.

The strength or value of the work depends very much upon a
judicious selection of the points that are to be used as camera
stations, in order to bring the enitre terrene under proper control
and to be enabled to construct the map by the method of inter-
sections of lines of direction. Other methods for plotting topo-
graphic features and details being employed only when the method
of intersections fails on account of insufficiency of data to give the
requisite number of horizontal directions (the camera stations
not being well situated) for a good location of points by intersec-
tions.

Each camera station should be marked with a signal before
leaving it, not be shown on the pictures, but to be observed upon
with the transit or altazimuth from the triangulation stations,
in order to locate the position of the camera station upon the plot-
ting-sheet.

Frequently it will be of advantage to set the camera up eccen-
trically over a triangulation station, in order to include certain
parts of the landscape in the views. The position of the eccentric
camera station with reference to the triangulation point can
readily be ascertained (by azimuth and distance) and should always
be recorded.

134 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

Complete panorama sets are not taken at every station ; it is
preferred, rather, to increase the number of stations, often occupy-
ing a special station to obtain a single view only, if by doing so
better intersections for the location on the plan of some special
feature may be obtained.

Multiplicity of stations demands but a small increase in labor,
either in the field in the extra observations of horizontal direc-
tions for their location or for plotting them in the office, and
enough stations should always be occupied to give a full control
of the relief of the area to be surveyed.

A certain section of the terrene may be so located that it will
be a difficult matter to select more than one station, whence it
may be seen. In such a case the method of "vertical intersec-
tions" may often become useful; .two or more views of such area
may be taken from stations at different devations: the greater the
difference in altitude between such stations the longer will the
base line be, and the better will be the intersections which
locate the features in question (if the latter are not too distant).

As enough plates should be exposed to completely cover the
ground, the camera stations will have to be distributed in such
a way that. all valleys, sinks, and depressions that may be repre-
sented in the scale of the map are well controlled (i.e., are seen
from different camera stations). It is evident, therefore, that
the number of stations to be occupied for the topographic develop-
ment of a certain area will depend upon the character of the
terrene and upon the scale of the chart.

Two or three well-defined points (" reference points ") in
each panorama section (covered by one plate) are observed
with the transit or altazimuth, noting the vertical and horizontal
angles upon the outline sketch that is made for every exposed
plate.

These sketches serve far better to identify points with cer-
tainty than a mere designation (by name or symbol) or descrip-
tion of the points observed upon. The general triangulation
notes are kept in the usual manner.

GRAPHICAL ICONOMETRICAL PLOTTING METHODS. 1 35

Vertical angles are observed to check the position of the
horizon line on every photograph and to correct errors due to
small changes in the level adjustments of the camera that may
arise during the transportation of the instruments over a rough
trail.

The horizontal angles are needed, both for the location of
the camera stations and for the orientation of the pictures (pic-
ture traces) on the plotting-sheet for the subsequent map con-
struction.

2. General Remarks on the Iconometric Plotting of the Survey.

The field notes of the phototopographic surveys made in
the Northwest Territory of the Dominion of Canada by the
Topographical Surveys Branch of the Department of the Interior —
under Capt. E. Deville, Surveyor- General of Dominion Lands —
are plotted on the scale of i : 20000, but the maps are published
on the 1:40000 scale with equidistant contours of 100 feet.

The phototopographic reconnaissance in southeast Alaska exe-
cuted by Dominion Land Surveyors — under W. F. King, Alaskan
Boundary Conmiissioner to Her Majesty — was plotted on the
scale of 1:80000, with a contour interval of 250 feet, and it
was published on the 1:160000 scale.

After the triangulation has been computed and the points
have been plotted, and after the computed elevations have all
been affixed to the marked points on the plotting-sheet, the tri-
angle sides of the secondary triangulation scheme executed
during the phototopographic survey are computed, the cor-
rections to the horizontal angles, indicated by the closing errors,
having been applied. The latitudes and departures from every
secondary point to the nearest primary station are then com-
puted and the secondary stations are plotted by their latitudes
and departures (unless the primary sides are too long).

All camera stations not already included in the secondary
triangulation scheme are now plotted with reference to the tri-
angulation points, using either a table of chords or a station-

136 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

pointer (vernier protractor). If many points had been observed
upon from the camera station, the horizontal angles are pref-
erably laid oflF on a piece of tracing cloth or paper and this
improvised multiarm protractor is used, like a station-pointer,
to locate the plotted position of the camera station.

The surveyor should endeavor to obtain at least one direction
from a triangulation station to every camera station; the plotting
will then be less troublesome and far more accurate. Photo-
graphs should not be used for plotting the positions of camera
stations; enough angles should always be observed in the field
to locate (trigonometrically) every occupied station in the manner
just indicated.

From the original negatives copies are made, enlarged to
9 J by 13 inches, on heavy bromide paper (more recently so-
called " Platino Bromide " paper has been used by Capt. Deville).
The enlargement adopted in Canada for these bromide prints
b about 2.1 times, which ratio was selected to utilize the full
width of the paper foimd in market. These enlargements,
being extensively used in the map construction, should be made
with great care to reduce distortion to a minimum.

After the prints have been developed (with iron oxalate),
well washed in acidulated water and fixed, they are again thor-
oughly washed and dried in a flat position, under special pre-
cautionary measures to control the contraction or expansion, in
such a way that the final size of the dry-prints have imiform
dimensions. Slight distortions that would arise from a play of
the negative carrier in the enlarging camera or from the bromide
paper not lying perfectly flat on the surface of the copying-screen
are best reduced by using a copying-lens of long focus.

Before using the prints for the map construction any distor-
tion due to the enlarging process should be ascertained, which
is best done in the following manner:

Fig. 56, Plate XXXI.— Jom the middle notches H and if',
P and P', and with a set-square* test these two lines for perpen-
dicularity. Take with a pair of dividers the distance of the

GRAPHICAL ICONOliETRICAL PLOTTING METHODS. 1 37

two notches A and -B, which should be one half of the enlarged
focal length and equal to the distance between the two notches C
and D. Apply one of the points of the dividers in P and the
other should come in E and F. Transfer the point to P and
check FG and FJ. If the print stands all these tests, it may
be used iconometrically; if it does not, it is returned to the pho-
tographer with the request for a better one.

3. Orienting the Picture Traces on the Woriong-plan.

Every photograph contains at least one, generally several,
of the triangidation points plotted on the working-sheet and
the traces of the picture and principal planes are oriented and
plotted on the plan as follows:

The distance or principal line PSy Fig. 57, Plate XXXI,
is made equal to the focal length and the pictured point a of
the reference point A is projected upon the principal line (=«')
and upon the horizon line (=a).

If 5i, Fig. 57, Plate XXXI, is the plotted position of the
camera station on the plan, and if 5i-4i represents the horizontal
direction to A from the station 5, make Siai equal to Sa (taken
from the photograph on the " photograph-board ") and from ai
as center, with aa'^Pa as radius, describe an arc to which
5i^ is drawn tangent. SiP will be the trace of the principal
plane (or the distance line) and the perpendicular to SiP through
ai=^pai will be the picture trace. Instead of making this con-
struction on the " photograph-board " (which will be described
under section i) it may be made on the plan.

On Si A I take SiB, Fig. 58, Plate XXXI, equal to the focal
length, erect BC perpendicular to SiAi and make it equal to
the abscissa (=aa'. Fig. 57) of the reference point. Join
SiC and take SiP equal to the focal length; at p erect a per-
pendicular to SiC and it will be the trace of the picture plane,
SiC being the trace of the principal plane.

Another simple method for orienting the picture trace of a
photograph having the image of a reference point C is as follows

138 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

(Fig. 59, Plate XXXII) : Take a triangle of hard rubber or wood
and mark off along one side the focal distance SP, Fig. 57, Plate
XXXI, equal to ab, Fig. 59, Plate XXXII, from the right-angle
comer a. Carefully notch the triangle side at b so that the cen-
ter of a fine needle marking the plotted station point will fit
into the notch. From the photograph take the abscissa (of a = -f a,
Fig. 57, Plate XXXI) of the pictured reference point (a) between
the points of a pair of dividers, move the triangle about the needle,
marking the plotted station b with the left hand until ac, Fig. 59,
Plate XXXI, is equal to the distance a'a (abscissa of reference
point), held between the points of the dividers. The triangle is
securely held in this position and lines are drawn along the tri-
angle sides ab and ac. Produce ac beyond a and check the dis-
tance ac again to be equal to a'a. The line be represents the
horizontal direction from the plotted station b to the plotted
reference point C (its image on the negative, Fig. 57, Plate XXXI,
was a) and we will now have:

6a = trace of the principal plane;

ac = trace of the picture plane;
a = projection of the principal point on the plotting-sheet.

The trace of the principal plane = aft is preferably marked
by a short line only, bearing an. arrow pointing toward the plotted
station whence the picture was taken, and the principal point a
is marked to correspond with the designation of the print, and
it may be remarked here that as few constructive lines as pos-
sible are drawn on the working-sheet to avoid confusion and
mistakes (see photograph-board, section 10, page 148).

4. The Identification of Pictured Points in Photographs
Representing Identical Points of the Terrene.

The survey being plotted, analogous to a plane-table survey,
mainly by intersections of horizontal lines of direction, points
controlling the same area must be identified on pictures taken
from different stations. When selecting such points on a photo-
graph, preference should be given to such that best define the .

GRAPHICAL ICONOMETRICAL PLOTTING METHODS. 1 39

surface or terrene like characteristic points of mountain ridges>
peaks, saddles, points at the changes of slope, bends in streams, etc.,
each point being marked by a dot in red ink on the photograph
and aflSxing a number or sjrmbol to it. It will now be necessary
to identify as many of these points as possible on other photo-
graphs covering the same area, marking these also by dots and
giving identical points the same designation, by number or by
symbol in red ink, as on the first photograph.

The identification of geodetic points on several pictures oflFers
no serious diflSculty, and with some practice as many points
as may be needed for a full development of the terrene, even
under different illumination of the pictured areas, may be picked
out with rapidity and precision. In case of doubt, beginners
may resort to Prof. Hauck's method, which has already been
mentioned several times in the preceding pages.

5. Appucation of Prop. Hauck's Method for the Identification of
Terrene Points Pictured on Several Photographs.

The two photographs, picturing the same areas as seen from
different stations or points of view, are pinned side by side on a
drawing-board. The images of the camera stations whence the
pictures were obtained are kernel points; if they fall outside
of the limits of the pictures their projections on the picture
traces may be determined from the plotting-sheet or working-
plan. The parallels to the principal lines, on which the scales
are to be placed, are drawn as explained in Chapter IV, para-
graph VII, section 2, and the scales are fixed in position.

A fine needle is inserted into the drawing-board through each
of the kernel points and the loop at one end of a fine silk thread
is dropped over the needle, the other end of the thread being
fastened to a small weight by means of a slender rubber band
(see Fig. 60, Plate XXXII).

A well-defined point is now identified on the two photo-
graphs sufficiently far from the kernel points, and one thread
is moved (by taking the small weight in one hand) to pass

I40 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

through the point identified on the photograph; the weight
is deposited on the drawing-board, holding the thread in this
position under slight tension of the rubber band b.

The same operation is repeated with the other thread and
the other photograph, when the two threads should intersect
the scales at the same division mark; if they do not, one of the
scales is to be moved until identical division marks are bisected
by the two threads. The identification of other geodetic points
pictured on both photographs may now be proceeded with:

Having selected a characteristic geodetic point on one of
the photographs, the corresponding thread and weight are moved
until the thread bisects that point. Noting the point of intersec-
tion on the scale by the thread in this position, the other thread
is now moved to bisect the corresponding graduation mark on
the second scale. The second thread will then also bisect the
corresponding image of the same geodetic point on the second
picture.

6. Plotting Pictured Terrene Points as the Intersections of Lines
OF Horizontal Directions. Iconometric Plotting of Terrene
Points in General ("Horizontal Intersections").

After enough pictures have been selected to develop a cer-
tain area, and the identification and marking of the images of
corresponding geodetic points have been completed, -projec-
tions of all these points on the horizon lines of the pictures are
marked and transferred to the straight edge of a strip of paper,
including in this transfer the marking on the strip's edge of the
principal point of every photograph.

The strips are given the same designation as the pictures
to which they belong (by number or symbol) and they are
then placed upon their picture traces on the plotting-sheet in
such a manner that the principal points of picture trace and
paper strip coincide. They are secured in this position on the
working-sheet by means of small weights or fine thumb-tacks.

Tq plot the horizontal projection of a geodetic point, shown
: J d marked on two photographs and marked on the correspond-

GRAPHICAL ICONOMETRICAL PLOTTING METHODS. I4I

ing paper strips, two fine needles are inserted into the plotted
stations / and //, Fig. 60, Plate XXXII, of the two photographs.
A fine silk thread is attached to each needle. The other end
of the thread is connected with a small weight W by means of
a fine rubber band 6.

The thread looped over station needle / is now moved over
the paper strip (indicating the picture trace on the plan) until
it bisects the projection a' of the geodetic point's image. The
weight is now placed upon the paper, holding the thread under
slight tension of the rubber band in this position.

The second thread, connected with the needle in station //,
is placed over the horizontal projection a" of the image of the
same point A. The point of intersection A of the two threads
will be the desired position on the plan of the point A.

After this position of A upon the plotting-sheet has been
checked by means of another photograph taken from a third
station ///, and containing the image a'" of the point A^ its
plotted position is marked by a dot in red ink and its designa-
tion, corresponding with that given on the prints, is also afiixed
in red ink. A sufficient number of points having been plotted
in this manner, and all having been supplied with the same let-
ters or numerals (in red ink) that had been given their images
on the photographs, their elevations are determined and also
affixed to the points in red ink. Frequently the designation
of the point by letter or numeral is added ih pencil on the
working-sheet, to be erased after the elevation of the point has
been affixed to it in red ink.

In case the strips of paper on the picture traces should over-
lap, as shown in Fig. 61, Plate XXXII, the part CD of the pic-
ture trace PQ is marked oflF on the strip MN lying under it,
the band of paper PQ is then placed in proper position and the
marks on its edge are transferred to the line CD. The strip PQ
is now placed under MN^ the marks on the latter, along CD,
serving the same purpose as those of PQ.

When a station, A, Fig. 62, Plate XXXII, falls so close to

142 PHOTOTOPOGRAFHIC METHODS AND INSTRUMENTS.

the edge of the working-sheet that the trace QR of the picture
plane falls outside of the limits of the plan, then the trace AC
of the principal plane may be produced to B, AB being equal
to i4C= focal length of the picture, and MN, drawn perpen-
dicular to BC or parallel to QR, will occupy with reference to QR
the same position as the focal plane of the camera does to the
image plane of the perspective.

The direction of a point of the photograph projected in Q
on the picture trace is found by joining NA and producing
to the opposite side of A.

As already mentioned, the intersection of two lines of direc-
tion, establishing the plotted position of a geodetic point, pic-
tured on two photographs, should be checked either by a third
line or otherwise, before the position of such point should be
accepted on the plan as correct.

Such intersections may, for instance, be checked by deter-
mining the elevation of the point from both photographs. Unless
the point has been correctly plotted, these two heights will not
agree. This check, however, does not guard ' against slight
errors in position.

A check may also be obtained by drawing a line, on which
the point may be situated (for instance, the shore line of a river
or lake), with a perspectograph or perspectometer; still, the best
check will always be afforded by a third intersecting line of direc-
tion obtained from a third photograph.

7. ICONOMETRIC PLOTTING OP PICTURED TeRSENE PoINTS BY SO-CALLED

"Vertical Intersechons."

We have seen how the base line between two stations is pro-
jected into horizontal plan when using the method of intersec-
tions of horizontal lines of direction, hitherto considered, but
when two camera stations are occupied at different elevations,
and not far apart horizontally, to locate geodetic points by inter-
sections of lines of direction, the so-called " method of vertical
intersections " may be employed with advantage.

GRAPHICAL ICONOlfETiaCAL PLOTTING METHODS. 143

With this method the base line — ^its horizontal projection
being either too short or more frequently falling into the same
direction with the distant points to be located by the intersec-
tions of lines of direction — is projected upon a vertical plane.

The greater the diflFerence in elevation between the two sta-
tions the greater will the length of this vertically projected base
line be and the more accurate will be the iconometric location
of the points by lines of direction.

We have, with reference to Fig. 63, Plate XXXIII, two camera
stations A and B, two photographs A^ and Bn obtained from
them and containing the image dji and ds of the identical geodetic
point Z7. It is assumed that the horizon plane through the
lower station £ be the grqund or plotting plane, and that the
principal plane of the photograph A ^ he Hie vertical plane of
projection which is revolved about its trace into the horizon
{dane of B.

a— horizontal projection of station A ;
a5«« horizontal projection of the base line AB;
H.4P-ff'^AP« picture trace of photograph As in horizon plane
of B (plotting-plane);
Hb Hb^— picture trace of photograph Bn in horizon plane of B;
aPyi'=5PB'«= constant focal length of the negatives Aj^

and Bs]
aP>i'-= trace of principal plane passing through aP/ in
horizon plane of B.

To plot the position d' of a point D (pictured in i4J^r as rf^
and in 5jy as rf^) in the plotting-plane the rays Ad a and Bds
are projected upon the vertical plane (revolved about aP/ into
tbe ground plane), when (di) will represent their point of inter-
section d projected into that same vertical plane (revolved about
aP/ into the plotting-plane).

The ray Adji^AD intersects or penetrates the picture plane
iljv at a distance— (f/id'^B vertically above the ground plane
(above the picture trace or ground line HabH'ab of picture A^).

144 FHOTOTOPOGBAFHIC METHODS AND INSTRUMENTS.

This ordinate is laid off upon P>i'-ff>ifl=PA'(d>i), when dji
will be the projection on the vertical plane of pictured point dj^.

The vertical through a projected upon the vertical plane is
represented as a(i4), and if we make

a{A)^Pj^P'AB (picture i4iv)

=diflFerence in elevation between the two stations B and A,

(A) will be the upper camera station A projected into the ver-
tical plane, and the line (i4)(d^) will be the projection of the
ray -4d^, or AD, upon the vertical plane (revolved about aPj^'
into the plotting-plane).

The ray Bds—BD intersects the second picture plane Bff
in ds- If we draw through ds (projection of rf^ on ground line
HbHb') a perpendicular to aP j^'^ds'diB, dis will be the pro-
jection in the vertical plane of the horizontal projection in the
picture trace of the pictured point ds- Producing dB%B beyond
diB and making diB{dB)^dBdB' (measured on the negative Bs)
will locate at (ds) the projection of the pictured point da upon
the vertical plane.

The perpendicular to aPy^ ' through B locates the projection
into the vertical plane =61 of the plotted station B, hence the
line 61(^5) will be the projection into the vertical plane of the
ray Bds'^BD.

The intersection (di) of 61(^5) with A{dA) locates the pro-
jection into vertical plane of the point d, and the horizontal pro-
jection of the point D (plotted on the groimd plan) will be on
the line (di)d', which is the vertical through d (perpendicular
to aP^ in our case) passing through (di) and produced beyond
d\y and either horizontal line of direction ad^ or BdBt pnxiuced
to intersect this perpendicular (di)di\ will locate the position d'
(of the point D) on the plotting-sheet with reference to the plotted
stations A (or a) and B.

(The location of d' as the intersection of the horizontal direc-
tions adji^ and Bds^ would not be very accurate in our case,
and far less so for points pictured on the other side of the prin-

GRAPHICAL ICONOMETSICAL PLOTTING METHODS. I45

cipal point P^, the angle of intersection of their horizontal direc-
tions being even smaller than at df.)

The point di being the projection into the vertical plane
of the point d' (the horizontal projection into the ground plane
of the point d), the length {d\)d\ (measured on the plotting-scale)
will represent the elevation of the point D above station B (or
above the ground plane).

8. IcoNoifETSic Determination op the Elevations op Pictuiied
Terkene Points.

Generally speaking, one perspective is insuflScient to deter-
mine the elevation of a point, although there are exceptions,
like the points on the horizon line of a photograph which have
the same elevation as the camera station. A single photograph
would also suffice if the distance from the camera station to
the point to be determined vertically be known; for instance,
Fig. 64, Plate XXXIV, the horizontal projection d of the point D
being known, its height H above the ground plane will be the
fourth proportional to the three known lines Bdu BdiB and disids) :

Bdi» horizontal distance between the plotted station B
and the plotted point, measured in the plotting-scale
of the working-sheet;
BrfiB= horizontal distance between station B and projection of
pictured point ds in the ground line H^Hb^ meas-
ured on the plan;
<^ii(rfB)=" A =• ordinate of pictured point dp, measured on the pic-
ture plane {-^ds'dB, Fig. 63, Plate XXXIII, pic-
ture Bii)j

and the value for H may be computed from the equation

JodiB

If we now project the plotted point d\ and the pictured point ds
into the principal plane and revolve the latter about the prin-

146 PHOTOTOPOGRAPmC METHODS AND INSTRUMENTS.

cipal line BP into the plotting-plane, we will haVe with refer-
ence to Fig. 64, Plate XXXIV,

Pids') "^height of pictured point ds above the horizon plane«ft;
(<^b')== pictured point ds, projected into the principal plane
and revolved with the latter into the horizon or plot-
ting plane;

(d')di'= vertical distance of the point d above the horizon
plane =1?.

This height, fl", is the fourth proportional to the three known
lengths Bdi, Bdis and h;

BP=focal length of the print =/;
-P(^bO== ordinate of the pictured point above the horizon line (to
be measured on the photograph), and
Bd\'^j-\-Pd\j where Prfi'= vertical distance between the plot-
ted point d\ and the picture trace HsHs^did (to
be measured on the plotting-sheet),

its value may be found with the aid of an ordinary sector, Fig. 65,
Plate XXXIV, in the following manner:

Take with a pair of dividers the (ordinate) distance from
the pictured point ds to the horizon line (on the photograph)
place one point of the dividers on the division c of the sector,
when CO = focal length of the photograph, and open the arms
of the sector until the second point of the dividers coincides with
the corresponding division D of the other sector arm (OD being
equal to 0C = focal length). Now take with the dividers the
horizontal distance {di'P=did, Fig. 64, Plate XXXIV) of the
plotted point di from the picture trace HbHb', place one of
the points in C and note where the second point of the dividers
intercepts the scale OCj say at A. Turn the dividers about this
point A (maintaining the opening of the sector unchanged) and
place the second point of the dividers upon B on the scale OD —
B corresponding to -4, or OB=OA — ^when AB^ measured on
the plotting-scale, will represent the height, -ff, of the point d
above the horizon plane of the station B.

GRAPHICAL ICONOMETRICAL PLOTTING METHODS. I47

g. looNoiiETSic Detekionation of the £lev*ations op Pictured Ter-
SENE Points by Means op the So-called ''Scale op Heights."

Another method for determining the elevations of plotted
points iconometrically consists in the use of the so-called " scale
of heights," Fig. 66, Plate XXXV.

Make SP equal to the focal length of the photographic per-
spective, erect PA perpendicular to SP in P, and divide both
lines into equal parts. Join the points of division on PA to S
and through those of SP draw lines parallel to PA.

To use this scale of heights with a pair of dividers, take
from the photographic perspective the (ordinate) distance from
the pictured point to the horizon line and transfer it to the line
PA « Pfji. The point fi may be found to correspond to the line Sfiy
passing through the division mark 9 of the graduation on PA.
With a pair of dividers take the vertical distance from the hori-
zontal projection of the point to the plotted-picture trace (measured
on the working-sheet) and transfer it to SP to the right or to
the left of P according to the position of the plotted pomt with
reference to the picture trace, whether beyond Ae picture trace or
between the same and the plotted station.

In Fig. 66, Plate XXXV, it is shown as falling between the
station and the picture trace into m. The line mBy parallel
with PAy is intersected by Sfi in Af , and the distance mM, meas-
ured on the plotting-scale, will be the height of the. point M
above (or below) the station horizon.

A scale. Fig. 67, Plate XXXV, is conveniently pinned, some-
where on the plotting-board, perpendicularly to a line AB; the
division C of this scale, bisected by the line AB, corresponds
to the height of the camera horizon. Placing one of the legs
of the dividers with which the length AB was taken off the
" sector," Fig. 65, Plate XXXTV, or with which the length mM
was taken off the " scale of heights," Fig. 66, Plate XXXV,
in C, Fig. 67, Plate XXXV, the division D of the scale, coincid-
ing with the other point of the dividers, will indicate the height

148 PHOTOTOFOGRAPHIC METHODS AND INSTRUMENTS.

of the point above the plane of reference or datum plane This
height is entered in pencil on the plan, inclosed in a small circle
to distinguish it from the number of the point. It is checked
by means of a second photograph, and when the discrepancy
between several values for the elevation of the point falls within
the limits of permissible error, their mean is entered in red ink
on the plan and all pencil figures are erased.

Any marked diflFerence in the values for the height obtained
from two photographs would indicate that the two points of
which the elevations were determined are not identical points
or that an error had been made in plotting the same or in deter-
mining its height.

A third intersection would dispose of the first two alternatives
and a new measurement of the height will show whether an
error has been made, or whether the discrepancy is due to una-
voidable errors.

10. The Use of the So-called " Photograph-boakd."

The various constructions described in the preceding pages
if made directly on the photographs would obscure many details
and produce confusion through the intricacy of the auxiliary
lines. Capt. Deville, therefore, had a special drawing-board
prepared on which as many of the construction lines are drawn,
once for all, as would have to be repeated for the different prints
of imiform size (which were, of course, obtained with the same
camera).

This so-called " photograph-board " is an ordinary drawing-
board covered with tough drawing-paper the surface of which
is to represent both the picture plane and the principal plane
(both planes revolved into the horizon plane), and it is used in
conjunction with the photographic perspectives, using the nega-
tives when great accuracy is required, or using solar prints for
general plotting.

Two lines DD and 55', Fig. 68, Plate XXXV, are drawn
at right angles to each other; they represent the horizon and

GRAPHICAL ICONOMETRICAL PLOTTING METHODS. 149

principal lines, while PD^Piy =PS=PS' are equal to the
focal length, so that Z?, Zy, 5, and S' represent the left, the right,
the lower, and the upper distance points respectively.

The photographic perspective is placed in the center of the
board, within the rectangle TYOZ, the principal line coinciding
with SS' and the horizon line with DlXy and it is secured in
this position by means of small thumb-tacks, pins, etc. The
four scales forming the sides of the rectangle OTYZ serve to locate
lines parallel with either 55' or Diy on the perspective (with-
out actually drawing those lines).

At a suitable distance from D' a line QR is drawn perpen-
dicular to Diy, and on it are laid oflf , by means of a table of tan-
gents, the angles formed with DQ by a series qf lines drawn
from Z7 as a center. This scale, QR, is employed when measuring
the altitudes or the azimuthal angles of points pictured on the
perspective, as will be explained in a following paragraph.

From 5 as a center with SP as radius an arc of a circle PL
is described and the latter is divided into equal parts. Through
the points of division of PL lines converging to 5 are drawn
between PL and PI/. The lines MN are drawn parallel to
the principal line, as shown in Fig. 68, Plate XXXV, and these
lines are all used in connection with the scale of degrees and
minutes QR.

The studs of the centro-lineads are fixed in A, B, C, and £,
the lines AB and CE joining their centers, and those needed
for adjusting the centro-lineads are drawn and used in the man-
ner to be explained in Chapter X.

A square, FGKH, is constructed on the four distance points,
Fig. 68, Plate XXXV.

zi. IcoNoiiETRic Plotting of the Tkace of a Figuse's Plane.

If one wishes to use a perspective instrument for convertmg a

figure — situated in an inclined plane of which the perspective

photograph) is given — into the projection of the figure into

I50 PHOTOTOFOGRAPmC METHODS AND INSTRUMENTS.

horizontal plan it will be necessary to locate the traces of the
figure's plane in both the principal and picture planes.

We may distinguish between two cases frequently met with
in practical work:

(i) The inclined plane containing the figure may be given
by its line of greatest slope.

(2) The inclined plane containing the figure may be given
by three points.

First Case. — The mclined plane of the figure may be given
by the line of greatest slope, which may be an inclined
road-bed, the drainage line of a straight valley (thalweg),
the surface of a glacier, etc.

This line of greatest slope may be represented on the plan
by a line aft, Fig. 69, Plate XXXVI, the altitude of a being known.

The photographic perspective is pinned to the photograph-
board, and the ground line XY is drawn, taking the horizontal
plane through a as ground plane.

On the plotting-boaid aO is drawn through a perpendicular
to the horizontal projection ab of the line of greatest slope A By
and it is produced to its intersections L and O with the prin-
cipal line 5i^i and with the picture trace XiYi.

On the photograph pE is made equal to pib, at £ a perpen-
dicular to XY is erected and produced to the intersection ^ with
the pictured line a^, representing the line of greatest slope AB.
If we make pN, on the photograph-board, equal to piO of the
plan and join N with ^ on the picture, this line Nfi will rep-
resent the trace of the required plane on the picture plane. If
pQ is made equal to piL and Q is joined with Af , MQ will rep-
resent the trace of the required plane, revolved about 55', on
the photograph-board, into the picture plane, the station 5
falling in D.

Producing MQ to if, DR will represent the vertical distance
of the station 5 above the plane RMfi.

Second Case. — The inclined plane containing the figure
is given by three points.

GRAPHICAL ICONOMETiaCAL PLOTTING METHODS. I5Z

Take for groiind plane the horizontal plane containing one
of the points, a, Fig. 70, Plate XXXVII, and draw the ground
line -YF on the photograph. Join a on the plotting-sheet to
the two remaining points and produce these lines to their inter-
sections E and F with the picture trace. On the photograph
make piK equal to pE and draw KL perpendicular to XY; join
the perspectives a and fi of the points shown as a and b on
the plan and produce to the intersection with KL. Take piT
equal to pF, draw TN perpendicular to XY and produce to
the intersection N with the line joining the perspectives a and j-.
Join N and L, when NL will represent the trace of the required
plane on the picture plane.

Produce LN to O and take pG equal to piO; join a and G
and make piQ equal to pH. The line MQ will represent the
trace of the required plane on the principal plane revolved about
SS' into the picture plane, the station being in D. Here also DR
is the vertical height of the station above the plane containing
the three given points.

12. loONOMETRIC CONTOUMNG.

After the heights of a suflScient number of points have been
determined to give a good development of the terrene that is
to be mapped, the contour lines are drawn in by interpolation
between the points of which the heights had been established.

In a moderately rolling country a limited niunber of points
of known elevations will suflSce to draw the contour lines with
precision, but in a rocky region, where abrupt changes and
irregular forms predominate, it is almost impossible to plot
enough control points to enable the iconometric draughtsman
to render a faithful representation of the relief of the broken
terrene, and it is here that a close and minute study of the photo-
graphs becomes indispensable to modify the courses of the con-
tours to represent the characteristic features of the terrene.

The value of photographic views for the cartographic delinea-

152 PHOTOTOPOGRAPHIC METHODS Ato) INSTRUMENTS.

tion of the topography of a mountainous area is generally acknowl-
edged by experienced topographers, even when using instrumental
methods exclusively for all the control work. A minute study
of the pictured terrene will alwa)rs be of great aid to the draughts-
man (when inking the topographic sheet), to draw the contours
of which the main deflections had been located instrumentally>
with a more natural and artistic reproduction of nature's forms^
than could be attained by mechanically inking the pencilled
lines as obtained by instrumental measurements and free-hand
sketching alone.

Instead of drawing the contour lines at once upon the plan,
the draughtsman may begin by sketching them on the photo-
graphs first, following the same rules for their location (by inter-
polation), as if he were drawing them on the plan, for the image
of every plotted point is already marked on the photographs
and its elevation may readily be taken from the working-plan.
By adopting this plan he will be enabled to follow the inequali-
ties of the surface very closely and the perspectives of the con-
tours thus drawn on the pictures will greatly facilitate the draw-
ing on the plan of their horizontal projections. They may also
be transferred to the plan by means of the perspectograph or
perspectometer if accuracy is to give place to rapidity in the
map production.

A suflScient number of tertiary points having been plotted
by the method of intersections, there will be little difficulty in
drawing the contour lines by interpolation between such points.
It may happen, however, that the control points are too few
in number and too far apart to give a good definition of the ter-
rene (in a topographic reconnaissance), and then it will become
necessary to resort to less accurate methods for locating the
contours on the plan. For example, the ridge abed of a mountain
range, pictured on a photograph as afiyd, Fig. 71, Plate XXXVIII,
may be divided by the contour planes by assuming it to be
contained in a vertical plane.

On the plan we produce the projection ad of the ridge to

GRAPHICAL ICONOMETRICAL PLOTTING METHODS. I S3

the mtersection JP with the picture trace and draw through the
station Si the line SiC parallel to ad.

The photograph having been pinned to the photograph-
board, take from the principal point on the horizon line PV
equal to piC and PG equal to piF. At G place the scale of
equidistances perpendicular to the horizon line if i?', the division
at G corresponding to the height of the station, and join the
marks of the scale (corresponding to the elevations of the con-
tour planes) to the vanishing point V.

Having thus located the points of intersection of the ridge
by the contour planes, their distances (abscissae) from the prin-
cipal line are now marked upon the edge of a strip of paper and
their directions plotted in the usual way. The intersections of
the radials (drawn from Si to the points marked on the paper
strip) with ad will give the intersections of the contour lines with
the ridge ad.

When the mountains have rounded forms showing no well-
defined ridges, the visible outline, silhouetted on the photograph,
may be assumed to be contained in a vertical plane perpendicular
to the line of direction drawn to the middle of the ridge outline,
or silhouette.

The construction may be made by drawing, on the photo-
graph-board, SV perpendicular to the direction SM of the middle
of the outline. Fig. 72, Plate XXXIX; PiMi on the plan is made
equal to PJIf, and from the projection a of the summit of the
mountain a perpendicular ac is let fall on 5iAf 1, which represents
the projection of the visible outline. It is produced to the inter-
section N with the picture trace. PQ is taken equal to ^iJV and
the scale of equidistances is placed at Q, perpendicular to the
horizon line. The points of division are joined to V, these
radials are produced to intersect ay, and the plotting of the con-
tour points along ay is done in the same way as described in the
preceding case, or the directions of the intersections of ay by
the contour planes may simply be plotted and the contour lines
drawn tangent to these directions.

154 FHOTOTOPOGRAFHIC METHODS AND INSTRUMENTS.

The horizon line, containing the perspectives of all points
of the same elevation as the camera station, represents the per-
spective of a contour line when the horizon plane coincides with
a contour plane.

The topographic draughtsman should pay particular atten-
tion to geologic forms and to the originating causes of the topo-
graphic features, as without such knowledge the correct inter-
pretation of such forms by means of contours and a faithful
cartographic representation of the various terrene forms would
require the cartographic location of a vast number of control
points.

Although the terrene forms often result from the successive^
or from the combined, actions of many agencies, they will yet
have similar characteristic shapes when resulting from the same
causes, and the cartographic representation of such typical
terrene forms (produced by identical agencies) should also show
a corresponding characteristic similarity in the contour forms.

13. The Use op the So-called "Photograph-protractor."

The angle included between the line of direction to a point
of a photographic perspective and the principal and horizon lines
(the altitude and azimuthal angle) is sometimes wanted in arc
measure.

The azimuthal angle of the line of direction to a point A
may be obtained at once on the photograph-board by joining
the station 5, Fig. 73, Plate XL, to the projection a of the pic-
tured point on the horizon line.

If required in arc measure, the distance Pa is transferred
to the principal line =PG, D is joined to G and produced to
intersect the scale of degrees and minutes BC, where the gradu-
ation mark K indicates the value of the azimuthal angle in arc
measure.

When many such angles are to be measured, the horizontal
scales TY and OZ, Fig. 68, Plate XXXV, may be divided into

GRAPHICAL ICONOMETRICAL PLOTTING METHODS. 1 55

degrees and minutes by means of a table of tangents, using the
focal length SP as radius.

The altitude is the vertical angle at 5, Fig. 73, Plate XL,
of the right-angle triangle, having for sides Sa and aa. To
construct it, take DF equal to 5a, draw FE parallel and equal
to aa, join D and E and produce DE to the scale (BC) of degrees
and minutes.

This construction will be facilitated by the lines previously
drawn on the photograph-board. With a pair of dividers take
the distance (abscissa) from a to the principal line, carry it from P,
Fig. 68, Plate XXXV, in the direction PIT, and from the point
so obtained take the distance to the arc ML, measuring it in
the direction of the radials marked on the board, which will be
the distance PF. Then with the dividers carry aa to FE, which
is that one of the series MN of parallel lines. Fig. 68, Plate XXXV,
which corresponds to the point F. The construction may now
be completed in the manner already explained.

A protractor- may be constructed to measure these angles
direcdy. It consists of a transparent plate on which lines are
drawn parallel to the principal line containing points of the
same azimuth and curves containing points of the same altitude.

The azimuthal lines are found by plotting the angles in 5
and drawing parallels to the principal Une 55' through the points
of intersection with the horizon line.

If we take the horizon and principal lines as axes of coor-
dinates and denote the altitude of a point pictured as a by h,
the equation of the curve of altitude h may be written

This also is the equation of a hyperbola of which the prin-
cipal and horizon Unes are the transverse and conjugate axes
and of which the principal point is the center.

One of the hyperbola's branches represents the points above
the horizon and the other branch those of equal altitude below
the horizon.

156 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

The asymptotes are lines intersecting each other at the prin-
cipal point and making angles equal to h with the horizon line.
This hyperbola is the intersection by the picture plane of the
cone of visual rays forming the angle h with the horizon.

These hyperbolic curves of equal altitude may be obtained by
computation, using the preceding formula and substituting
different values for A, or they may be obtained graphically by
plotting a series of points for each curve, reversing the construc-
tion given above for finding the altitude of the pictured point a,
Fig. 73, Plate XL. The angular distance between the lines
representing points of equal azimuths or those of the same
altitude depends on the degree of precision aimed at.

The complete protractor is shown in Fig. 74, Plate XL. It
may be made in the same manner as mentioned for the per-
spectometer by drawing it on paper on a laiige scale, reducing
it by photography, and finally making a transparency by bleach-
ing the negative in bichloride of mercury.

t). Method 0} V. Legros for Locating the Horizon Line of a
Vertically Exposed PUUe.

Commandant Legros reconunends the use of these hyper-
bolic curves for the location of the horizon line of a vertically
exposed plate.

When the camera with the photographic plate adjusted in
vertical plane is rotated horizontally, the plate remaining ver-
tical, any point a. Fig. 74, Plate XL, will describe a hyperbola
aa' in the picture plane (on the ground -glass plate). The
nearer a approaches the horizon line the smaller the curvature
of its hyperbolic trace on the ground-glass plate will become,
and that point, a®, which traverses the groxmd-glass plate in
a straight line, HH\ will have the same elevation as the second
nodal point of the camera-lens — ^its angle of elevation will be— -l-O,
or HH' will be the horizon line of the plate.

To locate the horizon line experimentally in this way the

GRAPHICAL ICONOMETRICAL PLOTTING METHODS. 1 57

ground glass is best provided with a series of equidistant hori-
zontal and vertical lines, after the manner of Dr. Le Bon^s ground -
glass plates.

E. Prof. S. Finsterwalder's Method jot the Iconometric Plotting
of Horizontal Contours,

Prof. Finsterwalder's method for plotting horizontal contours
is well adapted for the development of the terrene forms of a
moderately rolling country and it is based upon the following
consideration:

The pictured outline of a terrene form may be regarded as
the trace of the terrene surface in a plane (picture plane) ver-
tical to the plotting or ground plane.

The camera stations should be specially selected with refer-
ence to the use of this method with a view toward obtaining
pictures with a sufficient number of such outlines, or silhouettes,
of the terrene forms to enable the iconometric draughtsman to
give a good definition of the relief of the terrene to be plotted.

These terrene-form silhouettes may be regarded as falling
within vertical planes and the rays drawn from the point of
view to the pictured points of the silhouette will form a cone,
with apex in the second nodal point of the lens (or point of view),
its base being formed by the pictured outline (silhouette) of the
terrene. A horizontal plane containing a contour A will inter-
sect such a cone of rays in a curve 5, the latter touching A in
one point.

If we designate by h the difference in elevation between
the station (whence the picture was obtained) and the hori-
zontal contour -4, by /? the vertical angle of each radial or visual
ray drawn to each point of the silhouette, then the curve B may
be plotted on the working-sheet by laying off, upon a few rays,
from the plotted station to points of the pictured outline the
corresponding distances

h cot p,

158 FHOTOTOPOGRAPHIC METHODS AND INSTRUICENTS.

and the points thus located on the radials drawn from the sta-
tion point, if connected by a continuous line, will represent the
curve Bt plotted in horizontal plan.

The direction of the silhouetted outline is now plotted on
the plan, and where it bisects this curve B will be a point of
the contour A. As we, naturally, would draw not only one
curve B but a series of them corresponding to several horizontal
planes, passing through a series of contours A of various ele-
vations, the construction may be simplified, inasmuch as the
curves B — being the lines of intersection of the same cone of
rays with a series of parallel planes containing the horizontal
contours-r-will all be similar in shape, their corresponding points
having the same relative positions with reference to the plotted
station, and the value hcotp need only be determined for one
point of the remaining curves B if one ciure B had been drawn;
the others will be parallel to it

CHAPTER Vn.
CAMERA-LENSES.

The general theory and laws of optics as applied to lenses
are the same whether the latter are to be moiinted in telescopes
or in photographic cameras. The camera may even be regarded
as an incomplete telescope, lacking only a suitable eyepiece to
convert it into a telescope.

Still, photographic lenses are to fulfill requirements differing
widely from those of telescopes, the main difference being in
the field commanded by either. As only the central part of a
telescopic lens is utilized for observing, comprising a field of
but a few degrees, spherical and chromatic aberration do not
a£Fect the latter. Phototopographic lenses, however, should
command as wide an angle as possible (over 60®) and still pro-
duce geometrically true perspectives without distortion, with a
sharp definition, a uniformly bright illumination for the entire
plane surface of the sensitive plate, and with a great depth of
focus.

Rapidity in the action of the camera-lens being desirable,
but not of essential importance for surveying purposes, the
quality of the lens will in a great measure determine the value
of the photogrammeter or photographic surveying camera.

A. The Refractive Index.

With reference to Fig. 75, Plate XLI, we designate by AB
the refractive surface, by SI the incident ray, by IP the refracted
ray, by CCi the perpendicular to the refractive surface in the
point where the mcident ray SI enters the second medium, by a

159

l6o PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

the angle included between the perpendicular CCi and the inci-
dent ray 57, and by /? the angle of refraction.

IS being equal to IP=r, the ratio of the sines of the angles a
and /? may be expressed by the ratio of the lines DS and £P,
or, in other words,

For all angles a, larger and smaller than the one indicated
in Fig. 75, Plate XLI, the ratio between DS and EP will be
the same for the same two substances.

This constant ratio is termed the " refractive index " of the
two substances that are separated by the refractive surface AB.
The incident ray, the refracted ray, and the emergent ray (coming
from the same source) are all in one plane.

When speaking of the refractive index of any one medium
in optics, it is always to be understood that the incident ray
has passed through air (or space). Thus we have, approxi-
mately, if the refractive index for air or space be assumed as
imity, the refractive index for

Water, about 1.3

For crown glass, about • 1.5

For flint glass 1.6 to 1.9

For diamond, about 2.4, etc.

This means, for instance, that for any angle a (for any
incident ray 57) the vertical DS is 1.5 times as long as PE if
the ray passes through air and is refracted by crown glass.

B. Refraction of Light-rays.

The preceding consideration enables us to find the means
for changing the course of light-rays by refracting them to any
amount desirable.

With reference to Fig. 76, Plate XLI, we have AB and
j4i5i = refracting surfaces of a piece of plate glass.

The incident ray 57 arrives at the surface -4 J5 under an angle
-a with the perpendicular IC (perpendicular to AB in 7). Glass
being denser than air the ray will be refracted toward the per-

THE OPTICAL LENS. . l6r

pendicular 7C, continuing in a straight line IE as long as it passes
through this second medium (glass) ; arriving at E it passes from
the denser medium into air and at E it will be refracted away
from the perpendicular ECi (under an angle a) and continue
in the direction EP, parallel to the incident ray SI.

By changing the direction or position of one or of both sur-
faces of the denser medium (glass) the final direction of EP
may be given any course, since the equation

-: — ^=»== refractive index
sm p

must always be fulfilled.

It becomes plain that the change in the direction of EP from SI
will increase directly with the angle included between the two
refractive surfaces AB and AiBi.

In Fig. 77, Plate XLI, this change in direction is shown for
three different glass prisms shaped in such a way that their
refractive angles not only decrease from A toward B, but have
been given such values that the three rays emanating from a
certain luminous point 5, after refraction, converge to one
point P.

A point P, where "several converging rays (originally ema-
nating from a point S in space) intersect one another, is termed
an " image point."

C. The Optical Lens.

If the directions not only of three, Fig. 77, Plate XLI, but
of an infinite number of light-rays emanating from a luminous
point 5 are to be so changed that all will converge into a point P,
we will have to superimpose an infinite number of prisms one
upon the other. The heights of these prisms will have to be
made infinitesimally small and the refractive angles of two neigh-
boring ones will differ by an infinitesimal small amount.

This means that the broken lines AB and ABu Fig. 77,

l62 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

Plate XLI, will become curves, and a piece of glass with its
two faces shaped in such a manner that all light pencils ema-
nating from the same point S will converge to meet in its image
point P is termed an optical lens.

Evidently such a lens is a body formed by rotating the fig-
ure ABBi, composed of an infinite nimiber of prism sections,
about the line BBi as axis. This axis of rotation is termed
the optical axis of the lens, and the latter may be considered
as composed of concentric zones or rings with spherically shaped
outer surfaces. The question now arises what form should
be given the figure ABBi to obtain a lens that will produce optical
images of luminous points.

Opticians can produce in the manufacture of lenses only
spherical surfaces with any degree of precision; therefore all
optical lenses are inclosed by spherical surfaces. Still, spherical
lenses produce well-defined and sharp images of luminous points
only within certain limits, limits between which the spherical
surface approaches very closely that ideal shape which is best
adapted for the purpose m view, but which cannot be manu-
factured owing to mechanical difiiculties encountered in the
grinding or cutting process of the lens. In our superficial treat-
ment of the laws of optics — considered inasmuch as they apply
to phototopography only — ^we shall assume that the spherical
lenses are optically perfect and of a small thickness.

The deduction of the optical laws governing the action of
lenses of various shapes would require complicated computa-
tions; still,, at least a general consideration of certain optical
laws and facts should not be omitted in this treatise on photo-
topography, in order to better elucidate the formation of the
optical images and to determine such elements of the photo-
graphic lens as will be needed in iconometric plotting.

Generally speaking, we meet with so-called simple lenses
and with combinations or sets of lenses in photography. The
symmetrical combinations are preferable for topographic sur-
veying purposes, as they command a wider field or larger view

OPnCAL DISTORTION, 163

angle and as they are less affected with distortion and aberration
than is generally the case with the simple or single lenses.

D. Optical Distortion.

So-called " spherical aberration " is more conmionly pro-
duced by those light rays which pass through the marginal zone
of the lens, as this part of the lens is less perfect than the cen-
tral part. Spherical aberration may be reduced by decreasing the
effective diameter of the lens which is generally done by insert-
ing a so-called " diaphragm " between the lenses forming the
combination, or by a reduction of the curvature of the faces of
the lens.

Lenses corrected for spherical aberration are known as
aplanatic lenses.

In so-called " chromatic aberration " the different color
rays which compose the white light are unevenly refracted, and
colored, ill-defined images are the result.

Lenses corrected for chromatic aberration are termed achro-
matic lenses.

Probably the greatest source of error introduced into photog-
raphy is due to distortion of the image when using an inferior
lens. It is caused primarily by the greater refraction — ^in the
direction toward the optical axis — of those light-rays which pass
through the marginal or border zones of the lens. When the
image on the ground glass of a test-screen of the form shown
in Fig. 78, Plate XLII, assumes the form indicated in Fig. 79,
Plate XLII, so-called " pin-cushion distortion " has taken place,
whereas an image of the form shown in Fig. 80, Plate XLII,
is produced by *' barrel-shape distortion." A lens affected by
either is imfit for phototopographic purposes.

Commandant Moessard has invented an ingenious little con-
trivance — the tourniquet — by means of which the field or
angle that is affected by distortion of the image may readily
be determined experimentally.

l64 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

Astigmatic distortion in an image is produced when well-
defined images of the lateral points of an image may be obtained
for two different positions of the groimd-glass plate, and yet
neither of these two images of the same points will represent
the true shape of the original. Using a test-screen of the shape
shown in Fig. 8i, Plate XLII, radial distortion will be shown^
Fig. 82,. Plate XLII, for one position of the focusing-glass;
the distortion will be in directions radiating from the center
of the ground-glass plate. In the other position of the focusing-
glass tangential distortion will be observed, Fig. 83, Plate
XLII; the distortions will appear in directions at right angles
to the directions radiating from the center 6i the ground-glass
plate. Both radial and tangential distortions increase from the
center toward the extraaxial zones of the lens.

Lenses corrected for astigmatic distortion are termed anastig-
matic lenses.

The distortion shown in Fig. 84, Plate XLIII, of the image
of the test-screen. Fig. 81, Plate XLII, is. due to imperfect regis-
tering of two lenses composing a double lens; the component
lenses are not " centered."

The Zeiss anastigmatic lens has a perfectly flat field. That
is to say, if the ground glass has been focused for the sharp
definition of a central point, extraaxial points will also be well
defined on the focusing-plate.

Nearly all the older lens types were characterized by more
or less curvature of the field, which means the focal length when
focusing for a central point would be longer than when focusing
for sharp definitions of a marginal point shown on the image
plate.

NODAL POINTS AND NODAL PLANES OF A LENS. 165

E. Nodal Points and Nodal Planes of a Lens.

Formerly the thickness of a lens was disregarded when inves-
tigating its action upon light-rays passing through it and it was
generally assumed that the central rays — those passing through
the so-called " optical center " of a lens — suffered no change
of direction.

Lenses are generally regarded as being bounded by two
spherical surfaces. If both sides are convex (the lens is thicker
in the center than at the edge) it is termed a biconvex or
positive lens, Fig. 85, Plate XLIII.

If the spherical surfaces are concave on both sides of the
lens (its center- is thinner than its edge) it will be a biconcave
or a negative lens, Fig. 89, Plate XLV.

Fig. 86, Plate XLIII, represents the cross-section of a con-
cave-convex or a periscopic convex lens, the convex surface
having a shorter radius than the concave surface, the lens
being thicker at its center than at its margin. When the con-
cave smiace has the shorter radius the lens would be called
convex-concave. The principal elements of a lens (Figs. 85 and
86, Plate XLIII) are:

First. The geometrical centers; they are the centers C
and Ci of the spherical surfaces forming the faces of the
lens.

The line passing through C and C\ is termed the principal
axis of the lens. "

Second. The vertices A and B of the lens are the inter-
sections of the principal axis with the two spherical lens
surfaces.

Third. The thickness AB oi the lens is the distance between
the vertices of the lens.

A lens is centered " when the plane PP, passing through the
circumference of the lens — passing through the circular line of

1 66 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

intersection of the two lens surfaces — ^is intersected by the prin-
cipal axis at right angles.

A lens-combination is centered when the planes PPi of the
individual lenses are parallel and if they are intersected by the
principal axis at right angles.

The foci of the separate lenses should also fall upon the
principal axis or the images of the discs shown on the test-screen,
Fig. 8i, Plate XLII, will show so-called " flare spots," some-
what like those represented in Fig. 84, Plate XLIII.

A large flare spot, or halo, in the center of an image or
picture may be produced by halation, caused by light- rays that
have passed through the diaphragm aperture being reflected from
the lens surfaces.

There exist certain relationships between the curvature of a
lens, the distance of a luminous point from the lens, and the
distance of its image from the lens which we will now briefly
consider.

An incident ray SI, Fig. 85, Plate XLIII, will be refracted
at / toward the radius R (=Co/), glass being a denser medium
than air; it will continue through the lens in the direction IE,
and the emergent ray EP will be parallel to the incident ray SI.
The radii CI and CiE are also parallel. The point Co, where
the refracted part IE of the light-ray intersects the optical axis
of the lens, is known as the optical center of the lens and
the following relation exists between its distances from the geo-
metrical centers and the radii of the two lens surfaces:

CCo _R
CiCq Ri

The triangles IC(/I! and JECoCi are similar.

Every lens has two nodal points N and Ni, Figs. 85 and
86, Plate XLIII, on the optical axis of the lens. The rays reach-
ing the first nodal point N from luminous points S in space
are parallel with the rays connecting the second nodal point Ni
with the corresponding images P of the limunous points.

NODAL POINTS AND NODAL PLANES OF A LENS. 167

Hence a negative produced by an optical-lens system may
be regarded as a central projection or as a perspective image
(the center of which coincides with the second nodal point Ni),
Fig. 88, Plate XLIV.

The positions of the nodal points will be constant for all
rays that make a small angle with the optical axis of the lens
(for all rays passing through the small aperture of a diaphragm).
The distances of the nodal points from the corresponding ver-
tices of the lens are constants and their values are given by the
equations (Fig. 85, Plate XLIII)

BCo C^

where n is the refractive index from air into glass, or n=f.
As the distances of the nodal points from the optical center
(Co2V and CoNi) will be small — they may sometimes become

inappreciable or=o — ^we may omit the factors -p^pr and Jt J^

from the equations (when CqN^o and CoiVi=o the factors

7^7^ and -prpr- will become =1), hence

ACo

AN"'

A close approximation to the distance between the nodal
points will be

l68 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

The planes and the points of a lens system are numbered
in the sense of the direction of the incident rays. With refer-
ence to Fig. 87, Plate XLIV, the light is supposed to be coming
from the left, hence

iV= first nodal point;

J7= first principal focus;
i^G= first focal plane;
H^NK={iTSt nodal plane, whereas
iVi= second nodal point;

J7i= second principal focus, etc.

Lengths are considered plus, or positive, if they extend in
the sense of the direction of the incident rays, and minus, or
negative, if they extend in the opposite direction. With refer-
ence to Fig. 87, Plate XLIV, where the light comes from the
left, we have

FN^-f and FiNi^+fu

The nodal planes (passing through the nodal points and
intersecting the principal axis at right angles) coincide with the
principal planes if the extreme or outer medium of the optical
lens system is the same, which is the case in photography where
air surrounds the lens.

F. Principal Foci and Focal Planes of a Lens.

The principal foci F and Fi of a biconvex or positive
lens. Fig. 87, Plate XLIV, are two points on the optical axis —
one on either side of the lens — where those incident rays con-
verge which arrive at the refractive lens surface in a course
parallel to the optical axis.

In Fig. 87, Plate XLIV, the ray 5'/^^, coming from a lumin-
ous point 5' at infinite distance from the lens, traverses a path
parallel in its course to the optical axis FFi of the lens, and

THE FOCAL LENGTH OF A LENS. 169

after refraction converges to the point Fi; while a similar ray
P£", coming from the opposite direction in a course parallel
with the optical axis, converges to F.

The planes FG and -FiGi, passing through the foci F and Fi
and intersecting the optical axis FFi at right angles, are termed
focal planes.

G. The Fo€al Length of a Lens.

FN and FiNu Fig. 87, Plate XLIV, are termed focal
lengths and they are generally designated by the letter /. The
value of the focal length is expressed by the equation

/.-/-

(n-i)(i?i-i?+^-4s)

"When this value for /i becomes positive it is an indication
that the incident rays, when coming from infinite distance (parallel
with the optical axis), are refracted to converge to the principal
focus of the lens.

A negative value for /i would indicate that the rays entering
the lens in a course parallel to its optical axis diverge from the
principal focus.

For thin lenses (the distance between A and -B, Fig. 85,
Plate XLIII, is very small in comparison with the lengths of
the radii R and Ri and it may be assumed "o) the formula
for the focal length would read

'""(n-i)(iJi-i?)* ""^
1 (n-i)(Rr-~R) /i i\

7" — rtr^ ^^""^^^R-Rir

170 PHOTOTOPOGRAPHIC METHODS AND INSTSUMENTS.

After substitution of J for n, the approximate value for the
refractive index of glass, the approximate value of the focal
length for thin lenses would be

I I I

H. The Biconvex or Positive Lens.

The image of a point at infinite distance from the biconvex
lens is on the opposite side of the lens and falls together with
its principal focus.

When the distant luminous point approaches the lens the
image will recede, at first slowly, but more rapidly the nearer
the luminous point advances toward the lens, and by the time
the original point will have reached a distance from the lens
equal to its double focal length, its image will have moved to a
point beyond the lens, also at a distance of the double focal length.
When the luminous point continues to approach the lens within
the double focal distance range, its image moves faster and
faster beyond the double focal distance on the opposite side of
the lens, and when the luminous point finally falls together
with the first focus (F) the image will disappear at infinite dis-
tance.

The relation which exists between the position of a luminous
point (5) and that of its image (P) may be briefly expressed in
the following equation:

I i_ i_

where, with reference to Fig. 87, Plate XLIV,

/« principal focal length of lens;

a -distance (SH^SiN) between the lens (its first nodal plane)

and the luminous point (5) ;
^—distance (NiPi) between the lens (its second nodal plane)

and the image plane PPi.

CONJUGATE FOCI AND CONJUGATE PLANES. 171

From the above equation we deduce

h ^-^

With these simple formulae any question concerning the
dbtance between the image and the lens (its focal planes) may
be solved.

We may have, for example, a lens of 15 cm. focal length
and the object (the luminous point 5, Fig. 87, Plate XLIV)
may be 50 cm. away from the lens. It is desired to find the
distance between the nodal and image planes (the distance PiiVi).

For/— —15 and a— -50 we find,

fa
from 6 =-7 — ,
/-«

• 15-50

6— — "^-7 — — 21.4 cm.
-15 + 50

L Conjugate Foci and Conjugate Planes.

The image Pu, Fig. 87, Plate XLIV, of a limiinous point U,
situated on the optical axis of a biconvex or positive lens, will
also be on the optical axis of the lens, but on the opposite side
of the latter. The incident ray Uluf emanating from the axial
point U, will be refracted beyond luf and its course may be found
by drawing a ray FH', parallel with Ulut through the first prin-
cipal focus F, which ray, after having traversed the lens, will
emerge in a direction H'Guy parallel to the direction of the
optical axis. This fictitious or auxiliary ray H'Gu intersects

172 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

the second focal plane FiGi at Gu, and if we draw a line from
Ku through Gt/ it will represent the path of the emergent ray
originally emanating from Z7, and the intersection Pu of FFi
with Ki/Gu will locate the image Pu of the luminous axial point U.

The point U and its image Pu (when axial points) are termed
" conjugate foci."

The planes TuPu and TU, both vertical to the optical axis
and passing through the conjugate foci U and Pu, are termed
conjugate planes.

K. To Find the Image of any Luminous Point for the
Biconvex Lens.

The image Tui Fig. 87, Plate XLIV, of any luminous point T
in the conjugate plane UT may be found by locating the point
of intersection Tu of the emergent rays of:

First, an incident ray TI^^My drawn parallel to the axis FFi;

Second, an incident ray TNy drawn through the first nodal
point Ny and,

Third, an incident ray TF, drawn through the first principal
focus F.

If the conjugate plane TuPu had already been located,
the drawing of the third ray TF would sufiice to locate the image
Tu of r, as it is in the intersection of TuPu with the emergent
ray H'Tu of the incident ray TF.

Knowing how to locate the image P of any Ivuninous point
5 or r for the biconvex or positive lens we can now locate the
images of lines (as these may be regarded as a series of an infinite
number of points), and also of surfaces (being composed of an
infinite number of Imes), provided the objects are not too far
away from the optical axis of the lens.

The image P of any luminous point 5, Fig. 87, Plate XLIV,
not on the optical axis is found graphically by locating the point
of intersection (after refraction) of the three following specific
rajrs emanating from the point S:

THE BICONCAVE OR NEGATIVE LENS. 1 73

First. Draw a ray from 5 through the first principal focus F
and produce it to the intersection with the first nodal
plane at K^ whence it continues in a direction parallel
to the principal axis (having passed through the first
principal focus).
Second. Draw a ray from 5 parallel with the optical axis
of the lens to its intersection H\ with the second nodal
plane, whence it converges to the second principal focus jFi
(having arrived at the lens in a direction parallel to its
optical axis) and produce it to its intersection in P with PK.
Third. Draw a ray from 5 to the first nodal point N\ it
will pass through the second nodal point iVi, and it will
emerge at £' in a direction E'P parallel to the direction
of the incident ray SPI^.
The image of any point 5 situated in a plane SS\ perpen-
dicular to the optical axis will fall within the conjugate plane P\P.
In Fig. 90, Plate XLV, where similar points are designated
by the same letters correspondingly used in Figs. 87 and 89,
it has been shown how the image P'PiP of a line SSiS' may
be found if the incident rays are refracted by a biconvex lens.
The preceding definitions and formulae are applicable only
to light-rays which make small angles with the optical axis (for
lenses with diaphragm stops), and they serve to illustrate and
explain the formation of images.

More rigid (and consequently more complicated) formulae
would have to be applied to ascertain the best shape of lenses for
special purposes.

L. The Biconcave or Negative Lens.

The biconcave or negative lens produces upright virtual
images of originals which are beyond the principal plane, whereas
the biconvex or positive lens, as has been shown in Fig. 90,
Plate XLV, produces inverted real images of objects.

If the object UTy Fig. 91, Plate XL VI, is situated between

174 PHOTOTOPOGRAPmC METHODS AND INSTRUMENTS.

a positive lens and its principal focal plane FG, its rays will
produce a virtual upright image PuTv

Incident rays that are parallel to the optical axis of a posi-
tive lens will converge to the principal focus of the lens, but
with the negative lens such rays will, after refraction, diverge
in directions coming from the principal focus.

It will readily be seen, with reference to Fig. 89, Plate XLV,
that the image PPi of an object SSi is upright and virtual
The paths of the light-rays are given in full lines and similar
points are given the corresponding designation as in the pre-
ceding figures for the biconvex lens.

M. To Find the Image of a Luminous Point for a
Biconcave Lens.

To find the image P of a luminous point 5 beyond the prin-
cipal focal plane -FiGi, Fig. 89, Plate XLV, three incident rays
are drawn:

5/1, parallel to the optical axis;

5/2, through the principal point F\

5/3, through the first nodal pomt N.

The intersection of the backward prolongation of the three
corresponding emergent rays, PEu PE2, and PEz^ will locate
the image P of the luminous point 5. These two points, P
and 5, are termed conjugate points, the same as mentioned for
the positive lens.

N. Lens Combinations.

In Fig. 92, Plate XL VI, a combination of a single positive
and one negative lens is shown. The positive lens may have:

FN=FiNi = focal length;
FG and Fid = focal planes;
NM and iVi^p= nodal planes.

LENS COMBINATIONS, 1 75

The negative lens may have:

FN' ^Fi'Ni'^iocal length;

i^C and Fi'Gi'= principal focal planes;

N'K and iVi'L= nodal planes.

To find the principal focal planes of the lens combination we
proceed in the same way as with a single lens, bearing in mind
that the incident ray of the second lens is now the emergent ray
of the first lens.

The line SI represents an incident ray arriving at the positive
lens in a direction parallel to the optical axis; it is produced or
continued in its course until it reaches the second nodal plane
of the positive lens, where it changes its direction to one bisecting
the second principal focus of this lens. In this direction it is again
produced until it reaches the first nodal plane, in K, of the nega-
tive lens, whence it continues to the second nodal plane Ni'L
to the point L, the line KL being parallel to the optical axis.

Now we draw through the first principal focus F' of the nega-
tive lens the auxiliary ray FF' parallel with HFi, YX, drawn
parallel to the optical axis, intersects the second focal plane FiGi
of the negative lens in X, and XL will be the direction of the final
emergent ray; its intersection F/' with the optical axis is the second
I principal focus of the lens combination.

This point, Fi", may be checked with a second incident ray
5i/i, and Fi"Gi" will be the second principal focal plane of this
lens combination.
I In a similar manner two incident rays PF and PJi arriving

; at the negative lens from the other side of the combination, under

a direction parallel to the optical axis, will locate the first prin-
cipal focal plane F''C of this lens combination.

The first nodal plane of this combination (==iV"a/?) is located
by determining the intersections a and ^ of the original incident
rays PF and PJi with the final emergent rays F'T and FTi
respectively.

The second nodal plane of this lens combination is fixed bj

176 PHOTOTOPOGRAPHIC METHODS AND INSTRUl^ENTS.

the intersections of the original incident rays SI and 5i/i with
the final emergent rays Fi'X and Fi'Xi respectively.

0. Diaphragms or Lens Stops.

It had already been mentioned incidentally that diaphragms
are used to reduce the aberrations of light-rays which arrive at
the marginal zones of a lens, by excluding them from action
upon the photographic plate.

By selecting a sufficiently small aperture in the diaphragm
all rays may be excluded from reaching the interior of the camera,
which make an angle with the optical axis larger than the angle
controlling the limit of the central field of the lens that may be
regarded free from distortion.

This would comprise that effective circular disc of a lens for
which the preceding optical laws and rules have been given, as
the conditions are diCFerent for the extra-axial zones of a lens,
and those rules are not applicable to the latter. The laws given
in the preceding pages become less and less true the nearer the
outer margin of the lens is approached.

By the insertion of a diaphragm stop, a more or less great
amount of light will be excluded from action upon the sensitized
film of the photographic plate, and the smaller the aperture in
the diaphragm the longer the exposure will have to be made in
order to reduce a given amount of silver in the sensitized film.

Generally speaking, the quantity of light admitted into the
camera will be proportional to the square of the diameter of the
diaphragm aperture.

P. Rapidity of a Lens.

Lenses with comparatively short focal lengths will produce
brighter images than such with long focal lengths, the brightness
of the image being inversely prop)ortional to the square of the
focal length. The more light is allowed to enter the camera the

DISTORTION PRODUCED BY DIAPHRAGMS. 1 77

quicker the reduction of the chemical compounds of the sensi-
. tized fihn will take place; the rapidity of a lens depends in a great
measure upon the quantity of light which the lens will suffer to
reach the plate.

Small apertures necessarily will permit more light to reach,
the central part of the plate than reaches its extra-axial parts,,
and photographs obtained through small diaphragm apertures
often are darker and lack good definition on the edges.

If the sensitive plate could be made less sensitive to the action
of the light in its central part than it is on the edges this draw-
back would be overcome, in a great measure at least; practically^
however, the sensitized coating is of a uniform character.

If d represents the diameter of the diaphragm aperture, and
if / represents the focal length of the lens, then the rapidity of the
lens (or the brightness of the image produced with that lens)
win be proportional to the fraction

(r-

Q. Length of Ezposuie.

Generally speaking, the length of exposure that should be given
a plate is inversely proportional to the rapidity of the lens, hence .
proportional to the fraction

a)-

R. Distortion Produced by Diaphragms.

When a diaphragm is placed in front of a positive lens so-called
"barrel-shape" distortion (Fig. 80, Plate XLII) frequently
ensues in the border regions of the image, and a diaphragm placed
behind the lens is apt to produce so-called "pincushion" dis-
tortion (Fig. 79, Plate XLII). It has been sought to compensate

178 PHOTOTOFOGRAPmC METHODS AND INSTRUMENTS.

these distorting effects by using two lenses and inserting the
diaphragm between them.

S. Chromatic Aberration of Light-rays.

The researches of Dolland, made with a view to reduce or
overcome chromatic aberration of telescopic lenses, led to the
combination of different glass compounds in the same lens, or
better, to the combination of two or more lenses each of which
was made of a glass mixture of different but well-known qualities
regarding both dispersoin and refraction. He was successful in
thus eliminating from the old-style lenses the greater part of the
chromatic aberration which shows itself in the more or less
pronounced appearance of colors on the borders of an image with
a simultaneous indistinctness of outline.

The improvement in this respect of all modem photographic
lenses is principally due to the results obtained in the optical
factory of Zeiss in Jena, where extensive experimental researches
were made by Dr. Schott by direction of Prof. Abbe. By a
judicious selection and combination of the glasses obtainable
from the works at Jena, opticians can now produce lenses more
fully answering the different requirements for the various uses
to which photography may be applied than has heretofore been
deemed possible.

Still, so-called achromatic photographic lenses cannot yet be
made free from all chromatic aberration, as no two kinds of glass
have yet been compounded to precisely counteract or neutralize
the refractive errors inherent to each.

That amount of aberration with which so-called achromatic
lenses still remain affected is known as secondary chromatic
aberration. It has been reduced to such a degree that its dis-
turbing effect in achromatic telescopes — where small angles of
the field are only used — disappears altogether, but in lens com-
binations for photographic work, and particularly in phototopog-
raphy (where large field angles are used), this permanent defect

CHROMATIC ABERRATION OF LIGHT- RAYS. 1 79

is still seriously felt, particularly when short focal lengths of the
camera-lenses become desirable.

Achromatic photographic double lenses are composed of
two or three lenses each, the glass of the single lenses being care-
fully selected with a view towards overcoming the chromatic
aberration of the light-rays as much as possible.

So-called white or colorless light is composed of a series of
colored light-rays intermingled in such a way that their joint
effect is that of colorless light. The main characteristic of these
color rays with reference to our subject is " each of the diflferent
light-rays that form the component parts of white light has a
diflferent refractive index for the same medium;" or, light-
rays of diflferent colors will be refracted under diflferent angles
for the same refractive medium. Red rays, for example, are
less refracted than yellow rays, and these again are less refracted
than the blue and violet light-rays for the same refracting medium.

If a pencil of white light be intercepted by a glass prism
it will become separated into its component color rays; each
different color, representing a different wave-length, is differently
refracted by the prism, each color having its own special index
of refraction.

The prism separates and disperses the different color rays
which compose the white light, and when the refracted rays
are cast upon a white screen in a 4ai-k-room, the band of colors
appearing on the screen is termed the spectrum of the particular
light used.

If a pencil of sunligRt had been used in the experiment the
solar spectrum would appear on the screen and the red ra)rs
will be less refracted than the orange, the green, the blue, and
the violet rays.

We had seen (Fig. 77, Plate XLI) that a biconvex lens may
be regarded as a series of concentric prismatic rings, one super-
imposed upon the other, and -it will be evident that a chromatic
lens will retract red rays less strongly than the orange, green,
or violet rays. The lens will have a shorter focus for the violet

l8o PHOTOTOPOGHAPHIC METHODS AND INSTRUMENTS;

and blue rays than it has for the orange or red rays, and the focal
length of such a lens will vary according to the particular color
of the emergent ray (Fig. 93, Plate XL VII).

As the retina of our eye is more sensitive to the yellow and
green light rays than to those of another color, we will, whe*
focusing upon a landscape view, perceive the best definition
when the focusing-plate falls together with the focal plane of the
lens focus for the yellow or light-green light-rays.

Generally speaking, however, the ordinary photographic
dry-plate emulsion is less sensitive to yellow light and more
sensitive to blue or violet light-rays, and in order to obtain a
good negative, the plate should be exposed in the focal plane
of the blue or violet light-rays instead of being exposed in the
focal plane of the yellow rays, as it would be if exposed in the
position as determined with the ground glass for the best ocular
definition.

The distance D, Fig. 93, Plate XL VII, between the focal
planes of the yellow rays (active rays optically or visually) and
the blue-violet (or chemically active) rays is termed the chro-
matic aberration or variation of the rays. Lenses affected with
chromatic aberration have a chemical focus, differing more or
less from the optical or visual focus, according to the more or
less great amount of chromatic aberration by which the lens may
be affected.

Compositions of glass of different indices of refraction will
separate white light into spectra of different lengths. Lenses
made of glass having a small refractive index will show a small
difference in the focal variation Z>, Fig. 93, Plate XL VII (between
the focal lengths), of the red and violet light-rays. Such glass
is generally known as crown glass and its index of refraction is
from 1.5 to 1.6.

Lenses made of a composition of glass having a strong refrac-
tive power will show a greater focal variation. Such glass is
known as flint glass and its index of refraction is from 1.6 to 1.9*

Any crown-glass positive lens of a given focal length may

CHROMATIC ABESRAXION OF LIGHT-BAYS. l8l

be matched with a flint-glass negative lens of a larger focal length
(Fig. 92) that maybe of such a refractive power to bring the chem-
ical focus of the combined pair almost into coincidence with the
optical or visual focus without annulling the entire refractive
power of the positive lens. A lens combination of this character
which still retains the characteristics of the positive lens (posi-
tive focal length and real image of a distant object) and which
is almost free from chromatic aberration is termed an achro-
matic lens combination.

. CHAPTER VIIL

PHOTOGRAMMETERS OR PHOTOGRAMMETRIC
INSTRUMENTS.

Photographic surveying instruments have already under-
gone many changes and various patterns are in use in differ-
ent localities. Until quite recently photogrammeters were not
procurable in open market. Nearly every observer who made
use of the photographic method for topographic surveys had
an apparatus constructed especially for his particular needs and
according to his personal ideas. Thus we find:

First. The ordinary photographic field camera (with bellows
extension) converted into a surveying camera by a few
simple additions and mechanical modifications.
Second. A specially constructed surveying camera' with a
constant focal length and special devices for leveling (to
bring the sensitive film into vertical plane).
Third. A surveying camera combined with some geodetic sur-
veying instrument (with a surveyor's compass, a transit,
or with a plane table). Such combination may be per-
manent, or it may be effected in such a way that the camera
is detachable and both may be used independently and
separately.
The practical value of a photogrammeter depends largely
upon the quality and general uniformity of the lens or lens com-
bination upon the rigidity of the component parts of the apparatus,
its transportability, and upon the rapidity with which it may

be adjusted and placed in position for use.

182

FHOTOGRAMMETBIC INSTRUMENTS. 183

The principal lenses that have been employed for photo-
topographic purposes are Dallmeyer's rapid rectilinear, Stein-
heil's aplanatic, Busch's pantoscopic, Gorz's double anastig-
matic, Voigtiander's collinear, and more recently Zfeiss's anastig-
matic lens.

The nodal points, the focal length, the arc of visibility, and
the arc which is perfectly free from distortion of any kind should
be known for every lens used for phototopographic purposes,
and the manufacturers of lenses of good quality are best fitted
to determine these values with great precision.

General Requirements to be Fulfilled by a Topographic
Surveying Camera.

A good surveying camera or photogranmieter for topographic
work should produce negatives that are geometrically true per-
spectives. The elements of the latter should be known and the
following desiderata should be fulfilled:

First. The photographic plates should be adjustable in ver-
tical plane.

Second. The distance between nodal and image plane
should be maintained unchanged for all exposures.

Third. This distance — the constant focal length — must be
known, or will have to be determined for every instru-
ment.

Fourth. Means should be provided to trace the horizon line
(line of intersection of the horizon plane with the vertical
photographic plate) upon every negative.

Fifth. Means should be provided for locating the principal
point (the point of intersection of the horizontal optical
axis of the camera with the vertical sensitive plate) upon
every n^ative.

Sixth. A ready orientation of the photographs for icono-
metric plotting should be possible; and we may add as

184 FHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

Seventh. Enough characteristic stations (outside of the tri-
angulation scheme) are to be occupied with the camera
to give a full development of the terrene, which is to be
mapped.

L Ordinary Cameras (with Extension Bellows) Converted into
Surveying-cameras.

These surveying-cameras have been constructed primarily for
economical reasons and their use should not be extended beyond
preliminary work or beyond surveys made for experimental study.
For extensive use the results will not be sufficiently uniform and
accurate.

Such cameras are generally supported by three leveling-screws
upon a tripod and they are provided with a circular level /, Fig. 95,
Plate XLVIII, or with two cross-levels L, Fig. 94, Plate XL VII,
for adjusting the sensitive plate into vertical plane. The dis-
tance between nodal plane and photographic plate is made invari-
able, generally by means of two metal rods /?, as shown in Fig. 94,
Plate XL VII (Werner's apparatus made by R. Lechner in
Vienna, Austria).

In Fig. 95, Plate XLVIII (apparatus of Dr. Vogel and Prof.
Doergens, made by Stegeman in Berlin, Prussia), this has been
accomplished by means of the clamp M. After the bellows
have been extended sufficiently to establish the desired focal
length, which may be read off on the vernier n, the screw M is
securely clamped. The pinion K with rack movement zz serves
to give the needed slow motion (when extending the bellows)
to set the vernier n.

Dr. G. Le Bon also used a modified field camera for his
archaeological researches in India, which were carried on under
the auspices of the French Ministry of Culture.

The braces H in Fig. 95, Plate XLVIII, and R in Fig. 94^
Plate XL VII, give the plate receivers a vertical position upon
the level extension boards.

SURVEYING-CAMERAS WITH CONSTANT FOCAL LENGTHS. 185

The short brass points m in Fig. 95, Plate XL VIII, locate
the principal and horizon lines by their reproduction on the
negatives. They are brought into actual contact with the plate
(before exposure) by turning the buttons A, thus producing a
sharp image of their outlines on the margins of the negatives.

Fig. 96, Plate XL VIII, shows an arrangement for setting
the four teeth (which locate the horizon line hhi and the prin-
cipal line tn/i on the negative) close against the sensitive film
surface, which has been used a great deal in Germany. By
turning the arms r the teeth may be brought into direct contact
with the plate, and when the camera should be used to obtain
pictures for their pictorial value only the teeth may be removed
from the plate by turning the buttons a, 6, c, and d back.

The original Coast Survey Camera was provided with a
device which would operate all four teeth together by turning
but one button.

n. special Survejring-cameras with Constant Focal Lengths.

Among the numerous patterns of this class of instruments
that devised by Dr. Meydenbaur is probably the earliest form.

A. Dr. Meydenbaur' s new small Magazine Camera,

This instrument is represented in Figs. 97 and 98, Plate
XL VIII. The camera weighs 750 grams and the plates are
9X12 centimeters in size. The camera-box is mounted, by
means of ball-and-socket joint, upon a vertical rod which is joined
at the bottom to three short legs in such a way that the four
pieces may be folded together to form a stout cane 0.85 m. long.
The lower ends of the three legs of this tripod and the upper
end of the supporting staff are connected by twisted violin-strings.
Tension may be given them by turning the ratchet-wheels indi-
cated in Fig. 98, Plate XL VIII, thus producing a very light
and yet rigid tripod. The plates contained in the magazine M

l86 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

are exposed successively by being pressed against a metal frame
having marks indicating the principal and horizon lines. This
frame is securely fastened at a constant distance from the lens,
establishing the constant focal length of the camera. Each
plate, after exposure, may be dropped into the leather pouch
b, Fig. 98, Plate XL VIII, secured underneath the camera, by
manipulating a button a'. The leather pouch, together with
one dozen plates, weighs about 500 grams. Dr. Meydenbaur
has used a pantoscopic lens, made by E, Busch in Rathenow,
Prussia, which is said to produce geometrically correct per-
spectives of an angular field of 105®. A circular level L serves
to adjust the plates into vertical plane before exposure, using
the ball-and-socket joint for this purpose.

B. Capt, E. Deinlle's Surveying-camera (new Model).

The following description of Capt. E. Deville's new survey-
ing-camera is taken from Deville's *' Photographic Surveying,'^
second edition (1895). The camera is shown in Fig. 99, Plate
XLIX, and Fig. 100, Plate L; Figs. loi and 102, Plate LI,
represent sections of the instrument.

The camera proper is a rectangular metal (aluminum) box
AB, Figs. loi and 102, Plate LI, open at one end. It carries
the lens L and two sets of cross-levels CC, which may be read
through openings in the outer mahogany casing. The metal
box is supported in the wooden casing by wooden blocks and
by a wooden frame FFi held in position by two bolts DD.

The plate-holder is made for single plates; it is inserted
into the carrier EE, which may be moved forward and back-
ward by means of the thumb-screw G.

A folding shade, of wood, HH, Figs. loi and 102, Plate LI,
hooked to the front of the camera, and diaphragms KK inside
of the metal box, intercept all light-rays which do not contribute
to the formation of the image on the photographic plate.

SURVEYING-CAMERAS WITH CONSTANT FOCAL LENGTHS. 1 87

The camera rests on a triangular base, Fig. 103, Plate LI,
with foot-screws, in shape exactly like the base of the transit
theodolite which is used in conjunction with Deville's survey-
ing-camera, so that either instrument may be placed on the same
tripod at any station. The camera may be set up on the tri-
pod with either the long or the short side vertical.

Both transit and tripod are carried by the surveyor, and one
camera including one dozen plates (in the single holders) with-
out a jtripod are taken by one of the men who always accom-
pany the surveyor as packers. The assistant has a second
camera with plates and a separate tripod.

The legs of these tripods when folded together are twenty
inches long. They are placed in separate cases and one is carried
under the box of the transit, to be carried on the back of the sur-
veyor, and the other is attached to the sole-leather case of the
camera in charge of the assistant surveyor.

The lens of this camera is a 2feiss anastigmat, No. 3 of series V,
141 mm. focal length, with a deep-orange color-screen in front.

Having mounted the camera on the tripod, the plate-holder
carrier E is moved back as far as it will go by turning the screw G,
Figs. loi and 102, Plate LI; the plate-holder is inserted through
the opening M, the slide is withdrawn, and the carrier moved
forward by revolving the screw G until the plate is brought into
contact with the back of the metal box AB. In order to secure
a perfect contact, the carrier E has a certain amount of free
motion. The camera should now be turned in the proper direc-
tion; the field embraced by the plate is indicated by lines drawn
on the outside of the mahogany casing. The camera Is then
carefully leveled, the exposure given, and the plate-holder is
withdrawn by repeating the same operations in the inverse order
as described for its insertion.

For the sake of rigidity and to reduce the number of adjust-
ments to be made in the field to a minimum, the levels CC have
been fixed rigidly to the metal camera-box (without any device
for subseqeunt adjustment). They are, however, very closely

1 88 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

adjusted by the maker of the instrument. For this purpose he
takes the metal camera-box out of the mahogany casing and
deposits the same on a piece of plate glass which previously
had been leveled like an artificial horizon. By filing down
one end or the other of the levels' outer case, each bubble is
brought as near to the middle of its tube as possible. The
graduation on the latter is numbered continuously from end to
end, as illustrated in Fig. 104, Plate LI.

Each camera is accompanied by a piece of plate glass J inch
thick and 11 inches long, which can be inserted into the carrier
in place of the plate-holder. That end of the glass plate which
projects beyond the camera limits when it is thus inserted is
coated on the back with a varnish composed of gum guaiacum
dissolved in alcohol to which some lampblack had been added.
This coating has very nearly the same refractive index as glass
and is well adapted for precluding all reflections from the back
of the glass plate.

After the camera has been received from the maker the exact
readings of the levels, when the back of the metal box — ^against
which the photographic plate is pressed — is vertical, should
be ascertained. To do this, the bolts P, Fig. 102, Plate LI,
next to the opening M, are loosened and removed. Q may
then slide backwards and be taken out. The piece of plate
glass is now inserted in the carrier £, Figs. loi and 102, Plate LI,
and pressed into contact with the metal box. The camera is
placed on its tripod and leveled. Immediately in front and
at the same height a transit (or a leveling instrument) T, Fig. 105,
Plate LI, is set up, and after carefully adjusting it, a distant
point P is selected on the same level with the transit and camera.
The intersection of the cross-threads in the telescope is brought
to coincide with P, and the telescope is clamped to the vertical
circle. Turning it around in azimuth, the image of P, reflected
by the plate glass, should appear in the intersection of the tele-
scope's cross-threads. If it does, the face of the plate glass is
vertical and the position of the bubble in the tube of the level,

SURVEYING-CAMERAS WITH CONSTANT FOCAL LENGTHS. 1 89

directed at right angles to the plate glass, is the correct one for
adjusting the instrument in the future. If it does not, the camera
must be tilted forward or backward by means of the foot-screws
imtil coincidence is established. The middle of the bubble
of the level may or may not now be in the middle of the tube,
but its position, whatever it is, will be the correct one for adjust-
ing the camera in its subsequent use. The divisions of the
graduation between which the bubble rests should therefore
be ascertained and the middle reading be recorded, and when-
ever the camera is to be leveled, it must be remembered that
the middle of the bubble is to coincide with the recorded middle
reading.

This determination of the level-reading is to be made for
the two positions of the camera in which it is used (horizontal
and vertical position).

I. Determination of the Focal Length, the Horizon Line, and the

PRiNaPAL Line.

The next step is to fix the position of the principal point
on the photographic plate and to ascertain the length of the
distance line. Select a station so that a series of distinct and
well-defined distant points may be found on the horizon line
as it is laid down by the maker of the camera. The selected
view may comprise the distant shore line of a lake, a large build-
ing or a row of buildings. Set up the tripod and adjust the transit.
Find two points E and F, Fig. 106, Plate LII, on the horizon
line (with a zenith distance of 90®) that both come within the
field of the camera, when set horizontal, apd fall near the two
vertical edges of the plate. Measure the horizontal angle w
between them. Find two other points G and if, also on the
horizon line and such distance apart that both come within the
field of the camera when the same is set up vertical. Now replace
the transit by the camera in the horizontal position and turn
it so that E and F will appear within the limits of the plate, level

IQO PHOTOTOFOGRAFHIC METHODS AND INSTRUMENTS.

carefully, and expose the plate. Set the camera in the vertica,
position and turn it -in azimuth to take in G and H^ level care-
fully, and expose another plate.

The first plate, after development, shows the two points
E and -F on a line very nearly parallel to the edges AB and CD,
Fig. 1 06, Plate LII, of the metal box. The principal point, of
course, will be on this line, which is cut into the film, using a
fine needle point and straight-edge for this purpose.

The second plate, exposed in the vertical position of the
camera, will give another horizon line GH, which may be trans-
ferred to the first plate by means of the distances AK and CL
to the comers of the metal box. This (principal) line is like-
wise cut through the film of the photographic plate with a fine
needle point and straight-edge. The principal point P will be
at the intersection of these two horizon lines EF and GH.

The length of the distance line (5P=/), or the focal length
of the camera, may be computed from the horizontal angle a>,
included between SE and SF, together with the distances EP=a
and PF--b.

Let 5, Fig. 107, Plate LII, be the second nodal point of the
camera-lens, a and ^ the angles ESP and PSF, when

The lengths of a and b are measured directly on the plate.
If we designate the focal length PS by / we have:

a
tan a =-7-,

, b

tan^=j,

tanaXtan/?=-p.

SUKVEYING-CAMESAS WITH CONSTANT FOCAL U:NGTHS. I9r

Hence

tan ^a+j?)— tano)

a b

111
ab'

♦

p-p^j-ab^o.
^^ ' tan tt> '

Resolving this adfected quadratic equation we find

' 2 tan tt> N 4 tan^ w

Having now found the focal length and the principal point,
reference marks should be made on the edges of the metal box
to indicate the horizon and principal lines as well as the focal
length on the prints from the negatives.

Measure the distance m, Fig. 106, Plate LII, from P to AC.
From the corresponding comers A and C, Fig. 108, Plate LII,
of the metal box lay out m on AR and on CT, With a very-
fine and sharp file, held in the direction toward the lens, cut
into the edge forming the rear frame of the metal box a clean
and sharp notch at T and another at R.

Repeat the same operation from the comers A and B with the
distance n from P to AB.

The lines OQ and RT will be the horizon and principal lines
of the photographs, when the camera has been leveled to bring
the bubble into its proper position as mentioned in the foregoing.

From R and T measure the distances Rr, Rr', T/, Tf, equal

to one half of the focal length ( =~)« From O and Q measure

Oo, 0</, Qq, Qg^t equal to one quarter of the focal length, and
at each one of these points make another notch with the file

192 PHOTOTOPOGRAPHIC METHODS AND INSTRUBIENTS.

held in the direction of the lens. Every photograph will now
show twelve triangular projections reaching into the dark border
of the photograph. Four of these projections serve to fix the
horizon and principal lines; the remaining eight give the focal-
length value.

2. Adjustment of Camera Spiiut-levels.

It now remains necessary to find the correct readings of the
transverse levels (placed parallel with the sensitive plate) when
the horizon and principal lines pass exactly through their cor-
responding notches of the metal box.

Set up the camera again, facing the same distant view as
before, but in adjusting it bring the bubble of the transverse
level near one end of the tube; note the reading of the level-
tube graduation and expose a plate. When developed, it will
give a horizon line EF, Fig. 109, Plate LIT, cutting the border
of the negative in A and 5, at some distance from the pictured
notches O and Q, Now change the adjustment of the camera
by bringing the bubble of the transverse level to the other end
of the tube, note the reading of the level and expose another
plate. This when developed will give another horizon line
N'Py cutting the border of the negative in C and D.

Great care should be exercised in both cases to keep the
other level (the one at right angles to the sensitive plate) at its
proper reading, in order to expose both plates while in vertical
plan.

After measuring CO and OA or BQ and QD^ a simple pro-
portion gives the proper reading of the transverse level which
will bring the horizon line of the vertically exposed plate through
the two notches O and Q of the metal box.

The correct reading of the transverse level of the second
set of levels is found by the same method, with the camera in
the vertical position.

All these operations must be executed with great care and
precision (and with the help of a microscope of moderate power),

SURVEYING-CAMESAS WITH CONSTANT FOCAL LENGTHS. 1 93

as the subsequent iconometric plotting of pictured points is
based upon the determination of the ordinates and abscissae of
such points, on the pictures, with reference to the principal and
horizon lines which serve as a system of rectangular coordinates.
It had been assumed that the levels were placed very nearly
in correct adjustment by the maker as mentioned before. If
found too much out, they must, of course, first be approximately
adjusted by setting the metal box on a well-leveled plate. For
this purpose the plate glass supplied with each camera may be
set on the camera base and leveled like an artificial horizon.

3. Use of the Instruments Comprised in the Canadian
phototopograpmc outfti.

The instruments and tripod of the Canadian instrumental
outfit being very light, steadiness may be secured by means of
a net suspended between the tripod legs, into which a heavy
weight (rock) is placed. With this device, photographs of good
definition and better observations may be obtained than with-
out it, and there is no risk of the instruments (secured to the
tripod) being blown over during one of the sudden and strong
gusts of wind so frequently encountered on elevated and exposed
mountain peaks.

After the phototopographer has arrived at a triangulation
station he adjusts the transit and observes the azimuths and zenith
distances of all signals (marking the triangulation points and
camera stations already occupied) in the vicinity that may be
visible from his position. If accompanied by his assistant,,
each reads one vernier and records the readings independently^
of the other into separate record books. After the observa-
tions at that station have been completed the two surveyors
compare notes and any discrepancy that may be discovered in
the recorded data is corrected on the spot by a careful repetition
of the doubtful observations.

The Canadian camera is carried in a sole-leather case which
also contains twelve filled double plate-holders; when more

194 PHOTOTOPOGRAPHIC METHODS AND INSTRtJMENTS.

holders are needed for a day's work they must be carried in a
separate receptacle. Taking the camera from its case the level-
ing base, Fig. 103, Plate LI, is secured to it by means of the
central clamp-screw, and the camera is then placed upon the
tripod (from which the transit had been removed) without dis-
turbing the position of the latter.

The shade or hood if, Fig. 99, Plate XLIX, is now unfolded
and attached to the hooks at the front of the camera. A plate-
holder is inserted into the carrier and the number of the plate,
in position to be exposed, is noted upon a rough outline sketch
of the panorama views (commanded by the field of "the camera
image as indicated by the converging lines cut into the exterior
sides of the camera-box), entering also such notes as may be
of value for the subsequent development of the plate, for the
iconometric plotting of the topography photographed upon the
latter, or for the lettering of the finished map.

Having made sure that the cap is secure on the lens, the
slide is withdrawn from the plate-holder and the sensitized sur-
face of the plate is brought into direct contact with the frame of
the metal box by turning the screw G, Figs. loi and 102, Plate LI,
devised for this purpose. The surveyor next turns the camera
in azimuth until the lines on the upper face of the wooden casing
show that it is properly directed, or oriented, to include the
panorama section to be photographed between the lines. The
field of view should, of course, in each case coincide with the
outline sketch bearing the number of the plate in position to
be exposed. Sighting along the lines (up and down) shown
on the side face of the wooden camera casing, the observer can
assure himself whether the view on the image plate reaches high
or low enough to control the landscape; if it does not, he either
puts the longer dimension of the camera upright, or, if already
in that position, he may have to occupy a secondary camera
station, either above or below the one occupied, as the case
may be.

The observer next levels the camera carefully in the manner

SURVEYING-CAMERAS WITH CONSTANT FOCAL LENGTHS. 1 95

previously described and exposes the plate. Whenever the
sunlight appears inside of the front hood, the latter should be
shaded off during the exposure of the plate by holding some*
object (plate-holder slide or hat) above the hood. Under no
circumstances must the sun be allowed to shine upon the lens.

Every evening after the return to camp the surveyor replaces
the exposed plates (in the dark-tent) by new ones, using a ruby-
colored light for this purpose. He also marks the exposed plates
in one comer close to the margin, before removal from the holder,
with his initials, with the number of the dozen and nimiber of
plate, using a soft-lead pencil for this purpose. N. N. — ^IV — 5,
for instance, means plate No. 5 of the fourth dozen and exposed
by N. N. This would be the forty-first plate exposed in that
season.

The exposed plates are placed into a double tin or copper
box, Fig. no, Plate LII, which can be closed hermetically and
which will float when filled with two dozen plates (should it
be accidentally thrown into water by the capsizing of a canoe).
These boxes, as soon as filled, are shipped to the head office
in Ottawa, where the plates are developed by a specialist.

The data obtained by aid of the transit theodolite for tri-
angulation purposes are recorded in the field-books in the man-
ner customary for such work.

The horizontal angles observed with the transit (or altazi-
muth) to the terrene points (so-called " reference points ")
marked on the outline sketch, which should accompany each*
n^ative, serve not only for the orientation of the horizontal
projection of the plate on the working-plan — ^to " orient '* the
so-called " picture trace " — but they also aid materially to ascer-
tain the amoimt of (and to coimteract in a measure) the dis-
tortion of the paper prints or photographic enlargements. The
vertical angles, tc^ether with the plotted distances to such refer-
ence points, serve to check and verify the position of the horizon
line on the different photographs as given by the camera alone^

Important camera stations are occupied by the surveyor.

196 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

secondary stations by the assistant with a separate camera. No
trigonometrical observations are made by the assistant when
occupying secondary and . tertiary .camera stations, as he is not
supplied with a transit. The surveyor locates such stations by
observing upon the signals, erected by the assistant before leav-
ing the station, from his own station^, and he subsequently com-
putes their positions as " concluded points." All views are
taken with the same stop, j/7^6.

C. The U. S. Coast and Geodetic Survey Phototopo graphic Cameras.

In the preceding we have already referred to the desirability
that phototopographic surveying instruments designed for use
in rough moimtain districts, where transportation facilities are
restricted and generally confined to portage over rough trails
(or to transportation up steep mountain sides on the backs of
packers), should be made as simple as possible to indefinitely
remain in perfect adjustment.

The phototopographic party generally reaches the mountain
station, after an exerting climb of several hours, in a more or
less fatigued condition, and to obtain the best results the observer
should not be required to spend much time in assembling and
adjusting the instrument before the actual survey work may be
begun. Then, too, the atmospheric conditions are rarely stable
for any length of time, making it most desirable to utilize favor-
able conditions at once. The fields of labor in S.E. Alaska, where
the Coast and Geodetic Survey camera work has been done,
are peculiarly well adapted for the application of the phototopo-
graphic methods, on accoimt of the prevailing cloudy condition
of the atmosphere in the higher altitudes. Distant peaks and
mountain groups, without apparent .warning, become suddenly
shrouded in drifting mists, which soon gather into a thick cloud
stratum, shutting the mountains out from view for days and
weeks at a time. During the summer months the prevailing
southerly, vapor-laden winds drifting inland from the Pacific
Ocean find their moisture condensed on approaching the snow-

SURVEYING-CAMERAS WITH CONSTANT FOCAL LENGTHS. I97

and ice-fields or hanging glaciers in the higher altitudes. Clear
days are generally accompanied by calms or they occur when a
northerly drift in the air-currents prevails.

Phototheodolites or instruments in which the elements of a
transit and a camera are assembled into a single apparatus, all
parts being merged into a composite instrument, to remain
united during the various operations of observing and plate
exposures, mostly have the objectionable feature of unstable
adjustments, requiring frequent tests and readjustments of their
component parts. They are, furthermore, more or less cumber-
some and heavy, making them more liable in transportation
than instruments that may be divided into two or three parts
(each section being complete in itself) and carried by two or
three helpers.

The original type of the U. S. Coast and Geodetic Survey
camera, used in connection with the topographic reconnaissance
made in S.E. Alaska under the U. S. Alaskan Boundary Com-
missioner, was similar in form to Capt, Deville's original survey-
ing camera, except that it was provided with a separate tripod
with ball-and-socket adjustment and that the teeth or index
marks which serve to fix the principal and horizon lines on the
negatives could be pressed into direct contact with the sen-
sitized film of the photographic plate simply by turning a button
after withdrawal of the slide. This camera was provided with
a groimd glass, enabling the observer to inspect the entire field
controlled by each plate before exposure was made.

The camera proper was a plain rectangular box, made of
well-seasoned mahogany, 6fx6fX9J inches in size. It was
always used in the same position, with the short faces vertical.
A circular level attached to the upper camera side served for
leveling the instrument, bringing the photographic plate into
vertical plane.

The bamboo tripod legs were made in three sections, each
sixteen inches long and screwed together at the joints. When
dismembered, the tripod was carried in a sole-leather case or

198 PHOTOTOPOGRAPmC METHODS AND INSTRUMENTS.

knapsack, together with the camera, six double plate-holders,
note-book, barometer, etc. A yellow color screen could be
attached to the inner side of the camera-box just behind the
lens mount. The four-inch transit used in conjunction with
this camera had a separate tripod.

With a view toward simplicity in structure and a light weight
to be transported in the mountains, the new Coast and Geodetic
Survey phototopographic instrument has been made into three
distinct parts or sections, the transit, the camera, and one tripod,
serving for both.

The superstructure, embracing the Y's, telescope, and ver-
tical circle, may be lifted off the horizontal circle, to which it
ordinarily is secured by two capstan-head screws, uniting the
base-plate of the Y support with the vernier plate of the hori-
zontal circle. Plate XCVIII shows the transit as used for
trigonometric observations.

The camera, complete as such, may be mounted on the ver-
nier plate of the horizontal circle with the same capstan-head
screws that secure the superstructure of the transit. Plate XCIX
shows the camera-theodolite in its usual position (long sides of
the photographic plate horizontal). The truncated aluminum
cone under the camera-box is secured to the latter by means
of a central clamp-screw (within the hollow cone), and the
base-rim of the cone is then fastened to the vernier plate of the
horizontal circle with the two capstan-head screws already men-
tioned.

Both transit and surveying camera are used on the same
tripod. The understructure (with the horizontal circle) is con-
nected with the tripod by means of the triangular tripod plate
shown in upper part of Plate XCIX. This triangular plate is
screwed to the tripod and the three leveling-screws of the under-
structure are placed on the arms of this plate, a clamp device
securing the conical ends of the leveling-screws to the tripod
plate serves to prevent a possible disturbance of the under-
structure when the exchange from transit to camera is made.

SURVEYING-CAMERAS WITH CONSTANT FOCAL LENGTHS. IQQ

The adjustments of transit and camera are stable and with
ordinary care will suffer no frequent changes. To reduce weight
aluminum has been used when practicable without sacrificing
rigidity and strength.

The camera is packed in a stout packing-case, together with
eight double plate-holders, focusing-cloth, note-book, etc. The
transit is packed by itself.

A small triangular net or hammock, that may readily be
attached to the legs, should be provided when stations are occu-
pied in windy weather. These light instruments lack stability,
but by placing a suitable weight (a rock will do) in the net sus-
pended between the tripod legs no noticeable vibration will occur.

The irmer edge of the rear frame of the inner (aluminum)
camera-box is supplied with notches to mark the horizon and
the principal lines. The constant focal length (about 5^V32
inches) of the lens is also laid off on the inner edge of this rim,
one half to either side of the principal line and one half to one
side of the horizon line. All these notches will be printed on
the edges of the negatives, giving ready means for checking
distortions in the prints.

As will be noted, this camera-box is similar in form to Capt.
DeviUe's new model, having also the same lens (Zeiss anastigmat,
6JX8J, series V). The plates used in this camera are 5X8
inches. The outer box or casing is made of well-seasoned J-inch
mahogany reinforced with strips of brass. The sensitized film
of the photographic plate may be brought into direct contact
with the rear frame of the inner camera-box by means of a milled-
head screw attached to the front board of the outer box with a
coimteracting spiral spring similar to G, Figs. loi and 102,
Plate LI.

The lens is about 2V16 inches from one long side and 3^Vi6
inches from the other long side of the outer wooden case, making
the horizon line correspondingly nearer one long side of the
camera. When the main field of the terrene falls below the
station the camera is mounted with the lens Unv and vice versa

200 PHOTOTQPOGRAPHIC METHODS AND INSTRUMENTS.

Three sets of cross-levels are attached to the inner camera-box^
each being covered with a glass window inserted into the outer
wooden case, so that a set of cross-levels will appear on the upper
camera side for each of the three positions in which the camera
may be mounted. Plate C shows the vertical position of this
camera; it is used when the terrene to be pictured subtends
rather large vertical angles for both elevation and depression.

D, L. P, Paganinfs new Phototopo graphic Instrument for Re-
connaissance Surveys on Scales of i : 50000 and i : 1 00000
(Model 0} 1897).

To overcome the difficulties encountered in the topographic
reconnaissance work (i : looooo scale) in Eritrea (East Africa) —
due principally to the torrid climate — and in Sardinia (1:50000
scale) — on account of the danger of contracting malarial fevers —
L. P. Paganini has devised another surveying-camera, smaller,
more simple in form, and easier in manipulation than the type
just described.

This instrument (model of 1897) combines rapidity in the
field operations with a minimum expenditure of money, and it
materially reduces the period during which the operator has
to be exposed to the vicissitudes of climate and weather. It is
compactly built, essentially light in weight, and preserves the
various adjustments, when once made, almost indefinitely, at
least for a long time if the instrument is carefully handled.

This instrument, together with all accessories, compass,
frame, tripod head, shutter, dark-cloth, etc., may conveniently
be packed into a knapsack to be carried by a single packer, the
entire outfit weighing only about 15 kilogrammes.

This photogrammeter is composed of the following parts:

First. A photographic camera;

Second. A horizontal graduated circle attached to the ver-
tical axis of rotation, with superimposed alidade bearing
verniers and spirit-levels;

SXTRVEYING-CAMERAS WITH CONSTANT FOCAL LENGTHS. 20I

Third. An azimuth compass;

Fourth. A tripod with folding legs which may easily be
taken apart.

I. The Phototofogsaphic Camera Proper.

The camera of Paganini's latest photogrammeter is rigidly
constructed of aluminxmi. It has been given a prismatic form
with an equal-sided trapeze for base. The rear side of the
camera-box is formed by a metal frame which supports either
the ground glass or the sensitized photographic dry-plate. The
plates are 18X24 cm. in size and they are exposed with the longer
side horizontally.

Attached to the center of the camera front is a metal collar
or tube into which a tube may be screwed having a thread of
I mm. rise and holding the objective in the outer end. This
objective tube is provided with a flat ring soldered near the
objective end to the tube in such manner to leave a cylindrical
space between the ring and objective tube into which the fixed
collar— attached to the front side of the camera — ^may enter
when the objective tube is screwed into the camera collar. The
latter has a millimeter graduation on its outer surface extending
in the direction of the optical axis. The rear (beveled) edge
of the flat ring is divided into ten equal parts, and when the
objective is brought nearer to or farther from the image plane —
by revolving the lens tube about its axis — the beveled edge of
the flat ring will slowly be moved over the millimeter scale. Fur-
thermore (this beveled edge being divided into ten equal parts),
the position of this circular scale on the beveled edge with refer-
ence to the longitudinal millimeter scale will permit the focal
length to be read to tenths of a millimeter for any position of
the lens.

The objective of this camera is a Zeiss wide-angle anastigmat
with a principal focal length of 182 mm. With a small dia-
phragm stop it produces a picture of 40 cm. diameter, equivalent
to an angular field of 104°. With the diaphragm aperture //35

202 PHOTOTOPOGRAPHIC METHODS AND INSTRUliENTS.

it covers a plate of 20X26 cm. very well, and as the adopted
size of plate is only 18X24 cm. we obtain a very clear and sharp
definition over the entire plate even when using a larger dia-
phragm opening.

The perspectives obtained with this camera conmiand an
angular field of 67® horizontally and 53® vertically. With six
plates we can, therefore, cover ap entire panorama view (of 360°)
from which vertical angles of elevation or depression may be

deduced up to ^^ ( = 26° 30').

Two adjoining plates have a common vertical margin of
3° 30' in width. Horizontal angles between points falling within
this panoramic zone or belt of 53^ angular width may also be
deduced from the six plates iconometrically.

This instrument has been carefully constructed with optical
axis of the camera perfectly normal to the image plane, and
the point of intersection of optical axis with the image plane —
the principal point — ^is photographically transferred to every
photographic perspective as the point of intersection of the pic-
tures of two very fine silver wires crossing each other at right
angles. These wires are secured, quite close to the image plane,
in such manner to be easily removed and replaced by others
in case they should accidentally be ruptured. — Directly below
the camera-box, attached to its lower side or base, are three
Z-shaped metal arms with rectangular bends. One is placed
immediately below the objective and the other two in the rear,
below the cross-wires. The lower (horizontal) arms of these
metal Z bars are equidistant from the camera base, and they
are perforated by smooth circular openings which readily receive
three stout screw-bolts securely and permanently attached at
right angles to the alidade of the horizontal circle. Each one
of these bolts has two nuts of which the lower one supports its
corresponding Z-bar arm, while the upper nut has been added
as a locking device to secure the position of the lower nut together
with the corresponding Z-bar arm at any desired height, after

SUSVEYING-CAMERAS WITH CONSTANT FOCAL LENGTHS. 20$

the proper adjustment of the camera's position on the horizontal
circle has been made.

2, The Horizontal Graduated Circle.

The horizontal circle of this photogranmieter has a diameter
of 14 cm.; it is graduated into half-degrees and reads from o^
to 360®. The vernier reads to single minutes, but 30 seconds
may be estimated. The vertical axis of rotation of the instru-
ment projects above the plane of the alidade and the latter is
secured in a plane at right angles to the axis of rotation by means
of a stout collar covering the latter.

The instrument is supplied with three leveling-screws, clamp-
and tangent-screws, spirit-levels, etc., the general arrangement
being the same as for any other surveying-theodolite.

The metal tripod head supports a circular spirit-level for
an approximate adjustment into position of the vertical axis,
the final leveling being accomplished by means of the three
leveling-screws, which form the support of the instrument on
the tripod head, together with a pair of cross-levels attached
to the alidade surface.

The superstructure is secured to the tripod head, precluding
an accidental upsetting, by means of a central clamp-screw with
spiral spring and a long handle for an easy manipulation between
the tripod legs, the same as shown for the other Italian photo-
grammeters.

3. The Azimuth Compass.

On the upper surface of the camera-box is a cylindrical recep-
tacle of pure aluminum, to receive the magnetic compass which
is modeled after the so-called Dixey or Schmalkalder pattern,
so well adapted for topographic work. It has the usual pris-
matic eyepiece and vertical hair-sight, which are permanently
fixed in the vertical plane which passes through the optical axis
of the camera objective.

204 PHOTOTOPOGRAPHIC MEIHODS AND INSTRUMENTS.

4. The Tripod.

The tripod of this photogrammeter is practically the same
as has been described for the other Italian phototheodolites,
except that the legs are folding and may be carried together
with the instrument by one packer.

5. Adjustments and Use of this Instrument.

Before conmiencing the regular phototopogfaphic survey of a
given area with this instrument, certain observations should
be made which, however, will serve equally well for all sub-
sequently occupied panorama stations, and certain adjustments
should be made or verified which, owing to the stable and solid
construction, will also remain in stabiliment for a long time
thereafter.

Of course, the operator should, after all the adjustments
heve once been made, check them from time to time, particu-
larly when it may be supposed, or when it is known, that the
apparatus had been subjected to unavoidable shocks or care-
less handling by packers while in transit over rough roads or
trails. With an adjusted instrument and careful handling the
final adjustment that will be necessary to be made before occupy-
ing the camera station for observing reduces itself to a small
matter.

The first step when the phototopographic survey of a given
area is to be made consists in a most accurate determination
of the constant length of the focal distance, which is preferably
done on a bright day in the following manner: The objective
is moved back or forth until some far-distant but well-defined
points of the terrene appear well outlined and quite sharp in
definition on the ground-glass plate under the dark-cloth, using
a small magnifying-glass to ascertain the position of the plate
(the focal length) giving the best definition. It is advisable to
focus in this manner successively upon several well-defined
distant points, reading the scale indications for each point focused

SURVEYING -CAMERAS WITH CONSTANT FOCAL LENGTHS. 205

upon and entering the records in the field-book. We will thus
obtain a definite value for the principal focal length, as indi-
cated by the mean of the recorded scale- readings.

We next determine the diaphragm aperture which is to be
used for the entire panorama, selecting the smallest stop that
will yet give a uniformly good definition for the entire field con-
trolled by the plate — 60° in azimuth and 45° in altitude — Shaving
special reference to a good definition of the summits of distant
mountains that may be shown on the perspective.

The adopted diaphragm aperture — the instrument in ques-
tion has a revolvable diaphragm disc with numbered apertures
of graduated sizes — at different hours of the day, at different
altitudes, and under different illumination (" light-intensity ")
will only in part control the length of exposure, as the latter ako
depends in great measure on the rapidity ('* sensitometer num-
ber ") of the plate that may have been selected for use in the
survey, and on the so-called rapidity of the lens, together with
the color screen, if such is used.

A systematic exposure of a few trial plates under different
conditions regarding hour of the day, elevation of station above
sea horizon, and illumination — lens, diaphragm aperture, and
plate remaining the same — will give valuable data for future
reference and for judging the required length of exposure cor-
rectly under similar conditions.

One indispensable condition to be fulfilled when proceeding
to make the final adjustments of this instrument would be to
make a complete turn of the camera about the vertical axis,
very much the same as when exposing the plates for a complete
panorama. If the instrument is well assembled with all its
parts in rigid adjustment, and if the tripod is securely placed in
position on firm ground and no disturbing causes interfere, the
instrument should, while being thus rotated, maintain its axis
(and its image plane) perfectly vertical.

To realize this condition the cross-levels attached to the
upper surface of the alidade should be well adjusted. This

2o6 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

may be accomplished by means of the three levelitig-screws
which transverse the tripod head and form the support of the
superstructure, in precisely the same manner as such adjust-
ment would be made with an ordinary surveying-theodolite.

After establishment of the verticality of the axis of rotation
there still remains the adjustment into position of the rigid system
of the three orthogonal axes, which have their common origin —
point of intersection — in the principal point of the perspective.
This system of coordinates is composed of:

First. The optical axis of the photographic camera, repre-
senting the principal ray of the photographic perspective.
Second. The two axes intersecting each other at right angles —
one horizontal and the other vertical — which are visible
on the ground glass and which are photographically trans-
ferred to the photographic perspective, where they repre-
sent the horizon and the principal line respectively.
The rectangular intersection of these two lines (last named)
is perfectly obtained by the instrument-maker. Two very fine
silver wires are strung quite close to the image plane, where
they are held in position by thin metal plates, the required ten-
sion being applied by means of pressure-screws. The correct
position of these cross-wires with regard to the rear metal frame
of the camera-box is assured by four fine incisions made with a
dividing-machine into the rear metal surface of the frame against
which the ground-glass plate or the sensitized photographic
plate rests when they are in position.

The degree of accuracy with which this rigid system of axes
is placed into position (and the degree of accuracy in their direc-
tions regarding horizontality and verticality) in a great measure
controls or determines the attainable accuracy in the iconometric
plotting, based upon pictured points referred to that system
of lilies as coordinates.

The optical axis of the camera, which by construction inter-
sects the perspective plane — the image plane — at right angles
in the principal point, should be horizontal, and therefore the

SURVEYING-CAMERAS WITH CONSTANT FOCAL LENGTHS. 20J

optical axis should intersect the plane containing the two axes
of coordinates (the cross-wires) also at right angles. It is fur-
thermore required that the wire representing the horizon line
of the perspective be horizontal, or, in other words, the optical
axis of the camera and the horizon line of the perspective are
to be in the same horizontal plane — that is, in the horizon plane
of the camera station.

This condition may easily be fulfilled by using the screws
which support the three Z bars as three leveling-screws. The
three lower arms of these Z bars are first placed approximately
at equal altitudes above the plane of the alidade circle (which
previously had been leveled) by means of the supporting nuts,
having first loosened the upper counter nuts to permit the Z-bar
arms to move freely over the upright screw-bolts. Now observe
the image on the ground-glass plate and bisect a distant point
with the pictured intersection of the cross-wires. Change the
pointings to the distant bisected point to different positions along,
the line defining the horizon by revolving the camera in azimuth
from left to right, or vice versa, at the same time raising or lower-
ing the screws which support the rear Z-bar arms of the camera,
until a point is found bisected by the intersection of the cross-
wires, which, when turning the camera about the axis of rotation,
does not leave the horizon wire, rather continues to be bisected
by that wire while being moved over it from one extremity to
the other, falling neither above nor below that wire, during the
full azimuthal swing through the entire length of the ground
glass.

If this distant bisected point was not in the horizon plane
of the camera-theodolite, the curve which it describes on the
ground-glass surface — during the revolution of the camera through
an azimuthal field of 60° — will be traceable with the eye, the
point while in transit will pass over the vertical wire above
or below the point of intersection of the two cross- wires, according
to its position in nature, whether below or above the horizon,
plane of the instrument.

2o8 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

The farther removed from the intersection of the cross-wires
the crossing of the pictured distant point over the vertical wire
is the greater will be the curvature of its hyperbolic trace on
the ground-glass plate and the greater will be the vertical dis-
tance between the bisected point and the horizon plane.

From the preceding remarks it will be evident that the Z bars
supporting the camera in the rear may be adjusted by simply
watching the courses of points of different elevation as they are
traced (by the points while in transit) on the ground glass during
the azimuthal swing of the camera. If the distant point, having
the same elevation as the optical axis of the camera, leaves the
horizon wire and passes over the vertical wire above the inter-
section of both, the supporting Z bars (those under the rear
frame) arc to be lowered and vice versa. The distance between
the point of crossing and the point of intersection of the cross-
wires gives a measure for the amount of change to be made in
the elevation of the rear Z-bar supports.

After a few carefully made trials, the position of the horizon
^ire will be such that any point bisected at one extremity of
the horizon wire will continue to be bisected by that wire during
the revolution of the camera through a horizontal field of 60°.
"When this has been accomplished the counter-screws of the
upright screw-bolts are tightened to secure (the Z bars) the
camera in this position with reference to the horizontal circle.

This adjustment once made and secured will be maintained
undisturbed for a long time and the horizon wire will now coin-
cide with the horizon line of the photographic perspective for
all positions of the optical axis, when the camera is rotated about
its axis during the exposures of the plates comprising a pano-
rama, provided, of course, the phototheodolite's axis of rotation
has remamed vertical.

The final adjustments to be made in the field after the
phototheodolite has been placed in position at a carefully
selected phototopo graphic station may now be summed up as
follows:

SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 209

First. A^djust the camera's objective to make the principal
focal length, previously ascertained and recorded in the
field-book, agree with the reading on the " objective
scale."
Second. Verify the position of the rotary diaphragm in order to
have the particular aperture in position that may be required
for the particular brand of plate under the particular
conditions of the atmosphere at the place occupied. Also
verify the setting of the time-shutter to see that the expos-
ure will be correct for this particular diaphragm aperture,
plate, illumination, altitude, and subject, although the
latter plays a minor r6le here, as it seldom varies much ia
phototopography.
Third. Verify the level adjustments and see that the axis
of revolution of the instrument is vertical, as the hori-
zontality of the optical axis, the verticality of the image
plane and of the " principal " wire, as well as the hori-
zontality of the ** horizon " wire, depend thereon.
The preceding descriptions show that much has been done
in Italy towards pushing photographic surveying to a high state
of perfection, and we are particularly indebted to Paganini for
numerous improvements in phototopographic and iconometric
instruments, including methods of their use for topographic and
hydrographic surveys. Paganini's good results, his experience,
and advice have materially aided in the decision in favor of the
phototopographic method in a number of surveys, particularly
for the survey of the mountains of the Caucasus (and Tiflis)
under the direction of Baron von Steinem.

m. Surveying-cameras Combined with Geodetic Instruments.
(Phototheodolites, Phototachymeters, Photographic Plane
Tables, etc.)

The data obtained in the field with the photographic-sur-
veying cameras considered in the preceding paragraphs had to

2IO PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

be supplemented with instrumental field observations to gain a
complete topographic control of the territory traversed by the
phototopographic surveying party.

The idea of combining surveying instruments with a camera
into a compact and serviceable apparatus originated very early
with phototopographic workers. Refined phototheodolites are
to this day the favorite photographic-surveying instruments not
only in Europe, but they are also widely in use in other countries,
particularly when these are dependent on European mechanicians
to supply the demand for instruments of precision.

Phototheodolites have been devised to secure great precision
in the results obtained with them; refined methods are largely
employed in the field observations in the culling of data from
the photographic perspectives, and in the computations.

Generally speaking, the best results for topographic pur-
poses are obtained by methods that have been devised with
due reference to the fact that phototopography essentially
and primarily is a constructive or graphic art, based on graphic
or pictorial records, in the form of perspectives, .that are to
be transposed into orthogonal projections in horizontal plan,
instrumental observations being required only to furnish such
elements as may be needed to facilitate the graphic trans-
position of lines of direction and distances, to insure accu-
racy, and to obtain certain checks or a proper control for the
work in its entirety.

It has already been pointed out in the preceding chapters
that phototopography is based essentially on the same methods
which are followed in topographic plane-table surveys, and the
best results may be expected when the surveying-camera is used
with the same 'object in view which the plane-tabler essays to
obtain. To increase the degree of precision the plane-tabler
will occupy a greater number of stations, and similarly in photo-
topography any degree of accuracy may be attained by increasing
the mmiber of camera stations for any given area.

Photographic surveys have been conducted principally in

SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 211

regions where other surveying methods are either precluded or
where their application would entail great cost and consume
too much time and such regions are characterized chiefly by
a rugged and broken topography. The necessity, therefore,
lies close at hand to devise instruments which will not easily get
out of order or drop to pieces when transported over rugged
mountain trails; the more simple their structural composition
the better adapted they will be for the production of rapid and
accurate work.

In Europe phototopography has generally been employed
for surveys plotted on large scales, necessitating the occupation
of numerous stations, with a resulting slow progress from one
locality to another. Then too the instrumental outfit could
readily be brought very near, if not actually to the very place,
where the work was to be done, by convenient and safe means of
transportation. The instruments are very seldom exposed to such
primitive and rough methods of transportation over long dis-
tances, as generally has been the case on our continent when
surveying-cameras have been used.

It is evident that the combination of a camera and a surveying-
instrument into a well-united, well-balanced, easily manipulated,
and essentially light and withal rigid instrument is not easily
accomplished. It is not surprising, therefore, that we meet
with a great number of types of phototheodolites and other photo-
grammeters in which the difiiculties in construction have been
overcome, more or less successfully, by various devices. In
the following we shall describe the principal types of photo-
graphing-surveying instruments that are either of historical
interest or are in use at this date.

A. L. P, PaganinVs Photo grammetric Instrument (Model of 1884).

The Italian photogranmietric apparatus devised by L. P.
Paganini, model of 1884, is illustrated in Figs, iii and 112,
Plates LIII and LIV. It is supported by a tripod which may

212 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

be dismembered into the tripod head H and three " alpenstocks "
A, The instrument proper may be separated mto two parts,
the camera-box C and the Y supports with eccentrically located
telescope T,

5', 5', 5' indicate the 3-foot screws — only two are visible in
the illustrations on Plates LIII and LIV — which form
parts of the tripod head H and which serve to level
the theodolite.
•Si, 52> ^3 represent three leveling-screws which support the
camera proper and which serve to adjust the position
of the cross-vjdres affixed to the rear frame of the
camera-box. The camera C is connected with the
upper limb of the theodolite by means of a catch-
lever K in such manner that the azimuthal revolu-
tion of this limb will also rotate the camera hori-
zontally.

i is a spirit-level attached to the telescope T", both being
supported by an upright or Y support U secured at
right angles to the horizontal limb of the theodolite
and at one side of but close to the camera.

T is an ordinary surveying-telescope (astronomical)
and it is provided with the usual cross-hairs (one
vertical and the other horizontal), adjustable in the
customary manner.

C represents the camera-box. It is made of hardened
pasteboard, which is strengthened by a metal skele-
ton frame or casing 5.

The camera is supplied with an aplanatic objective
(" antiplanat "), made by Steinheil, having a focal
length of 244.5 ^^'
The aperture in the diaphragm has a diameter of 5 mm.

Regarding the general arrangement of this instrument it may
be said that:

SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 213

First. The optical axis of the photographic lens (objective)
is parallel with that of the telescope T and it always is
perpendicular to the picture or image plane.
Second. The intersection of the optical axis of the camera
and picture plane — the principal point of the photographic
perspective — is marked by the point of intersection P,
Fig. 113, Plate LV, of two very fine and adjustable plati-
num wires OO' and jf securely fastened to the rear frame
of the camera-box, very close to the image plane;
When the instrument is leveled and in adjustment one
of these fine wires {OO') will be horizontal, while the other
(//') will be vertical, and both will be in a (vertical) plane
parallel to the image plane.
The optical axis of this camera may be brought into hori-
zontal plane by rotating the same about the horizontal axis CCy
Fig. Ill, Plate LIII, and clamping the screw h. In this position
the image plane and plane containing the platinum cross-wires
OCy and //' will both be vertical.

The horizontal wire OO' may be adjusted into horizontal
plane, after the instrument has been carefully leveled, by find-
ing some easily identified and readily recognized point on the
groimd-glass plate, which is bisected by this wire OO'j and by
gently revolving the camera in azimuth. If the wire OO' is in
horizontal plane, the observed point will be seen to move over
the entire length of the wire while the revolving motion is given
the camera. Should the bisected point, however, appear above
or below the wire 00' at any time during the azimuthal revolu-
tion of the camera, the same will have to be adjusted into hori-
zontal plane by aid of the two front screws S2 and ^s, Figs. 11 1
and 112, Plates LIII and LIV.

The camera is provided with a short tangent or slow-motion
screw /, by means of which the same may be slightly moved
in azimuth, while the telescope T and horizontal limb of the
theodolite remain stationary. This arrangement will enable
the observer to place the optical axis of the camera parallel to

214 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

that of the telescope T, provided both had been adjusted in hori-
zontal plane. This correction is made by " pointing " the tele-
scope to some well-defined distant point and clamping the the-
odolite in this position. The camera is now moved by means
of the tangent-screw / to the right or left until the same point
appears in the intersection P of the two camera wires OO'
and //'.

The prints of the camera cross- wires OCy and jf appear on
every negative taken with this instrument, and as their plates
were exposed while vertical to the optical axis of the camera,
the perspectives that are obtained (after the instrument had
been adjusted as described) are in vertical plan, each showing
the principal point of view P, as well as the principal and horizon
line /'/ and 00' , intersecting each other in P at right angles. The
horizon line OO' on the picture represents the intersection of the
horizon and picture plane. All points on the picture bisected
by the horizon line have the same elevation (disregarding the
error due to curvature and refraction) as the optical axis of the
camera at the station whence the picture was taken.

In place of the fixed platinum wires some photographic-
surveying instruments have four sets of teeth (or a series of notches)
attached to the rear frame of the camera-box, dose to the picture
plane (Fig. 114, Plate LV). If prints are used for the map
construction instead of the plates, this arrangement is preferable
to the fixed wires, as the latter often obscure details and as the
prints may be distorted to such a degree that the lines OO' and
if may have to be corrected, thus giving two sets of lines across
the face of the print. When only the ends of the cross-wires are
indicated on the pictures by means of the teeth, the correct posi-
tions of the cross-lines may be ascertained or checked experi-
mentally and the lines are then drawn across the face of the
picture by very fine lines in red ink.

Great care should be exercised in the proper location of those
lines, as they form a rectangular system of coordinates to which
every pictured point that is to be mapped must be referred. They

SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 215

also play an important part in ascertaining the value of the focal
length of the picture, which is one of the principal elements
required in iconometric plotting.

Fig. 112, Plate LIV, shows the camera in a position to take
a picture of terrene so far below the camera horizon that the
plate when exposed in vertical plane would not "take it in."
The construction of the instrument will permit a depression
(or an elevation) of the optical axis of 30® below (or above) the
horizon by loosening the clamp-screw and revolving the camera
about the secondary axis of rotation. (See paragraph on in-
clined picture plates.)

Constant Focal Length of the Italian Cameras. — ^Fig. 115,
Plate LV, shows the longitudinal section of a surveying-camera
with the diaphragm AB in position between the lens doublets.
The aperture of the diaphragm is taken as 5 nun. in diameter.
Only such rays of light emanating from a point N in nature
will reach the point n on the image plate // that form a cone
about the central ray nON as axis, with apex in n and base in O.
For the case illustrated in the diagram (Fig. 115, Plate LV),
that base will be an ellipse with 5 mm. length for the short axis,
while a pencil of light emanating from a point C on or very close
to the optical axis of the objective would be intercepted by the
plane of the diaphragm AB 'm 3, circle of 5 nmi. diameter.

The Italian lens is so focused that even for the largest aperture
of diaphragm used, all points from 10 meters to infinite distance
from the camera-lens O, Fig. 115, Plate LV, will be clearly
photographed with a maximum error in definition of 0.06 nmL
(for 10 m. distant objects).

If a = distance of object from the point O (10 meters to infinite
distance);
/= principal focal length of the camera (240 nun.);
ft = focal distance, variable for different lengths of a;
we find from the well-known relation

J'a'^J'

2l6 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

a-f

By adopting 240 mm. as value for /, and substituting diflferent
values, from i meter to 300 meters, for a in the preceding formula,
we obtain the following values for b:

a (in m.) «=» i 10 20 30 40 50 75 icx> 2cx> 3cx> 00
b (in mm.)"> 315.8 245.9 242.9 241.9 241.4 241. i 240.7 240.5 240.2 240.02 240.00

The error, therefore, in maintaining the focal distance constant is
6 mm. if the object is 10 meters distant from the nodal point;
it is I mm. if the object is 50 to 100 meters distant and it is in-
appreciable if the object is 300 m. or more distant from the nodal
point of the camera-lens.

The value ( — ) of the error (lack of definition or distortion)

produced in the photograph for points or objects at different
distances (a), when maintaining a constant focal length, may be
seen from the following: Assuming again that the image plane //,
Fig. 115, Plate LV, be held in a fixed position and 240 mm.
distant from the nodal point of the lens, it will be evident that the
image plane // (Fig. 115, Plate LV) will intersect some of the
light pencils or cones of rays (passing through the aperture O of
the diaphragm) in a circle (or in an ellipse) instead of intercepting
their apex. We see from an inspection of the foregoing table that
this circle of diffused light will increase in size with a decreasing
distance (a) of the object to be photographed. The true point
would be the center of the circle and the length of its diameter
X may be ascertained from the following relation (Fig. 116,
Plate LV):

xiO^fiOf

f'O

x=- — .
a

SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 21 7

Again assuming (Fig. 116, Plate LV)

/=240 mm. (principal focal length),

a=i meter to infinite distance, and

0= diameter of aperture in diaphragm = 5 mm.,

we find the following values for x from the preceding formula;

a (in m.) =■ i

100

X (in mm.)=i.2o

0.12

0.06

0.04

0.03

0.025

0.016

0.012

a (in m.) ^ 200

300

400

500

700

1000

X (in mm.) —0.006

0.004

0.003

0.002

0.00 1

0.000

The diameter x of the circle (or ellipse) is evidently quite
small, and a constant focal distances may well be maintained for
all phototopographical work without producing any appreciable
error.

In order to enable the observer to obtain good definition
in the pictures of objects not very distant from the camera the
Italian apparatus was devised with a movable objective and pro-
vided with a metal scale (a, Fig. iii, Plate LIII) extending in
the direction of the camera axis, which reads zero (or rather 240
mm.) when the camera has been focused upon objects at infinite
distance. The millimeter graduation of this scale, extending
in the direction toward the sensitive plate of the camera, enables
the observer to measure the focal length directly if the same
had been changed at any time from the principal focal distance
( = 240 mm.). The objective cylinder Af, Fig. 11 1, Plate LIII,
may be moved in the direction of the camera axis by revolving
it within iV, both tubes N and M being connected by means
of a screw, the rise of its thread being i millimeter.

The circumference of N is divided into ten et[ual parts, and
the position of the metal scale a, passing over this graduation,
when the objective tube M is screwed into iV, will indicate the
tenths (and estimated hundredths) of millimeters which it had
been moved beyond the number of millimeters read off on scale a.

The focal length plays a very important r61e in all photo-

2l8 FHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

topographic work, and it is advisable to verify, at the beginning
of operations, the reading of the metal scale, and if the principal
focal length has been changed, the diflference must be entered
into the note-book, so that the proper correction may subsequently
be applied.

The distance of the point of view from the perspective plane,
the position of the principal line, and the correct position of the
horizon line can always be ascertained or rectified by instru-
mental observations and computations, or graphically (if the
picture plane has been exposed in vertical plan or if its deviation
from that position be known) as has been indicated, and as will
be shown more fuUy later.

It has been described how the optical axes of the telescope
and of the camera are brought into two vertical and parallel
planes. Both may be kept in this position and yet be revolved
about the vertical axis of the instrument in order to successively
expose the plates covering the entire panorama. The horizontal
limb of the theodolite is divided into 360^ with subdivisions reading
to 20', and by means of two verniers 30" may be read off. The
vertical circle is provided with the same graduation and similar
verniers. Thus the means are provided to ascertain the azi-
muthal positions of the camera axis (the principal ray) for each
perspective, or the means of " orientation " are thus provided
for. The magnetic azimuth of the principal ray of the perspec-
tives (i.e., direction of optical axis for each exposure) or the hori-
zontal angle which is included between said ray and any other
line passing through the station and some known point on the
photograph (e.g., trigonometrical point) may readily be ascer-
tained by observation.

All perspectives that are to be used for mapping must be
obtained from stations with known geographical positions. Gen-
erally trigonometrical points are selected for the camera stations,
but if points beyond these have to be occupied to better con-
trol the topography, the elements needed (horizontal and ver-
tical angles) to determine their positions with respect to sur-

SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 219

rounding triangulation points may readily be observed with
the theodolite before leaving the detached camera station.

B. L. P. PaganinVs new Phoiotheodolite {Model of 1894).

The following description of PaganinVs new phototheodoliie
has been extracted from his "Nuovi Appunti di Fototopagrafia,"
Roma, 1894.

Paganini's new phototheodolite, model of 1890, differs from
the one just described, although the general form and the dimen-
sions of the camera-box, as well as the focal length of the lens,
remain about the same as with the older model. The principal
change rests in the omission of the eccentric telescope, which
has been replaced by the centrally mounted camera, which may,
at will of the observer, be converted into a surveying-telescope.

The telescope which we generally find attached to surveying-
instruments consists of a tube, slightly conical in shape, having a
positive lens (or a system of convergent lenses) at one end, known
as the "objective," which produces within the telescope a real
and inverted image (the same as the camera-lens) of any object
towards which this telescope may be directed. The other smaller
end of the telescope-tube has a still smaller tube inserted into it
which may be moved in the direction of the axis of the tube.
This second tube also contains a system of convergent lenses —
it is the so-called " ocular " lens set or " eyepiece " of the telescope
— ^which serve to project an enlargement of the image — formed
in the telescope — upon the retina of the observer's eye.

In the image plane of the objective the so-called diaphragm is
placed; it is a ring-shaped metal disc to one side of which a pair
of cross-hairs is attached in such a way that the hairs (spider
webs or lines cut into the surface of a thin piano-parallel glass
plate) will coincide with the image plane. One hair is horizontal
and the other vertical, their point of intersection falling in the
optical axis of the telescope.

A suitable eyepiece had only to be combined with the objec-
tive of the older camera model to convert the camera into a

220 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

telescope. The eyepiece of the camera telescope, camera model
of 1890, consists of a positive lens set, known in optics as Rams-
den's ocular lens.

The inner wall surfaces of the camera-box should be well
blackened to avoid side reflections and a consequent dimness in
the appearance of the cross-wires of the camera telescope.

The camera proper consists of two parts, a truncated pyramid
A, Figs 1 1 7-1 1 9, Plates LVI-LVIII, and a cylindrical attachment
B containing the tube /. A second tube, placed v/ithin the tube
/, may be moved in the direction of the optical axis by means of
a screw the threads of which have a rise of i millimeter. By
rotating this inner tube the lens may be brought nearer to or
farther from the image plane, the lens remaining parallel to the '
image plane at any position that may be thus given it.

A scale a. Fig. 117, Plate LVI, graduated to millimeters, is
permanently attached to the tube / and it lies very close to the
ring n, the circumference of which is divided into ten equal parts
(this graduated ring n is soldered upon the cylinder u encasing the
camera-lens). This scale a (extending in a direction parallel to
the optical axis of the lens) has a mark coinciding with the index
rim of the ring n, thus indicating the focal length of the camera-
lens when focused upon objects at infinite distance. The milli-
meter graduation of the scale a, extending from the zero mark
towards the ground -glass, serves to ascertain the focal lengths
for objects nearer the camera. The graduation on the ring n
serves to read one tenth of one revolution of the tube w, which is
equal to an axial motion of the lens of o.i nun., hence the focal
length for any object focused upon may be read to single milli-
meters on the scale a and to tenths of a millimeter on the graduated
ring n.

The construction of this phototheodolite is such that the optical
axis of the camera-lens is always at right angles to the picture
plane (the ground -glass surface or sensitive film of the photo-
graphic plate). The intersection of the optical axis and the
picture plane (the so-called principal point of the perspective) is

SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 221

marked by the intersection P, Fig 113, Plate LV, of two very
fine platinum wires, 00' and //', one horizontal and the other
vertical. They are stretched across the back of the camera-box
as close as possible to the picture plane. The buttons ft, Figs. 117
and 118, Plates LVI and LVII, serve to impart tension to the
wires. The horizontal line 00' corresponds to the horizon line
and the vertical line //' corresponds to the principal line of the
perspective represented by the image on the ground-glass surface.

Fig. 119, Plate LVIII, shows the rear view of this instrument,
the ground -glass or focusing plate having been replaced by an
opaque plate stiffened by a metal frame, which supports the
Ramsden eyepiece in the center in such manner that its optical
axis coincides with that of the camera-lens. The cross-wires OO'
and //', at the rear of the camera-box, serve also for the astronom-
ical telescope, into which the camera may be converted by attach-
ing the opaque plate with central eyepiece as shown in Fig.
119, Plate LVIII.

The fitting of the eyepiece allows for axial motion to adjust
its position with reference to the cross-wires to avoid parallax.
The opaque plate supporting the eyepiece is composed of a thick
cardboard impregnated with chemicals to harden its fibers and to
render it impervious to moisture. The camera-box is made of the
same material and both are strengthened by a frame and ribs of
metal, as indicated in Figs. 117 and 118, Plates LVI and LVII.
The cylindrical tube B is inclosed by a metal collar C which is
held in position within the metal ring //' by four screws if, i?', 5, S',
The ring W is connected with the frame gg^ by means of two arms
Ig and I'g^, all being cast in one piece. The pivots q attached
to the frame gg^ serve as horizontal axis of rotation for the
camera.

This instrument is provided with a vertical circle, horizontal
circle, verniers, reading-microscopes, levels, clamps, and tangent-
screws, forming a complete transit with centrally located camera
telescope.

A cross-section of this instrument is illustrated in Fig. 120,

222 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

Plate LIX. The scale a, already described, is here placed on top
of the tube u, to better illustrate its use.
yy= uprights forming the supports of the horizontal axis of
rotation for the camera telescope;
fe= upper horizontal limb, or alidade, supplied with two verniers;
JT^ lower limb, or horizontal circle, bearing the graduation;
rr« tripod head, supportmg the instrument by means of three
leveling-screws (W);
a = casing for conical center;

5^= central clamp-screw entering a ball which is supported by
the hemispherical socket w of the lower part of a. ((
secures instrument to tripod-head and it guards against
an accidental falling off of the instrument.
The horizontal circle, having a diameter of 17 cm., is grad-
uated into 20 minutes and suitable verniers are supplied to read
horizontal angles to 30 seconds.

The vertical circle, with a diameter of 10.5 cm., is graduated
into 30 minutes and its verniers ^ead to single minutes.

The photographic plates are 18X24 cm., which is the same
size as for the older instrument, model 1884.

The objective lens, at first selected, was an aplanat of Stein-
heil, and it had a focal length of 237.7 nwn. More recently,
however, the ItaUan phototheodoUtes have been supplied with
anastigmatic lenses of Zeiss.

The column £, Figs. 11 7-1 19, Plates LVI and LVIII, forming
a prolongation of the lower arm /'^, is held in place by two
counter screws m and w', which serve to hold the horizontal axis
of rotation of the camera in a fixed position, preventing accidental
changes that might otherwise take place during the execution of a
set of panorama pictures.

By unscrewmg the nuts d\ Fig. 120, Plate LIX, the tripod
legs may be removed and they may then serve as *' alpenstocks '*
during the transportation of the instrument from station to sta-
tion. The " camera telescope " may be lifted out of the Y's and
packed separately, the lower part of the instrument — the sub-

SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 223

Structure — is packed in another carrying-case, while the plate-
holders and plates are transported in a third case.

C L. P. Paganini^s Photographic Azimuth Compass (PhotO'
graphic *' AzimuUUe ").

During the last years of Paganini's service as an officer in the
Royal Navy of Italy, while in command of the cruiser " Tripolis,"
he was engaged upon work connected with making descriptions
of the coast (*' coast-pilot work ''), and with hydrographic surveys
for the construction of harbor and sailing charts. This work,
imdertaken for the production of better navigation guides, en-
tailed a minute study of the approaches from the sea and a
thorough reconnaissance of the coastal belt of topography.

Landmarks available and useful for sailing- guides were to be
accurately determined and plotted upon the hydrographic charts.
Paganini was particularly instructed to obtain pictorial views of
certain coast regions, showing the appearance of the coast when
viewed from certain points oflF shore and giving the magnetic
bearings to certain reference points shown on the views from the
points marked on the charts whence the views were obtained.
The point of view was determined either by means of the three-
point problem or it was indicated by means of the estimated
distance from some well-defined landmark (lighthouse or other
prominent object) giving also the magnetic bearing.

The perspectives used to illustrate parts of the coasts, in oider
to facilitate the identification of the "landfall" by mariners
when approaching the coast from the open sea, were published
either on the charts or in so-called coast-pilot books. Formerly
they were obtained by drawing a free-hand perspective from the
deck of a vessel, including all prominent features and sailors'
landmarks, particularly lighthouses, lone trees, prominent bluffs,
and other characteristic features or marks.

Such perspectives (Beautemps-Beaupr^ and Porro had a
remarkable skill in producting accurate perspectives of such

224 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

terrene views by ofifhand sketching) could not be made with
mathematical precision, and very few draughtsmen have the gift
to draw these perspectives under such conditions rapidly and of
uniform scale. Their subsequent reduction to paper, under
application of empirical and often arbitrary rules, based on sex-
tant angles, magnetic bearings, and the mentioned defects in the
sketches, had but a doubtful value, particularly if the vessel,
during the time period which was consumed while these various
observations were made, had gradually changed its position, due
to winds and sea currents.

Paganini, fully appreciating these difficulties, soon recognized
the value of photography for obtaining such perspectives more
readily and far more accurately, if a suitable photographic instru-
ment could be constructed to be used on shipboard, the use of
an ordinary camera, of course, being precluded.

For several years Paganini made studies and experiments
with the above object in view, particularly since the instantaneous
photographic process had been perfected, and the photographic
azimuth compass (*'azimutale fotografico '') is the direct result
of his labors in this direction. It was devised to subserve the
demands touched upon in the preceding paragraphs, and the
instrument, described in Paganini's *'De nuovi Appunti," has
been made by Galileo in Florence.

This instrument may serve to locate with accuracy lighthouses,
buoys, and rocks awash, to obtain topo- and hydrographic views of
the coast (for coast, harbor, and wharf surveys) for explorations
and scientific expeditions, for the survey of unexplored coastal
belts, for naval reconnaissance, for picturing naval displays and
engagements, etc. It is furthermore adapted to determine the
geographical latitude of a vessel's position by photographing the
altitude of the sun above the sea horizon, including its magnetic
bearing at the moment of the plate's exposure. From a negative
showing the image of both the sun and the sea horizon the
declination and the azimuth (magnetic) of the sun may be de-
duced, and the time being known when the plate was exposed,

SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 225

the geographic position of the camera may be deduced from the
plate.

Paganini's photographic azimuth compass is shown, in a
general way, in Figs. 130 to 133, Plates LXV to LXVIII. The
photographic plates are exposed in this instrument with the long
sides (24 cm.) horizontal and the short sides (18 cm.) vertical.

The objective is very similar in arrangement to that described
for the Italian phototheodolite, having a graduated scale to enable
the observer to obtain the focal length directly. The two cross-
wires, with their point of intersection in the optical axis of the lens,
are secured in the image plane, the same as with the phototheo-
dolite.

The camera-box C is supported by two upright pieces 5, shaped
like an inverted U; at the top they are united by a horizontal plate
Z, extending from the two camera sides around to the front of the
camera, forming a horizontal connection in the shape of a horse-
shoe. Three projections d, one at the front and two at the sides
of the camera, serve to support the camera-box upon the horizontal
frame Z by means of three pairs of counter screws v. In the
sectional view of the instrument. Fig. 133, Plate LXVIII, r indicates
the vertical axis of rotation, gg the horizontal circle, B the azimuth
or ship's compass, VV leveling-screws (supporting the horizontal
circle on the heavy plate TT)j Q a heavy weight (to lower the
center of gravity of the apparatus and to increase the stability of
the same when used on the deck of a rolling vessel), m is the
handle of central screw (clamp) which passes through Q and enters
a spherical nut attached below the horizontal circle, to allow for
lateral swing when leveling the camera with the foot screws V.

When this instrument is used on land the gimbal support is.
replaced by a special surveying-tripod, the instrument resting on
the latter by means of the three leveling-screws V. The illustra-
tions show the instrument with the gimbal support the way it is
used on shipboard. The three leveling-screws V rest upon a
plate TT which is connected with the stout ring A by gimbals,
the ring A in turn being supported by four stout legs. The

220 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

weight Q is suflScientiy heavy to assure the vertical axis of rotation
r, Fig. 133, Plate LXVIII, to remain always vertical. This
apparatus is best adjusted and tested on shore, in order to adjust
the horizontal thread OCy by means of the sea horizon.

Below the under side of the camera-box (the latter is inclined
about 30°) another smaller camera c is placed, close to the middle
of the rear end, having a prism attachment by means of which a
section of the compass graduation is reflected upon the lower edge
of the photographic plate, the pictured graduation extending to
both sides of the pictured principal Une //', as indicated in Fig. 134,
Plate LXVIII. The shutters of both cameras, C and c, are
operated simultaneously by pressing the rubber bulb ft, Fig. 132,
Plate LXVII. The rubber tube attached to the bulb is forked,
a separate branch leading to each pneumatic shutter, as indicated
in Fig. 132. The optical axes of the cameras are at right angles
to each other and both are in the vertical plane containing the
principal line //'.

The diameter of the dial compass passing through the zero
mark of the graduation is identical with the magnetic meridian,
and the compass-reading, designated by the graduation mark
that is bisected by the prolonged vertical thread //' below the
picture, represents the magnetic azimuth of the optical axis of
the instrument at the moment of the exposure, or it will indicate
the angle of orientation for the picture. The vertical frame £,
Figs. 132 and 133, Plates LXVII and LXVIII, has a set of cross-
wires with their point of intersection in the vertical plane which
passes through the optical axis of the camera, A peep-hole O,
also situated in the vertical plane passing through the optical
axis of the camera, is affixed to the upper horizontal limb, and
with the cross-wires in E, it will enable the observer to direct the
camera to any point that is to be bisected by the principal line
when a plate is exposed.

Also this instrument is provided with a Zeiss anastigmat lens
of 250 mm. focal length. Eastmann's films or plates are used
(18X24 cm.) and the horizon may be covered by eight plates,

SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 227

allowing a liberal marginal overlap, each plate covering an angle
of about 50° horizontally.

D. Photogrammetric TheodolUe oj Prof. S. Finsterwalder.

This phototheodolite, devised by Prof. S. Finsterwalder after
many years of practical work and experience, gained in his alpine
surveys and studies of glacial motion, has been constructed by
Max Ott (A. Ott), Kempten, Bavaria. In the pursuance of this
work Prof. Finsterwalder early recognized the desirability of a
sxirve)ring camera of compact build, rigidly constructed in aU
its parts and yet having a minimum of weight. To avoid the
transportation of a separate transit or theodolite for the trig-
onometric location of the selected camera stations, he provided
the surveying-camera with means for observing horizontal and
vertical angles.

This phototheodolite is represented in Fig. 135, Pate LXIX,
and the total weight of the outfit, 10 kgr., is distributed as follows r
The instrument itself weighs 2.7 kgr., its carrying case 2.4 kgr., the
tripod 1.7 kgr., one dozen leather receptacles, including twelve
photographic plates 2.5 kgr., and the packing-case for the latter
0.7 kgr.

Prof. Finsterwalder has used both an anastigmat lens of Zeiss
and a double anastigmat of Goerz, with a focal length of 150 nun.
With this focus the lens will photograph perspectively correct a
plate of 160X200 nun. The plates used are 120X160 mm.,
giving an effective horizontal field of 53®, enabling the observer
to cover the entire panorama with seven plates.

For the central or normal position of the objective this camera
conunands an effective vertical field of +20® and -20®, or 40®
in all. Twenty degrees above or below the horizon of the camera
station would often be insuflScient, particularly when working in
mountainous terrene. It was deemed advisable, therefore, to
moimt the objective on a slide, permitting considerable change
in the vertical sense. Owing to this device objects subtending

228 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

angles of depression up to 35° (together with a vertical angle of
elevation of 5°) may still be photographed on the vertical plate.

In extreme cases, when it should become desirable to photo-
graph objects subtending angles of +35° and -35° (or 70° in all),
Prof. Finsterwalder recommends the successive exposure of two
plates, one with the maximum elevation and the other with the
maximum depression of the lens. Inclined plate-pictures are
thus not only avoided but the effective plate surface is utilized to
the fullest extent and the weight of the glass to be carried is
reduced to its minimum.

In order to obtain uniformly accurate and trustworthy results
with the relatively short focal length, maintaining a constant
distance between the lens and the sensitized surface of the plates,
the latter are not placed into plate-holders of the usual pattern
(where the variable thickness of the glass plates would affect the
so-called " constant " focal length), but they are pressed against
a metal frame instead, which frame forms the back of the camera-
box, an arrangement very similar to that of Capt. E. Deville's
camera. To insert the plate into the camera use has been made
of Dr. Neuhauss's leather plate-holders, formed like a sack, B,
Fig. 135, Plate LXIX. The inner edges of the metal frame are
graduated in order to locate the principal and horizon lines upon
the negatives.

These leather sacks have metal slot devices facilitating the
transfer of the plates from the sacks to the camera and vice versa.
By hooking the mouth of the sack to the upper edge of the rear
camera side and opening the slot while holding the bag in a
vertical position, the plate is allowed to slip from the sack into
the carrier. Springs attached to the rear of the camera-box
serve to check the sliding plate and prevent a too sudden drop of
the same into the metal carrier where the plate is to be exposed.
These springs also press the plate into perfect contact with the
metal frame at the back of the camera-box when once in
position.

By withdrawing the upper curved handle (Fig. 135, Plate

SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 229

LXIX) at the back of the camera the tension of the springs may
be reduced (or their action may be released entirely), when the
plate will glide into position for exposure. After exposure the
lower slide is withdrawn and the tension of the springs is again
reduced, when the plate will slip into the empty sack 5, which
had been hooked to the lower edge of the rear camera side as
shown in Fig. 135, Plate LXIX.

The eccentricity of the center of gravity, by applying the weight
of the sack, including plate, to one side of the camera, does not
afifect the general adjustments of the instrument sufficiently to
throw the photographic plate out of the vertical plane when the
exposure is made.

The camera is accurately balanced when no sack is attached,
in which form it is used to measure the angles that may be
required for locating the camera station, both in the geographical
and vertical sense, with reference to the trigonometric signals
in the vicinity.

In order to use this instrimient as a transit the back of the
camera is supplied with an eyepiece jE, Fig. 135, Plate LXIX,
of a magnifying power of from 7 to 8, forming a centrally mounted
telescope with the camera objective O. The eyepiece is supplied
with a cap or shutter to exclude the light when the instrument is
used for photographing. A diaphragm with the usual cross-hairs
is also attached to the eyepiece.

The camera-lens (or the objective of the camera telescope)
being movable in the vertical sense within a range of 100 mm.,
all objects falling within a vertical range of ±17° may be bisected
with the telescope. The definition of points to be bisected, when
above or below the camera horizon, would become very poor if
the eyepiece E were rigidly fixed in a horizontal position by
means of the arms NN, Fig. 135, Plate LXIX. However, it may
be revolved about a horizontal axis in such way that it will always
be directed toward the center of the camera-lens.

With the double anastigmat of Goerz, which produces a per-
fectly flat picture, with neither spherical, chromatic, nor astig-

230 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

matic distortion, a change in the focus of the eyepiece will
rarely be necessary.

Horizontal angles may be observed directly by means of a
horizontal circle H oi 120 mm. diameter, which is provided with
two verniers reading to single minutes. Experimental tests
made with this instrument have shown that horizontal angles
between points of considerable difference in altitude may be
observed within a limit of error of 0.4'. This instrument, there-
fore, gives residts sufficiently accurate to locate the camera stations
trigonometrically with reference to surrounding fixed points of
tnown positions, provided they are not too far distant to be
defined with this low-power camera telescope.

Vertical angles, however, cannot be measured directly; still,
by means of the scale and vernier attached to tHe lens slide or
front board of the camera, changes of the objective from its
central or normal position — values directly proportional to the
trigonometric tangents of the vertical angles — may be read to
0.05 mm. The slide motion of the front board is accomplished
by means of a rack and pinion, and experience has proven that
vertical angles may be observed with this device within a limit of
of error (converted into arc measure) of i'.

The three rods designated by A in Fig. 135, Plate PXIX, are
100 mm. long and they serve to elevate the instrument support,
together with the three leveling-screws 5, sufficiently high above
the tripod legs to allow full play for the leather plate-holders 5, when
they are placed in position to receive the exposed plate. The tri-
pod legs may be folded to half their lengths. No ground glass
being provided, a special finder has been devised correctly show-
ing the field controlled by the plate (in both the vertical and
horizontal sense) for any position of the camera-lens. (Zeitschrift
fur Instnunentenkunde, Oct., 1895.)

SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 231

E. PhototheodolUe for Precise Work by O. Ney.

In the construction of this instrument, Figs. 136 and 137, Plates
LXX and LXXI, it has been sought to fulfill the following
requirements:

First. The camera should be dimensioned for the exposure of
plates suflSciently large to produce clearly defined perspec-
tives.
Second. The general disposition of weight and mass should be
symmetrical. Both camera and telescope are to be mounted
centrally.
Third. The total weight of the instrument is to be reduced to
the minimum consistent with rigidity and sufficient strength-
to assure stability and permanency of its adjustments when
used in the field. The integral parts are to be formed
to permit a free and easy manipulation of the instrument.
Ney's instnunent is composed of two distinct parts — ^the
camera proper and the transit theodolite — ^which may be used
successively and independently one of the other, but always upon
the same tripod. The interchange between the two is readily
accomplished with accuracy and expediency.

The principal advantages attached to this disposition of the
component parts of Ney's phototheodolite may be cited as follows :
First. The symmetrical and central mounting of the camera
and transit telescope will insure permanency in the adjust-
ments with consequent accuracy in the results.
Second. By using the same tripod and horizontal circle T,
Figs. 136 and 137, Plates LXX and LXXI, for both
camera and transit their individual weight has been reduced
to a minimum.
Third. A possible disturbance of the adjustments of the instru-
ment support D and tripod may be guarded against by hav-
ing the plate inserted and the slide withdrawn before placing
the camera-box in position on the upper alidade limb A^
Figs. 136 and 137, Plates LXX and LXXI.

232 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

Accuracy and ease in the manipulation of this instrument
have been assured by suppl3dng all leveling- and clamp-
screws with spherical ends which rest upon suitable plates in
such a manner that a free play of motion will take place. These
spherical terminations of the screws were originally devised by
Reichel.

Fig. 136, Plate LXX, illustrates the phototopographic camera
mounted for use. Fig. 137, Plate LXXI, represents the transit
with a striding compass B.

D is the very rigid, yet essentially light, instrument support.
The three arms supported by leveling screws are cast in one piece
with the bearing for the conical pivot, which in turn is securely
attached to the alidade T. The instrument support D, hori2X)ntal
limb Ty upper alidade limb Ay together with the skeleton tripod,
are used in common for both transit and camera.

A large circular level R is permanently secured to the center of
the upper alidade limb A.

Three hardened plates are inserted into the upper surface of A
at 5. One has a plane surface, the second has a conical cavity, and
the third is provided with a V-shaped slot or groove; they receive
the spherical ends of the three screws which support the transit
and the camera.

The horizon lines of both instruments may be adjusted and
brought into the same horizontal plane by means of these sets of
three screws each, which are attached to the base of both camera
and transit.

Either instrument may be securely bound to the common
support A by turning the horseshoe-shaped clasps C, hinged to
Ay over the ends of the three screws and giving the levers E a
quarter-turn.

The transit telescope is arranged for stadia-reading (after
Porro's method), having 100 as the common multiplier. The
telescope-level is graduated to 20''.

The compass graduation reads to 30' and the horizontal circle
reads either to 10" or 2o'^ according to the size of the instruments

SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 233

The larger class instrument has plates 18X24 cm. and the small-
sized camera has plates 13X18 cm*

To avoid changes in the dimensions of the camera-box due to
bygroscopical influences of the atmosphere it is constructed
entirely of aluminum. The plate-holders and the movable carrier
are made of mahogany impregnated with paraflSne to render the
wood impervious against moisture.

To avoid any possible change in the constant focal length,
due to any uneven thickness of the plate-holders or of the photo-
graphic glass plates, the carrier may be moved forward in the
direction of the camera axis by means of the levers iJ, Fig. 136,
Plate LXX, until the sensitized-film surface is brought into direct
contact with a metal frame securely fastened to the walls of
the camera. This frame has a centimeter graduation filed into
its inner edges, and the distance of the rear surface of the frame
from the nodal point of the camera-lens constitutes the constant
focal length of the camera.

The centimeter graduation on the inner edges of the metal
frame, reproduced on the margins of the negatives, serves a double
purpose. By its means the principal and the horizon line may
be drawn on the face of each negative, and it also serves to ascertain
whether the sensitized films, or the paper prints, have under-
gone any change in dimension during the process of development.
If distortion has taken place the amount of correction to be applied
to the print may readily be found.

The camera is provided with a pair of cross-levels to enable
the observer to detect any change in its adjustment before expos-
ing a plate. These levels have a graduation corresponding to
20" of arc.

When the instrument is in perfect adjustment, the picture
plane will be vertical and the principal ray will be in the same
horizontal plane as the optical axis of the horizontal telescope
if the camera were replaced by the transit (without disturbing
the position of the tripod and instrument support).

When this camera theodolite is adjusted the vernier ilf, Fig. 136,

234 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

Plate LXX, will read zero for the normal position of the lens.
The objective may, however, be elevated or depressed by 35 nrni.,
and any change from its normal position may be read correctly
within 0.1 mm. on the scale and vernier M.

The pneumatic camera-shutter is arranged for either time
or instantaneous exposures; a special device guards against
the possibility of exposing a plate before it is brought into per-
fect contact with the graduated metal frame. Neither can the
plate-holder be withdrawn from the camera before the slide
has been replaced nor as long as the plate is in contact with
the graduated frame. (Zeitschrift fiir Instrumentenkunde, 1895,
P- 55.)

F. Phototheodolite of Dr. C. Koppe.

Dr. Koppe, Professor at the Technical High School in Bruns-
wick, Germany, is an ardent advocate of photogrammetry and
he has done much toward popularizing photographic surveying
in Germany.

This phototheodolite, Fig. 138, Plate LXXII, has a centrally
mounted camera K with the transit telescope T on one side and
the vertical circle C on the other side.

The horizontal axis has been widened between the wyes to form
a conical ring R into which the camera K may be inserted. Four
stout springs / press the camera securely against the ring surface,
forming the collar of the conical ring. After insertion into the
ring the camera is revolved, within the former, about its axis until
the end of the screw b abuts against the stop d, when the prin-
cipal line of the negative should be in vertical plane (the horizon
line horizontal).

The camera axis is parallel with the optical axis of the tele-
scope T, both axes being in the same horizontal plane when
they are level. This parallelism between the two axes is per-
manent.

The instrument will be in perfect equilibrium with the camera
cither attached or removed.

SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 235

The horizontal axis of revolution may be adjusted by means
of the striding level i, which, when required, may be replaced
by a compass very similar to that shown in Fig. 137, Plate LXXI.

Since the telescope (and camera) may be reversed in the wyes
the error of collimation and any index error of the vertical circle
may readily be found or eliminated.

Neither slides nor plate-holders are provided with this instru-
ment, the plates being inserted directly into the carrier of the
camera. This may be done in the field by aid of the packing-
case, Fig. 139, Plate LXXII, specially devised to serve as a dark-
chamber.

The case proper is made of wood with double doors, each
door having a circular opening -4, Fig. 139, Plate LXXII, filled
in with a flexible light- and water-tight material, forming sleeves,
in such a way that the hands of the operator may be thrust through
an elastic opening in the center of the apron. The fabric closes
tightly around the wrists, leaving the interior of the case in per-
fect darkness and permitting free play of the hands in the
interior L for manipulating the camera and plates within the
case.

The wooden box is incased in a tight-fitting sole-leather
covering having two flaps S to protect the openings A against the
admission of dust when the packing-case is transported on the
back of the instrument-bearer.

The entire instrument, except tripod, may be stowed away
in the case for transportation and safe-keeping. The packing-
box also contains two receptacles Ki and K2\ one contains the
unexposed plates and the other receives those that have been
exposed during the day's work.

When an exposed plate is to be exchanged, the camera C,
Fig. 139, Plate LXXII, is placed into the packing-case, and doors,
as well as the leather main flap, are securely fastened, the hands
are inserted into the sleeves A, and the exposed plate P is removed
from the camera and placed into its receptacle Ku closing the
door r. A new plate g, taken from the box Jf 2* is placed into

236 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

the camera and its back securely closed, when it will be ready
for another exposure.

The constant focal length of this camera is represented by
the distance between the second nodal point of the lens and the
rear surface of a graduated metal frame permanently secured
to the walls of the camera. The inner edges of this metal dia-
phragm, or frame, bear a centimeter graduation; the middle
graduation marks of the two horizontal sides locate the prin-
cipal line, while the middle graduation marks of the vertical
sides represent the termini, on the photographic perspectives, of
the horizon line.

The focal length, once determined, remains the same for all
plates. This instrument has been manufactured by F. Rand-
hagen in Hannover, Germany.

The Topographic Bureau of Switzerland has used a photo-
theodolite constructed after the model of Dr. Koppe's instrument.
The experience in Switzerland, however, seems to have decided
the Bureau of Topography not to replace the plane table by the
phototheodolite for general topographic surveys executed by that
bureau.

Dr. C. Koppe^s new Instrument and Method for Observing Horizon-
tal and Vertical Angles directly on the Photographic Negative.

We have seen, on page 125, that the iconometric plotting of
vertical and horizontal changes in the terrene were based upon
direct measurements of the coordinates of pictured points. Dr.
Koppe, in his recently published pamphlet on photogrammetry,
with particular reference to cloud photography, has given a new
method having many advantages for particular kinds of work.

He inserts the negative into the camera in the exact position
previously occupied by the plate during exposure and illuminates
the same from the rear sufficiendy to bring out all the details of
the negative. With a theodolite telescope directed through the
camera-lens he now observes the vertical and horizontal angles by

SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 237

bisecting the particular points on the negative in the same manner
as the surveyor uses a- transit for observing in the field.

Light-rays reaching the camera-lens from a distant point in
parallel directions are concentrated and directed to one particular
point on the photographic plate — the image point. With Dr.
Koppe's device the same phenomenon takes place only in inverse
order as the light-rays now emanate from the illuminated negative.
Light-rays coming from any point of the negative are refracted by
the camera-lens whence they enter the telescope of the theodolite
in paraUel directions. If the telescope had been focused for
infinite distance the pictured point observed upon will appear
sharply bisected by the cross-webs in the telescope, and if the
camera is securely fixed in position and the telescope is now changed
to bisect another pictured poiftt,-4he angle included between the
directions to the two pictured points successively bisected will
be identical with the angle included between the lines of direction
drawn from the first nodal point of the lens at the original camera
station in the field to the corresponding two points in nature.
We can, therefore, obtain the values for the vertical and horizontal
angles by reading the corresponding verniers of the theodolite in
both positions of the telescope when bisecting the pictured points.

In the practical application of this method two cases are con-
sidered. The camera is either stationary and the observing tele-
scope adjustable in position, or the telescope is stationary and
the camera is adjustable. • Dr. Koppe ha6 experimented with both
tj^pes of instruments.

If we insert the negative in the camera, giving it the same
position which the plate had during exposure, and if we level the
instrument, giving it likewise the same position which it had
when the plate was exposed in vertical plane, and finally, if we
adjust the observing telescope of the transit to bring its optical
axis in line with the horizontal axis of the camera, we can observe
horizontal and vertical angles by bisecting the pictured points and
noting the vernier readings.

The main difficulty in the practical solution of this problem

238 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

was the necessity of having to give the observing telescope a
decidedly eccentric position to enable the observer to bisect
pictured points at any place of the illuminated negative. The
construction of this instrument, therefore, is more cumbersome
than the ordinary theodolite or transit with centrally mounted
telescope.

Dr. Koppe declares the degree of accuracy attainable in the
application of this method to be the same as when the corresponding
angles were measured in the field with an instrument of equal
size and power. This assertion is based on practical tests and
experimental observations made by Dr. Koppe in connection with
the preliminary survey for the location of the railroad over the
Jungfrau in the Alps.

The principal advantages of this method, compared with
angular measurements made directly in the field, may be found in
the reduction of the field-work, in the possibility of measuring
angles between objects in motion (using instantaneous plates for
this purpose), in locating points of an evanescent character, like
prominent features of distant mountains which may be liable to
sudden disappearance under freshly fallen snow, or which may
be visible only for short intervals, due to the frequent and sudden
formation of ** cloud-caps " or ** hoods, '' etc.

An advantage of Dr. Koppe's method against the general
method of measuring the coordinates of pictured points rests in
the fact that correct values for the horizontal and vertical angles
may be obtained from negatives that are not geometrically true
perspectives, owing to distortion produced by imperfect lenses or
lens combinations, as the points of the illuminated negatives will
emanate light-rays from the first nodal point of the objective in
directions identical with those in which they originally arrived
there when the plate was exposed. The instrument used by
Dr. Koppe in his experimental tests was manufactured by Oskar
Giinther, in Braunschweig, after plans and specifications furnished
by Dr. Koppe. It is illustrated in Figs. 140-142, Plates LXXIII
and LXXIV.

SUKVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 239

The phototheodolite is essentially the same as has been de-
scribed with a horizontal circle of 14.7 cm. diameter and vertical
circle of 12 cm. Aperture of the eccentric telescope is 27 mm.,
with a focal length of 20 cm. and a Ramsden eyepiece magnifying
twenty times. The verniers read to single minutes, admitting
15 seconds to be estimated.

The telescope r. Fig. 140, Plate LXXIII, used for angulation,
in both the vertical and horizontal sense, upon pictured points
has an aperture of 18 mm., a focal length of 8 cm., and its
Ramsden eyepiece magnifies five times.

The focal length of the camera objective (double anastigmat
of Goerz, series III, No. i) is 144.9 °^^^-

Fig. 140, Plate LXXIII, shows the phototheodolite com-
bined with the telescope r ready for angulation upon the pictured
points of the illuminated negative in camera Q.

Fig. 141, Plate LXXIV, represents the various attachments
required to convert the phototheodolite, as used in the field, into
the form shown in Fig. 140, Plate LXXIII, as used for the angula-
tion in the oflSce. These attachments, of course, need not to be
taken into the field.

For the use in office glass positives are preferably made by
contact printing from the negatives, as it facilitates the identifica-
tion of points for the "angulation on pictured points" if these have
the same appearance regarding light and shadow as the natural
objects which they represent.

To identify corresponding points on two or more positives the
plates are placed side by side on a frame with a reflector below
them. Points selected for angulation are marked by fine " pricks "
made with a needle. Points of reference that have been observed
in the field are marked with red ink, giving identical points the
same designation, and points to be determined by angulation in
the office are marked with blue ink. A check for the correct
identification of these points may be obtained in the usual manner
by determining their elevations from two or more plates, using, of
course, in this case the vertical angles obtained by angulation upon

240 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

the pictured points with the telescope r, Fig. 140, Plate LXXIII,
in the office.

The "spring frame" P, Fig. 141, Plate LXXIV, serves to
give the various positive plates the positions in the camera corre-
sponding with the positions of the original plates during exposure.
This is readily accomplished by viewing the plate through the
objective of the camera and adjusting the plate, held against the
graduated frame at the back of the camera by means of the spring
frame P, in such a way that the pictured graduation marks coincide
with their corresponding marks on the graduated frame which is
permanently fixed in the image plane of the camera.

From the phototheodolites as used in the field we now
remove the eccentric telescope, including its vertical circle and
circular camera support, from the wyes and replace these with
the telescope r and vertical circle ,5 (Figs. 140 and 141, Plates
LXXIII and LXXIV). The secondary camera support T-/,
Fig. 141, Plate LXXIV, is then secured to the theodolite by
means of the clamp- screws SS^ in the manner shown in Fig. 140,
Plate LXXIII, and the camera Q, with positive plate in adjusted
position, is inserted into the circular camera support K (Figs. 140
and 141, Plates LXXIII and LXXIV) and adjusted in posi-
tion so that the first nodal point coincides with the point of inter-
section of the horizontal and vertical axes of rotation of the tele-
scope r.

The positive plate, if well illuminated by diffused light, will
now emanate rays from the marked points in the same direction,
beyond the objective of the camera, as the incident rays origi-
nally emanating from the corresponding points in nature at
the time of the plate's exposure.

We may now proceed to measure the horizontal and vertical
angles of the marked points of the plate with the telescope r,
provided the camera has the same inclination which it had when
the original plate was exposed. This inclination may be estab-
lished by clamping the telescope r in position when the verniers
of the adjusted vertical circle B, Fig. 140, Plate LXXIII, read

SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 241

the same as the recorded mean value for the inclination of the
camera in the field for the particular plate under observation,
and tipping the camera, first roughly by means of the support /,
Figs. 140 and 141, Plates LXXIII and LXXIV, then with the
slow-motion screw h, Fig. 140, Plate LXXIII, and Fig. 141, Plate
LXXIV, until the principal point of the photographic perspec-
tive is bisected by the cross-webs of the clamped telescope r,
which is then released for the angulation of the marked points
on the positive plate.

Excepting the angulation upon the triangulation and refer-
ence points in the field, the vertical and horizontal angles to any
number of pictured points may be observed in the office by this
ingenious device, by means of which the greater part of the field-
work, when using tachymetric methods, may be transferred
to the officcj making such detailed observations independent of
the length of the field season and of the vicissitudes of climate
and weather.

The computations are the same whether such observations
are made in the field or in the office, both methods giving prac-
tically the same results with instruments of this class.

With Dr. Koppe's instrument the angular values were obtained
in both cases within a maximum error of one minute if the
bisected points were not farther away than 3000 m.

H. Phototheodolite Devised by V. Pollack; Manufactured by
R. Lechner in Vienna, Austria,

With this instrument the camera C, Fig. 143, Plate LXXV,
is centrally located and mounted above the horizontal circle.
The telescope F and the vertical circle V are attached to one
side of the camera, but counterbalanced by the weight G. In
order to reduce the weight as much as possible aluminum has
been used extensively in the construction of this apparatus.
For instance the upright T is made entirely of that metal.

This instrument has been manufactured in two sizes; the
horizontal circle of the smaller one is graduated to 30' and the

242 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

verniers read to i', while the larger one has a circle graduated
to 20' and its verniers read to 20'^ The telescope F is mounted
somewhat like that of the Danish plane-table alidade.

The adjustment of the horizontal axis of revolution of the
telescope F is accomplished by means of a special striding level
Clamps and slow-motion screws are provided for both the hori-
zontal and the vertical circles. The telescope has a focal length
of 27 cm. and an aperture of 31 mm. with a magnifying power
of 9 to 18 diameters. The telescope is arranged for stadia-read-
ing, and it has 100 as the constant multiplier. The telescope-
level L is graduated either to 10" or 20". The vertical circle
is graduated to 20' and its two verniers read to 20". The
camera proper is made of aluminum and it is provided
with a Zeiss anastigmat. By means of the rack and pinion, 2,
the lens may be elevated or depressed by either 30 or 50 mm.,
according to the size of the instrumtent. The scale /, together
with the vernier n, serves to measure the vertical deviation of
the lens from its normal position. Also this camera is provided
with a metal frame the inner edges of which have either a
centimeter or a 5-millimeter graduation, which is reproduced
upon the negatives. This graduation serves not only to locate
the horizon and the principal line upon the photographic per-
spectives, but it also gives ready means for discovering any dis-
tortion that may arise in the perspective, due to the wet process
of development. The graduated metal diaphragm or frame is
brought into direct contact with the sensitive surface of the film
by a simple mechanical contrivance, in such a way that the
focal length remains constant for all negatives, even if the
plate-holders or plates should vary a little in thickness.

/. Phototheodolite Devised by Pollack and Hafferl.

This phototheodolite is shown in Fig. 144, Plate LXXV.
It has been used under the Imperial General Directory of

SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 243

Austrian State Railroads, and it was placed on exhibition during
the Ninth Convention of German Geographers in Vienna
in 1 891. This camera has no graduated metal frame, its horizon
and principal line being located by means of a set of four vanes
or index marks, which are pressed against the sensitized film
of the photographic plate by means of a revolving button, a
device similar to that mentioned in connection with the older
pattern of the U. S. Coast and Geodetic Survey camera. The
instrument is leveled by means of two leveling-screws s, coun-
teracted by two pivots /, Fig. 144, Plate LXXV, which are held
in position by two spiral springs.

K. R. Lechnefs Photo grammeter,

R. Lechnefs photo grammeier is shown in Fig. 145, Plate LXXVT.
(Lechner also manufactures Pollack's, Werner's, and Huebl's in-
struments.)

It is a rectangular metal camera, C, of constant focal length,
centrally moimted upon a graduated horizontal circle, K. Two
spirit-levels /, attached to the upper limb of the horizontal circle
(which is graduated into degrees), and three leveling-screws s
serve to adjust the position of the instrument. Bi is the vertical
axis of rotation for camera and horizontal circle. The tripod is
set up approximately level, the circular level x being provided
for that purpose.

The camera is connected with the upper limb of the horizontal
circle by four screws. Two of these are in the direction of the
optical axis and serve to adjust the image plate into vertical plane
(the levels / reading zero), the other two are situated in a line at
right angles to the direction of the optical axis; they serve to
adjust the horizon line into horizontal plane.

The objective, O, is a Zeiss anastigmat //18, and it may be
elevated or depressed by means of a rack and pinion, such dis-
placement being read on the scale, /, with vernier, n.

A metal frame with inner edges graduated in centimeters is

244 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

also provided for this camera, and a special mechanical device
serves to press the plate (in the plate-holder after the slide had
been withdrawn) against the rear surface of this frame so that
the centimeter scale is impressed on the margin of the negative or
photograph, thus providing the means to locate the horizon and
principal lines upon the perspective and also to eliminate any error
due either to a possible faulty or imperfect registration of the
plate-holder or due to distortion in the photograph (paper print).

In the middle of the back-board of the camera is an eyepiece
with cross-threads, very similar in arrangement to that described
for Paganini*s new phototheodolite, forming a telescope with the
object-glass of the camera. In this case, however, the cross-
threads are attached to the eyepiece and their intersection coin-
cides with the principal point of the perspective.

A dial compass, B, is attached to the upper face of the camera-
box, a being the catch to clamp the needle, or dial, when not in
use,

L. Phototheodolite oj Col. A. Laussedat (new Model).

CoL A. Laussedat^ s latest phototheodolite j manufactured by
E. Ducretet and L. Lejeune, Paris, France, is shown in Figs. 146
and 147, Plate LXXVI. Both transit telescope and camera are
centrally mounted, the latter above the former. The camera may
also be separated from the transit, and by means of a special pivot
or spindle /, Fig. 147, Plate LXXVI, it can then be mounted
upon the same tripod. The transit may be used for trigonometric
observations after removal of the camera. Fig. 146, Plate LXXVI,
represents the complete instrument.

^5s are the three leveling-scrcws and Ci is the central clamp;
C is the camera and B is the magazine for fifteen plates;
O is the objective of the camera (it is a rectilinear wide angle lens

of 75 millimeters focal length);
H is the sliding front plate provided with pinion and rack R to
elevate or depress the lens;

SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 245

F is a finder to show the extent of the field covered by the photo-
graphic plate, although a focusing-glass is also provided;
L is the transit telescope provided with stadia wires;
Ce is the vertical circle graduated to 30';
MM are the uprights supporting the horizontal axis of revolution

of the transit telescope and also supporting the camera;
A is the horizontal circle graduated into 30'; its clamp and slow-
motion screw are shown at P';
N is the adjustable level, and

Z? is a long compass, with slow-motion screw and clamp at P,
to read the magnetic azimuth on the horizontal circle -4.
Several loaded magazines, each containing 15 plates, may be
carried with the instrument. The plates may be changed in full
daylight without removing the camera. The plates are 6 J X 9 cen-
timeters. Enlarged prints are used for the iconometric plotting.
Six plates forming a panorama cover the entire horizon. The
lens is provided with an iris-shutter, and it may be focused for
short distances or infinity by turning a lever over a scale, show-
ing the distances in meters, attached to the front board Hy Fig.
146, Plate LXXVI. In Fig. 147, Plate LXXVI, the instru-
ment (camera) is represented with the magazine B removed and
replaced by the ground glass G.

The entire outfit, excepting the tripod, which is carried sepa-
rately, may be transported in a carrying-case (with shoulder-
straps) 39 X 28 X 1 7 centimeters. The weight of the carrying-case>
including instrument (complete), one magazine, and fifteen plates
amounts to 8 kilogrammes.

M. Phototheodolite oj Starke and Kammerer.

This instrument, represented in Fig. 148, Plate LXXVII,
somewhat resembles in construction the phototheodolite of Prof.
Finsterwalder; neither has a vertical circle and both have a
" camera telescope."

The camera is mounted on the horizontal circle like a theo-
dolite. An ordinary skeleton tripod supports the three leveling-

246 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

screws 5 of the horizontal circle, Fig. 148, Plate LXXVII, and
a central clamp-screw P with spiral spring connects the tripod
with the instrument proper.

H is the horizontal circle, graduated to 20'; its two verniers
with microscopes L read to single minutes.

The vertical axis, terminating in three horizontal arms Bu -B2,
and Bzi may be adjusted by means of the leveling-screws 5 and
the cross-levels h and h- The plate £>, forming the support
for the cross-levels, is firmly attached to the arm ^2.

E = upper clamp-screw ;
Jlf= upper tangent-screw for slow motion;
JPi, ^2* and -F3= three leveling-screws supporting the camera-
Jx)x; they rest in the grooves of the three
arms B\y J?2> and Bz\
Iz and /4=cross-leyels attached to the camera, as shown
in Fig. 143, Plate LXXVIII, which represents
the camera-box viewed from above.
These cross-levels, together with the screws i^i, F2, and Fz,
serve to adjust the photographic plate into vertical plane.

5= movable front board, or objective slide;
Q= handle to facilitate mounting of the camera;
2^1= head of pinion to elevate or lower the camera-lens, the
slide S having a corresponding rack for that purpose;
ir2=differential pinion for slow motion;
if = clamp-screw to secure the slide 5 in a given position;
m (Fig. 148, Plate LXXVII) = millimeter scale to measure
the vertical change of the lens from its normal or
central position, the vernier n permitting such change
to be read to 0.05 mm.

The camera may be securely fastened to the vertical axis
of the horizontal circle by manipulating the central clamp-screw
from the interior of the camera-box.

When the instrument is in adjustment the zero mark of the
vernier n will coincide with the 70 mark of the scale f», and the

SURVEYING-CAliERAS AND GEODETIC INSTRUMENTS. 247

lens will then be in its central or normal position. The slide S
may be moved 70 mm. up or down; from 70 to 140 the lens will
be above the normal position.

The lens of this camera is a Zeiss anastigmat, //18, with
a focal length of about 212 mm.

The camera -lens is suitable for phototopographic purposes
if the horizontal change in the distance between its second nodal
point and the image plane does not exceed

0.09 o.ii 0.15 0.22 0.45 mm. for distances of

500 400 300 300 100 m.

Hence, focusing may be dispensed with for general photo-
topographic purposes; still, in order that this camera, for special
purposes, may also produce sharp and well-defined pictures of
objects comparatively close to the camera, its lens has been
mounted to admit a longitudinal motion, in the direction of
the optical axis, within a range of 2 mm., thus giving the
means to focus objects that are only 23 m. distant from the
camera.

The external tube of the lens mount has a helical groove,
or slot, in which a small metal block /, Fig. 149, Plate LXXVIII,
provided with an index mark, may travel. This block / is attached
to the inner tube of the lens moimt, and a screw r, Fig. 149,
Plate LXXVIII, at one end of the slot serves to clamp the two
tubes together when the focal length is to be maintained con-
stant for any length of time.

Loosening the screw r and revolving the outer tube from left
to right will shorten the focal length, and when the block / has
passed from one end of the slot to the other, the focal length
will have suffered a change in length of 2 nun.

The two positions of the index mark on the block / for these
extreme limits are marked on the outer side o and 2, Fig. 149,
Plate LXXVIII; the interval is divided into twenty equal parts,
one division corresponding with an axial motion of the camera-
ipTis of 0.1 mm. A metal frame is attached in the rear to the

248 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS,

camera sides and the posterior surface of this frame coincides
with the image plane. The inner edges of this metal frame
are provided with a- centimeter graduation, the middle marks
of the two horizontal sides indicate the position of the principal
line, while the middle notches of the two vertical sides mark
the position of the horizon line. When the camera is adjusted'
these lines will intersect each other in the principal point of the
photographic perspective. The inner opening of the metal
frame is 17.8 and 22.8 cm., which, of course, is also the size of
the pictures.

The two frames / and //, shown in Figs. 150 and 151, Plate
LXXVIII, give the means to make a light-tight .connection
between the single plate-holders (or ground glass) and the camera-
box. The short bellows Wy connecting frames / and //, will
admit the frame // to be moved toward or from the frame /,
which is securely attached to the camera- box. Each one of these
frames / and // is provided with two hooks; frame / has an upper
hook hu Figs. 148 and 149, Plates LXXVII and LXXVIII,
and a similar hook near the lower comer diagonally opposite hi.
The hook A2, Fig. 149, Plate LXXVIII, is attached to the upper
comer of frame //, opposite hook Ai, and frame // has a similar
lower hook diagonally opposite A2 and directly below hook hi.

Fig. 151, Plate LXXVIII, represents the section of a rear
comer of the camera-box, showing the ground-glass-plate attach-
ment V (it also has the eyepiece forming a telescope with the
camera-lens, Fig. 152, Plate LXXIX).

Frame II is secured to frame I by means of the upper left
hook A2 and the lower right hook.. The ground-glass frame V
is supported by the screws Zi and Z2, Figs. 151 and 152, Plates
LXXVIII and LXXIX, the points of which rest upon the metal
plates ;r, Figs. 149 and 151, Plate LXXVIII, permanently
attached to the fixed frame /. The face of the ground glass G,
Fig. 151, Plate LXXVIII, is brought into contact with the rear
surface of the graduated frame R by means of the upper right
and lower left hooks.

SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 249

The position of the optical axis of the eyepiece may be adjusted
vertically by turning the screws Zi and Z^ until the line of col-
limation of the eyepiece and camera-lens coincide in the horizon
plane, the camera-lens being in its normal position, the zero
mark of vernier n coinciding with the 70 mark of the scale w,
Fig. 148, Plate LXXVTI. In this position points may be obser\'^ed
through the eyepiece of the ground-glass attachment. When the
camera-lens has been moved some distance up or down (away
from its normal position), however, the eyepiece can no longer
be used with its line of coUimation in a horizontal position, and
the clamps, or stops, P\ and ^2, Fig. 152, Plate LXXIX, are
unfastened and the eyepiece is tilted up or down — it rotates about
a horizontal axis XiX2^ Fig. 152, Plate LXXIX — ^until its optical
axis is directed to the center of the object-glass. The image of
the point to be bisected will then appear well defined.

The circular openings /o, shown in the ground-glass attachment,
Fig- 152, Plate LXXIX, serve to examine the middle notches (of
the inner edges of the graduated metal frame R) which define the
termini of horizon and principal line of the perspective. The
openings p give the means to test the positions of those lines with
reference to the middle notches; both should coincide. The outer
wooden frame F, Fig. 151, Plate LXXVIII, of the ground-glass
attachment is strengthened by two metal diagonals D joined into
a ring at their intersection, which ring supports the eyepiece,
revolvable about Xxx^ as axis.

Each holder contains but one plate, and Fig. 150, Plate
LXXVIII, shows a section through the upper rear part of the
camera with a plate-holder K in position.

P= dry-plate;

/= springs supporting the dry-plate at its four comers;

G= hard-rubber slide, which is completely withdrawn when a
plate is to be exposed;

i?= graduated metal frame permanently secured to the rear
of the camera-box;

C= section of camera-box wall.

250 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

To attach a plate-holder K to the camera, Fig. 150, Plate
LXXVIII, frame // is set free from / and K is hung to frame //
by means of the bent plate / (permanently attached to K) when the
beveled projecting frame or edge of K closes into the corresponding
rebate of frame //, producing a light-tight connection. K is
secured to // with the upper left and lower right hooks, giving
it the position shown in Fig. 150, Plate LXXVIII. After the
hard -rubber slide G has been withdrawn, the second pair of hooks,
upper right and lower left, are tightened to draw the holder K
forward until the sensitive-film surface is brought into contact
with the graduated metal frame R (at the back of the camera), the
springs / serving as a cushion to insure a perfect contact without
straining the glass plate P. The lens is now uncapped, the
exposure made, and the holder is withdrawn by repeating the
same operations in inverse order:, unfastening the pair of hooks,
upper right and lower left, inserting the slide G, and drawing back
the two last hooks, lower right and upper left.

JV. CapL von HubPs Plane-table Photogrammeter.

This instrument, manufactured by R. Lechner in Vienna, has
been described in Lechner's " Mittheilungen aus dem Gebiete der
Photographie und Kartographie," Wilhelm MuUer, Graben 31,
Wien. It is the result of Capt. Hubl's endeavors to reduce the
weight and costs of an effective photographic-surveying instru-
ment, to be easily adjusted and manipulated and to subserve
topographical purposes.

As the final result, generally aimed at in topography, reduces
itself to the graphical representation of the terrene, Capt. Hubl
combined the surveying-camera with a plane table by means of
which the directions needed for the orientation of the picture
traces, as well as those required for the location of the camera
station, may be plotted directly in the field. It is supposed that
a sufficient number of triangulation points had been provided
to locate the camera stations by resecting upon signals, of which at
least three should be visible from every camera station.

SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 25 1

For this purpose the camera top M (21X21 cm.) supports
the plane table sheet, which is securely held in position by four
metal comer clamps n, Fig. 153, Plate LXXIX.

C= camera-box (of constant focal length) made of aluminum;
7"= graduated horizontal circle with clamp- screw. It enables
the observer to turn the camera in azimuth by an equal
amount (panorama section) after each successive ex-
posure;
Fy= graduated metal frame;

JfJf^ correction screws for adjusting VV to bring the principal
point into the optical axis of the camera-lens;
ft = rubber bulb for operating the pneumatic shutter of the

camera;
/=head of pinion, which serves to elevate or depress the
camera-lens. Any such change from its normal posi-
tion may be read off on a scale with vernier, secured
at one side of the camera-lens;
d= spirit-level. Two are provided (at right angles) for
adjusting plane table M into horizontal plane (the
photographic plate will then be in vertical plane);
2?= movable plate-carrier;
ZIr= lever to move the plate-carrier toward the lens imtil the
sensitive-film surface of photograpic plate is brought
into direct contact with the graduated metal frame VV\
ir= plane-table alidade with vertical circle, arranged for

stadia-reading;
Z= pivot vertically above second nodal point of camera-lens.
The lever h serves to locate the principal point /, Fig. 154,
Plate LXXIX; when the edge of the alidade ruler LL abuts
against the upturned lever h the principal ray z/, bisecting the
angle ezgy Fig. 154, Plate LXXIX, may be drawn upon the plane-
table sheet.

Fig. 154, Plate LXXIX, represents the plane-table top abcd'y
it has two pivots z and 2f about which the alidade ruler LL may
be revolved.

252 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

z/= constant focal length of camera;
e^= horizontal projection of the picture trace;
€2^= horizontal angle commanded by each plate = panorama
section.

At e and g are two stops (corresponding to the ends of the
photographic field) representing the ends of the picture trace
on the plane-table sheet.

By placing the alidade ruler LL upon the pivot z the hori-
zontal projections of lines of direction, emanating from z as a
center, to such points of the landscape which serve to orient the
picture (so-called reference points) may be drawn upon the
paper within the sector limits ezg. The point 2, marked on
the sheet, may be regarded as the plotted station point. The
central pivot zf serves as vertical axis of rotation for the alidade
ruler LL when the horizontal directions to known points (signals
over trigonometric stations, visible from the camera station) are
to be plotted to locate the position of the camera station.
Point 7fy transferred to the plane-table sheet, is regarded as the
plotted station point.

2/ or zfj is the trace of the principal plane upon the horizontal
projection plane.

The plane table M with alidade K serves to locate the camera
stations in both the vertical and horizontal sense; it may also
be utilized for the location of tertiary points with the stadia-
rod and for sketching details in the neighborhood of the sta-
tion. The horizon line and principal line may be located upon
the perspectives by means of the centimeter graduation of FF,
or two very fine wires may be attached to the corresponding
graduation marks.

The instrument rests upon the three ends of the leveling-
screws 5, Fig. 153, Plate LXXIX, which fit into slots at the bot-
tom of the camera-box, the latter being firmly imited with the
tripod by means of a central screw with spiral spring.

This photographic plane table is well suited for topographic
reconnaissance surveys. The results obtained with it may not be

SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 253

as precise as those obtainable with the more complicated and
refined phototheodolites; still, it is more readily transported,
is very easily manipulated, and its adjustments are not subject
to frequent disturbance. The instrument is compact, well con-
ceived, and excellently executed.

The size of the photographic plate is 12X16 cm., giving an
effective picture, within the graduated margin, of 10X14 cm.

The cube-shaped camera weighs 3.5 kgr.

The packing-case (knapsack), including the entire outfit and
a stout tripod (with 3 folding legs), weighs only 11.5 kgr.

The cost of this outfit in Vienna is 400 florins.

• O. PhototheodolUe {'' Phototachiomhire'') Deinsed by J. and

H. VaUot.

In 1893, Joseph and Henry Vallot entered into correspond-
ence with Col. Laussedat with reference id the planning of a
phototheodolite, to be used in the topographic survey of the
Mont Blanc mountain group. The first instrument made ix
this purpose did not give results with the anticipated and desired
degree of accuracy; its adjustments were found too unstable tor
transportation in rough moimtains.

Owing to the defects in this instrument three hundred plates
obtained in 1893 were discarded and a new phototheodolite
with a constant focal length and with vertically exposed plates
was devised. J. Vallot designed the geodetic part of the instru-
ment and H. Vallot planned the camera. The manufacture was
intrusted to Brasset Frferes, who succeeded in furnishing an in-
strument of great stability, small weight, and general excellence.
It is shown in Figs. 155 and 156, Plates LXXX and LXXXI.
The latter illustrates the theodolite (" tach&)mfetre *') and the
former represents the surveying-camera. Both are mounted
upon the same tripod and one instnmient support with hori-
zontal circle R answers for both. The following desiderata
have been fulfilled by the makers of this instrument:

254 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

First. In order to give the least resistance to the wind, to
facilitate transportation, and to reduce the price the size.
of the instrument has been reduced to a minimum com-
patible with efficiency.

Second. To avoid vibration of the plate during exposure the
component parts of this instrument have been assembled
and joined together with the utmost rigidity.

Third. The instrument is well balanced to avoid strains in
the instrument itself and to give the results obtainable
with it the greatest possible accuracy.

Fourth. The geodetic parts of this phototachymeter have
been designed with a view to give results (vertical and
horizontal angles) as accurate as those obtainable with
Goulier's theodolite of to cm. diameter, which instrument
had been previously used for the triangulation of the
Mont Blanc group and which was now to be replaced
by this new combination instrument.

Fifth. The camera, with fixed focal length, commanding

. an angle of. 40 centigrades above and below the horizon,
, was to be made of metal and the correct position of the
horizon line should be readily obtainable for every photo-
graphic perspective.

Sixth. The component parts of the phototachymeter have
been assembled by the makers in such a manner that all
future adjustments may be dispensed with. Any dis-
crepancy may be discovered, however, by special obser-
vations, made for testing the adjustments, which discrep-
ancies may be neutralized by applying proper corrections
to the results.

Seventh. The lens selected should be as speedy as possible
to reduce the time of exposure, and the definition of all
points falling within the unfavorable parts of the plates
should be correct within o.i mm. (The diameter of
the circle of diffusion ^o.i mm.)

Eighth. If (orthochromatic) plates give better results than

SUEVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 25$

films the camera chamber should be arranged for the
use of the former. The definition should be good enough
to admit a twofold (photographic) enlargement with
no distortion to aflfect the accuracy of the iconometric
plotting.
Ninth. Means were to be provided to enable the operator
to inspect the field under control for each exposure with-
out having recourse to dark cloth and ground glass.
Tenth. Provision should be made to divide the panorama
automatically into sections of equal horizontal extensions.
Eleventh. The exchange of plates, when photographing a
panorama view, should be made without the least chance
of disturbing the adjustments of the instnmient.
At first Carbutt's orthochromatic films were used, and it
was found that they underwent a change retraiie during the
developing process, which, however, was uniform in character,
thus admitting a correction to be applied. Still, it appeared to be
desirable to avoid even such a uniform change in the dimensions
of the negatives, requiring a correction, and glass plates are now
used altogether in Vallot's work.

The orthochromatic plates are 130X180 mm. in size, with an
effective field for the perspective of 120X170 mm. These small
plates do not materially increase the weight of the outfit; one
cardboard box containing 12 plates weighs 1.4 kgr.

The double-plate holders (0.25 kgr. in weight) were made by
Balbreck. They may be inserted into the carrier and the sUdes
be withdrawn without imparting the least shock to the instrument.
The 18 double-plate holders, carried in a special packing-case,
are numbered from i to 36. The plates are marked in one comer
with a soft pencil before removal, giving series and number. For
instance: ^4, i to 36; 5, i to 36, etc.

While in the field, the exposed and imexposed plates (their
plate-holders) are kept separated in the case by placing the ground-
glass frame between them. The plates used at present are
Lumifere's orthochromatic, series -4 (sensitive to yellow and green).

256 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

The diagram given in Fig. 157, Plate LXXXII, represents a
vertical section through the camera chamber (in the principal
plane).

^B = nodal plane of the lens;
i4C = CJ5=5o mm.= range of possible vertical change of
lens above or below its normal position (at C) ;
3fiV= effective vertical width of plate = 120 mm.;
A = extreme upper position of lens;
5= *' lower '' " ''
C= normal or central position of lens;
A'M^B'N^i cm.;
AA^= CO = BB' = constant focal length = 1 50 mm.

The extreme vertical angle which a plate may contain will be
found from

tan ^MiV=^j^=— =0.733,

^A^ AN =40.3,^ (centesimal graduation; 400^=360*^ sexagesimal
graduation).

For the extreme lower position B the greatest vertical angle would
be

^B'BM= 40.3^ y

and for the central or normal position C the greatest angle above
and below the horizon is found from

tan C'CM=g^=— =0.4,

The horizontal field a of the plate may be found from

« 8s
tan -=-7- =0.567,
, 2 150 ^ ''

SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 2 $7

the effective width of plate being = 170 mm.

j=32.8; q:=65.6^.

If no overlaps are required a panorama may be photographed
on six plates,

6X65.6^=393.6^,

showing the omission of 6.4^ of the horizon. It often will be
desirable, however, not only to photograph the entire panorama,
but also to overlap two adjoining plates. The horizon has
accordingly been divided into six equal sectors of 60^ each and a
seventh sector of 40^.

In place of the customary slide (which is either too tight or
too loose and consequently not light-tight) the camera chamber
has been provided with three openings (Oi, O3, and O2, Fig. 155,
Plate LXXX), A, C, and 5, Fig. 157, Plate LXXXII, into either of
which the lens may be inserted and securely held there by a
bayonet catch. The lens is secured in the middle opening O3,
Fig. 155, when the vertical angles above and below the horizon
do not surpass 26^. It is inserted at O2, Fig. 155, Plate LXXX
(at B, Fig. 157, Plate LXXXII), when low grounds (valleys) are
to be photographed from high elevations and at A, Fig. 157,
Plate LXXXII, when mountain peaks are to be photographed
from lower stations.

The horizon line for each of these three positions of the lens is
located on the negatives by a set of two projecting points p, Fig. 155,
Plate LXXX, which are photographed on every plate.

When great differences in altitude are to be recorded, two
plates may be exposed in the same direction and from the same
station, one with the lens at A, the other with the lens at B, Fig.
157, Plate LXXXIL

The openings Oi and O2, Fig. 155, Plate LXXX, are closed by
caps which have the same bayonet catch as the lens mount O3.

258 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

The objective is an anastigmat of Zeiss, No. 2, series Ilia,

with revolving diaphragm and stop No. 5 ( — ). This lens is made

of Jena glassby Krauss in France, and it has a constant focal
length of 150 mm.

Enlargements of the negatives 50X60 mm. give very good
results for the iconometric plotting, which has been done on
scale 1 120000 for the Mont Blanc sur\^ey.

Behind the lens a yellow (plate) glass screen has been in-
serted with a tint sufficiently dark to necessitate the length of
exposure required for the normal plate to be increased fifteen times.
The insertion of this screen increases the focal length of the ob-
jective to 151 mm., the yellow light- rays being less refracted than
the blue and violet rays. This camera is not provided with an
instantaneous shutter, as the exposure will always require several
seconds.

The tripod is made of oak and it may be folded together to
0.75 m. length. On rocky peaks, where the unfolded tripod
would be too long or where it would require too much ground
space, it may be used in its folded condition.

The tripod head H, Figs. 155 and 156, Plates LXXX and
LXXXI, is made of aluminum, which metal enters greatly into
the composition of this instrument. The camera chamber and
lens-hole caps Oi and O2, Fig. 155, Plate LXXX, are also made
of aluminiun.

The horizontal circle and metal instrument support resembles
in form the horizontal circle adopted for the tachymeter-theodo-
lites used by the G^nie Corps of France. The three leveling-screws
i. Figs. 155 and 156, Plates LXXX and LXXXI, set into the
three arms A, are on the circumference of a circle of 0.17 m. in
diameter.

The horizontal circle i?. Figs. 155 and 156, Plates LXXX and
LXXXI, is o.io m. in diameter and is mounted above the
circular disc F which supports the cross-levels h and /2, used for
leveling the instrument.

SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 259

Its graduation (centesimal 400^=360® sexagesimal) is on the
vertical (outer) cylindrical surface, and it may easily be read
when the camera is mounted on the instrument support.

A conical pinion (in the prolongation of the vertical axis) on
the horizontal circle serves to support the camera or the tachym-
eter-theodolite, which are provided with conical bearings to fit
this conical pinion.

A special arrangement with stops insures the uniform azi-
muthal swings of the camera when exposing the plates forming
a complete panorama; six of these plates cover an angle of 60^
each and the seventh controls 40^ of the horizon. Each plate
covers or overlaps the adjoining one by a common margin of
15 mm. in width. When used as a theodolite the arresting stops
may be set inactive.

The definition of the photographic perspectives for all objects
from 10 m. to infinite distance is very clear (using stop No. 5).

A tele-objective (long-distance objective) has been provided
by means of which circular pictures may be obtained on the plate
of o.ii m. diameter and with a two and one half fold enlargement.

The metal points p and f. Fig. 155, Plate LXXX, have cir-
cular openings of i mm. diameter which serve to locate the
horizon and the principal line on such pictures where the image
of the plate has been obscured by the tint of the image close to
the point.

Blackened diaphragms Z>, Fig. 155, Plate LXXX, are inserted
into the darkened chamber to intercept all side-light rays that
may be reflected into the camera and cause the plate to be
fogged.

The heads of the three screws 5, Fig. 155, Plate LXXX,
used to secure the camera in its position on the horizontal circle,
are corrugated or checked and well blackened to prevent side
reflections of light-rays from their surfaces.

The ground-glass plate is used only when testing the adjust-
ments of this instrument.

The upper face of the camera is provided with three sight-

26o PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

frames Vu F2, and F3, which may be revolved about their
lower ends to lay perfectly flat on the camera top when not in
use.

Two vertical planes containing Vi, F2, and F1F3 include a
horizontal angle of 60^, the extreme width of the horizontal
field for one plate. The sight-frame Vi (near the lens) is pro-
vided with a vertical wire and the sight-frames V2 and F3 (at
the back of the camera, Fig. 155, Plate LXXX) have three
peep-sights each, which, together with the metal bridges of Fi,
are disposed at such distances to correspond with the vertical
angles, commanded by the lens on the plate, for the three differ-
ent positions that may be given the lens. By sighting through
the three peep-sights it may readily be determined into which
one of the three openings (Oi, O3, O2, Fig; 155, Plate LXXX) the
lens should be inserted to control the extension of a certain pan-
orama section in the vertical sense.

For executing the tertiary triangulation and for locating
detached camera stations (stations not already included in the
triangulation scheme) the surveying-camera, Fig. 155, Plate
LXXX, may be converted into a tachymeter-theodolite by replac-
ing the camera by an " alidade holomftrique " with " broken "
telescope, ab. Fig. 156, Plate LXXXI, like that of Col. Gou-
lier's pattern (with the exception that the ruler has been dis-
carded here). The base of this tachymeter has three screws 5,
Fig. 156, Plate LXXXI, corresponding with those of the camera,
to secure it to the horizontal circle.

The telescope, which magnifies about twelve times, is "broken"
to facilitate the measuring of large vertical angles. A finder e.
Fig. 156, Plate LXXXI, of a fourfold magnifying power is pro-
vided. It has the same form as the telescope ab.

Vertical angles are measured with the vertical circle E
(" r&limfetre"), Fig. 156, Plate LXXXI, which has a diameter
of 0.08 m. and reads to single grades (quadricentesimal gradu-
ation).

A special level is supplied to facilitate the ready adjustment

SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 261

of the telescope ah into horizontal plane when this tachymeter
is to be used as a spirit-level.

This entire phototachymeter (excluding the tripod) is packed
into a box having three compartments, which may be closed by
two doors like a wardrobe. One compartment contains the
tachymeter proper (" P&limfetre ") ET, Fig. 156, Plate LXXXI,
the other receives the camera C, Fig. 155, Plate LXXX, and
the third is reserved for the repeating circle R with its support A,
Figs. 15s and 156, Plates LXXX and LXXXI.

The folding tripod, together with a light folding stadia or
telemeter-rod, is carried in a separate packing-case. All pack-
ing-cases are covered with rubber cloth to guard against the
evil effects of dampness.

This entire instrument outfit may be strapped to a light
packing-frame, making a compact pack to be carried on the back.

The weight of this outfit has been distributed as follows:

Complete phototachymeter 5.3 kgr.

Packing-case and accessories 3.6

Water-proof case, including straps 0.8

Folding tripod 4.8

Folding stadia-rod 0.8

Water-proof case for both, including straps ... 0.7

Total weight of instruments 16 kgr.

Packing-frame, including straps 2 "

Total weight to be borne by one packer. . . 18 kgr.

The 18 double-plate holders (including 3 doz. plates) weigh
9 kgr.; they are stored in a wooden case which weighs 3.5 kgr.,
including the water-proof covering. These 12.5 kgr. are carried
by a second packer who has additional implements to take,
bringing the weight of his pack also up to 18 kgr.

For the survey of Mont Blanc it has been found that 36
plates fully suffice for a day's work. For a trip of several days*

262 PHOTOTOPOGKAPHIC METHODS AND INSTRUMENTS.

duration the observer carries extra plates (in their cardboard
boxes, but stowed away in a separate water-proof packing-case),
which are exchanged every night, using a small folding ruby
light and portable dark tent.

In 1894 four hundred negatives were obtained by Vallot
Bros., and in 1895 the number reached over 500. The season
of 1896 was so misty and rainy that few days were available
for this work, the mountain peaks being rarely visible, and also
lower parts of the mountains being more generally hidden from
view in a dense fog. This season's results again proved the value
of the phototopographic method above all others for surveys
in the higher altitudes of mountainous regions, as the results
obtained during the short periods of good weather could not
have been acquired in the short time available by any other
known topographic method.

The area of this survey is controlled by 300 triangulation
points (established between 1894 and 1896) which are connected
with a base line at Chamounix of 1785.824 m. length.

P. Phototheodolite Designed by J. Bridges-Lee,

This instrument has been patented in England and other
countries, and it is made by Louis P. Casella, who has published
a full description of the same, with instructions for its use in
the field (Description of a New Phototheodolite designed by J.
Bridges-Lee, Esq., M.A., F.G.S., etc. With full instructions
as to its manipulation in the field. L. Casella, maker to the
Admiralty, Ordnance, etc.. No. 147 Holbom Bars, London,
E. C), from which the following description has been abstracted:

This phototheodolite is shown in Figs. 158 and 159, Plates
LXXXIII and LXXXIV, the latter representing a view of the
interior of the camera-box, the ground-glass plate H having
been turned down. It will be noted that this instrument com-
prises:

(i) A complete, well-made theodolite reading to minutes;

SURVEYING-CAMERAS AND GEODET C INSTRUMENTS. 263

(2) A photographic outfit answering all ordinary demands;

(3) A good and large azimuth compass, its vertical scale

admittmg of very close readings to be made either

through the rear window /t' or through the lens B.

The combination of these three instruments into a symmetrical

well-made phototheodolite makes this one of the most generally

useful instruments that an explorer of unknown regions can

add to his instrumental outfit.

With reference to Figs. 158 and 159, Plates LXXXIII and
LXXXIV, we have:

A. Rectangular camera-box made of aluminum (cast metal);
its upper face supports the telescope E, with vertical circle F
reading to minutes. This box is permanently attached to
the leveling support T', Fig. 158, Plate LXXXIII.

B. Rectilinear photographic lens of excellent quality and sup-
plied with iris diaphragm. No focusing adjustments are
provided for this lens, and it should be set by the instru-
ment-maker very accurately in correct position with refer-
ence to the other parts of the instrument.

A second photographic lens, movable in a sleeve with rack-
and-pinion focusing adjustment, wiU be supplied if the mstru-
ment is to be used for ordinary photographic work at short dis-
tances.

C. Horizontal circle, graduated to half degrees with vernier
reading to single minutes.

The vernier is attached to the rear of the camera. Fig. 159,
Plate LXXXIV, its zero mark falling into the same vertical
plane which passes through the optical axis of the camera-lens.

D. Tribrach, or triangular piece connecting tripod head and
camera. It supports the terminating heads at the base
ends of the three leveling- screws. To insure stability the
terminal feet of the levelling- screws may be secured to the
tribrach by, means of the usual locking-plate connected
with the tribrach.

E. Telescope with cross- and stadia-inches in the body and

264 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

general adjustments for a surveying- telescope. The tele-
scope is mounted like that of a plane-table alidade to be
rotated about a horizontal axis in a vertical plane only. Its
plane of transit contains the optical axis of the camera-lens>
the zero mark of the vernier of the horizontal circle, the ver-
tical hair K, as well as the vertical axis of the needle sup-
port of the azimuth compass, when the instrument is in
adjust ment and leveled.

F. Vertical circle, graduated into half-degrees, firmly con-
nected with the telescope and provided with a fixed vernier
reading to single minutes. This circle controls a vertical
range sufficient for all ordinary topographic surveying pur-
poses.

G. Tubular spirit-level revolvable in a low cylindrical case
firmly attached to and sunken into the upper face of the
aluminum camera-box. This level serves to give the top
of the camera- box a horizontal position when the camera
has been oriented for an exposure to be made. This adjust-
ment is made in the usual manner by means of three levehng-
screws supporting the instrument on the tribrach.

H. Back of the camera containing the ground glass h and sup-
plied with hinges to enable the operator to swing the ground
glass back, as shown in Fig. 159, Plate LXXXIV. A win-
dow h' of polished glass is inserted into the ground glass h
to enable the surveyor to observe the vertical index hair K
and coincident graduation mark of the vertical compass
scale, with or without the use of the microscope O.
J. Rectangular metal frame supplied with stout backstays
(not visible in the illustrations) and securely attached to the
bottom plate which supports the compass box M. This
bottom plate may be moved in the direction of the axis
of the camera-lens, sliding on guides rigidly fixed to the
bottom of the camera-box.

When the camera is to be used for ordinary photographic
purposes the bottom plate (including the frame / and azi-

SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 265

muth compass) may be withdrawn and laid aside. A square
piece of black velvet is provided to be placed upon the metal
bottom of the camera-box. After the insertion of the second
lens with adjustable focal length the camera will be ready
to be used for photographing near objects.

The rear surfaces of the frame / are in vertical plane
which is perpendicular to the optical axis of the camera-
lens when the instrument is leveled and in perfect adjust-
ment.
jj. Transverse pinion extending across the bottom of the camera-
box and terminating in two milled heads J J. The frame /
may be racked backwards or forwards by a corresponding
turning of the milled heads //.

Pointers are attached to the milled heads, which serve
to indicate whether the internal structures are in the forward
or backward position when the falling back of the camera-
box has been let down to allow a double-plate holder to be
inserted into the camera.
The dimensions of the rectangular frame / are such that
it can be i^ked back (entering the plate-holder frame) to be
brought into direct contact with the sensitized film of the photo-
graphic dry-plate, when the cross-hairs K and K' will also actually
touch the film. Of course, the slide protecting the photographic
plate against light should be first removed before the frame /
is racked back. Two small stops in the form of sliding bolts
(not shown in the illustrations) are provided to prevent the frame 1
from being carried back too far or with too much force and
to secure imiformity of the focal length for all phototopographic-
plate exposures.
K. Vertical hair passing through two round holes in the frame
/and held in position by means of small wooden pegs.
When broken, a new hair can readily be inserted into the
holes and secured by small pegs. This hair serves to mark
on the negative the median vertical plane of the instrument
and the principal line of the photographic perspective. It

266 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

cuts the optical axis of the camera-lens at right angles and
it serves as index mark when the compass scale is read.
It is in the same plane as the vertical web in the telescope,
the optical axis of camera-lens, the vertical axis of revolu-
tion of the azimuth compass, and the zero mark of the vernier
of the horizontal circle.

K'. Horizontal hair secured to the frame in the same manner
as hair K. The point of intersection of both hairs is in
the optical axis of the camera-lens, and on the negative it
marks the principal point of the photographic perspective.
When the camera is in perfect adjustment and accurately
leveled the shadowgraph of K' on the negative will bisect
points having the same height as the optical axis of the
camera at the station whence the picture was taken.

The proper location of the small holes in the frame /,
fixing the position of the hairs K and iT', is ascertained by
the maker.
LL, Small tablets of transparent celluloid or xylonite. They
serve to receive notes giving particulars concerning station
mark, barometric reading, or determined elevation of camera
station, number of plate, time of exposure, etc., which it
may be desirable to record photographically, as shadow-
graphs, on the picture. Such notes are written upon the
tablets LL in the ordinary way with quick-drying ink.
The tablets when dry are placed upside down in small pock-
ets in the frame and the notes appear as shadowgraphs in
the comers bordering the sky portion of the negative.

Jf. Magnetic azimuth compass with vertical cylindrical trans-
parent scale, graduated to half degrees from o° to 360°.
This transparent scale in its revolution passes quite close
to the vertical index hair K, without ever touching it, how-
ever. The pivot of the compass is permanently secured
to the base or bottom plate already referred to, so that the
cylindrical compass-scale is always at exactly the same
distance from the vertical hair K,

SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 267

When the bottom plate supporting the frame / and com-
pass M is racked forward in the camera-box, a copper disc
is automatically raised, lifting the agate cup and pressing it against
the support m so that the compass is firmly clamped, preventing
injury to the pivot and other parts of the compass during trans-
portation. When the bottom plate is racked back to bring the
cross-hairs K and jRT' into contact with the sensitive film of a
photographic plate, the agate cup is lowered (automatically)
upon the pivot and the magnetic compass assumes its natural
position. The compass graduation being on a transparent
cylinder, the magnetic azimuth of the principal line of every
exposed plate will be recorded photographically in the sky region
of the negative as a shadowgraph.

N. Catch to hold the double-plate holder in place when inserted
into the camera. The frame of aluminum forming the
rear of the camera-box is faced with black velvet to guard
against the entry of extraneous light when the double-
plate holder is in position.
O. Microscope with universal joint movement to permit of
its being used either for reading the observed horizontal
angles (on the horizontal circle C with vernier), or for read-
ing the compass bearings through the window h' in the
ground-glass back h.
P. Adjustable microscope for reading vertical angles.
Q. Clamp- and tangent-screw for arresting azimuthal or hori-
zontal circle and giving it slow motion.
R. Clamp- and tangent-screw for camera.
5. Clamp- and tangent-screw for telescope.
T. Strong aluminum head of tripod with bronze clamping-
screws for folding legs. It is supplied with transverse bars
(not shown in the illustrations) of bronze, serving as attach-
ments for chains to safeguard the instrument when in posi-
tion at a station. These bars also serve to receive the hooks .
attached to a net into which heavy stones may be placed

268 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

to give stability and steadiness to the instrument when used
in windy weather.
U. Two cross-marks on the top of the box. Their distance
indicates for ordinary temperatures the permanent focal
length of the camera, to be used for the negatives.

A small hook for the attachment of a small plummet is secured
to the tripod head T in the central axial line of the instrument.
The telescope is supplied with an erecting as well as an inverting
eyepiece.

A color-screen of optically worked green glass may be fitted
inside the sunshade of the photographic lens. Yellow or orange
glass can also be supplied when desired.

Attached to the frame / (which carries the hairs) is a hori-
zontal transparent scale of angular distances (Fig. 159, Plate
LXXXIV), photographically prepared by aid of the identical
lens and instrument which it accompanies. It is used for sur-
veying purposes, as with its aid the exact angular distances of
points in the picture — to the right or left of the principal vertical
plane — may be read off directly with the aid of parallel rulers.
This scale also facilitates the determination of compass errors,
because if there are any points in a. picture whose true bearings
have been fixed with precision — trigonometrically or otherwise —
it is only necessary to add or subtract the angular distances of
those points, as read. on the horizontal scale of angles, to or
from the photographiccally recorded compass-bearings of the
points, and the difference between the compass- bearings and
the true bearings is the compass variation. This simple verifi-
cation can be performed in office at any time. This instru-
ment is supplied with 6 double-plate holders of good construction,
to carry one dozen plates, size 5X4 in., either horizontally or
vertically.

It fits easily and securely in a strong, well-made, brass-bound
mahogany case with catches, lock, and key.

The double-plate holders, extra eyepiece, extra camera-lenS;
color-screen, and plumb-bob all fit in the same case, which for

SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 269

greater security and convenience of transportation is placed into
an outer sole-leather case with pack-straps.

The tripod head is provided with a metal screw-cap, a suit-
able protecting cover of its own, and the legs can be strapped
together for easy transportation.

CHAPTER IX.

PANORAMA CAMERAS.

The older lens types gave correct perspectives only for small
angles, rarely exceeding 30 degrees, and Martens in Paris was
probably the first to devise a so-called panorama camera capable
of photographing wider sections of the horizon on one plate,
with lenses that ordinarily would cover but a small angular field.
He solved this problem by constructing a hemi-cylindrical camera
with a revolvable lens plate. If the objects are far enough away
to permit the use of a constant focal length of lens, and if the
lens may be rotated about a vertical axis in the second nodal
point of the lens system, panorama views may be obtained on a
sensitized surface of a daguerreotype plate bent into a half-cylinder
whose radius equals the constant focal length of the lens and
whose axis coincides with the vertical passing through the second
nodal point of the camera-lens.

I. The Photographic Plane Table Devised by A. Chevallier (1858).

Chevallier's " planchette photographique " may be men-
tioned here, as, in a certain sense, it also is a panoramic camera.
In this instrument the entire panorama view was continuous
and found representation on a single plate; the latter, however,
was exposed in the horizontal plane. The lens axis being hori-
zontal, a prism had to be interposed between plate and lens to
bring thd image into the horizontal picture plane. All verticals
of the landscape converged to one point, the center of the cir-
cular horizon line. For further deails of this historically inter-

270

THE ROCKWOOD-SHALLENBERGER PANORAMIC CAMERA. 2? I

esting instrument we would refer to the publications on this sub-
ject given under French phototopographic literature.

n. The Rockwood-Shallenberger Panoramic Camera.

A horizontal section through this camera (made just above
the camera-lens) is represented in Fig. i6o, Plate LXXXII., It
practically consists of two cameras C and c. The latter (smaller)
one contains the lens O and is revolvable about a vertical axis
passing through the latter. The main camera-box C forms a
semi-cylinder with a sensitive film stretched over the inner cylin-
drical surface, and that may be unwound from the magazine
roller B passing to the receiving roller A after exposure. As
the small camera is revolved, the light-rays entering the lens
act upon a narrow vertical strip of the film at a time. The con-
nection between the objective end of the small camera c with
the front board h of the main camera C is accomplished by means
of a pliable light-tight fabric e. The lens O has a long focus
and the panoramic perspective is entirely free from distortion,
only a vertical strip of one quarter inch width being exposed at
one time. The pictures are 8X40 inches and it takes from
three to five seconds for the lens to complete one revolution of
180 degrees. The speed in the swing of the smaller camera
is controlled by a clockwork, the rate of which may be increased
or retarded at pleasure, with due reference to the changes in
atmospheric conditions and character of subject.

in. R. Moessard's Topographic Cylindrograph.

The so-called cylindrograph of R. Moessard (commandant
du G^nie, attach^ au Service g^ographique de Tarmac) is
similar in construction to the apparatus just described; this
instnunent, however, is specially devised for surveying purposes.

The semi-cylindrical camera- box, Fig. 161, Plate LXXXV,
rests upon a tripod with leveling-screws to adjust the verticality
of the axis of revolution aa of the camera lens O, which axis coin-

272 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

cides with the axis of the half-cylinder, formed by the surface
of the sensitive film. For focusing purposes the latter may
be replaced by a semi-cylindrical ground-glass plate. By using
the sight-ruler 5 as a lever the camera-lens O may be rotated
about aa, allowing the speed of motion to be controlled by the
illummation of the landscape. By carefully examinuig the
panorama through PF while aa is being moved in azimuth,
the correct timing for the exposures of the different panorama
strips may be made. The space between the frame RR is filled
in with a soft and light-tight fabric, allowing an easy play for
the rotation of the objective O.

The upper surface of the topographic cylindrograph is pro-
vided with an azimuth compass C and a set of cross-levels A
and B. The bent frame forming the guide for the film is sup-
plied with graduations on the inner edges which form the mar-
gins of the panoramas. The divisions of the upper and the
lower scales (horizontal) correspond to degrees in arc, while
the divisions of the vertical marginal ends are graduated to read:

~— , where /== constant focal length of the lens O (= radius of
100

the cylindrical sensitive surface of the film). Four movable
indices are provided; two of these, H and -ff'. Fig. 162, Plate
LXXXVI, serve to mark the horizon line of the half-panorama
and the other two, N and E, serve to indicate the magnetic
meridian and the magnetic east and west line, respectively,
for each panorama view. The proper placing of the indices
for each half -panorama may be accomplished by means of the
azimuth compass C and sight-ruler or alidade S. Thus the mag-
netic azimuths of horizontal directions may be taken directly
from the picture.

The vertical angles are readily found by means of the ondinates
of the pictured points (above or below the horizon line HH')
measured in one hundredths of the focal length /, using the photo-
graphed scales on the vertical margins of the pictures for this
purpose.

R. moessard's topographic cylindrograph. 273

For instance, the angle of depression of the ray Oa (to the
foot of the pictured tree a, Fig. 162, Plate LXXXVI) is found
from

tan/?'=y,
or when ad is measured on the side scale and found to be 25 parts^

tan a = — =0.25.

^ 100 ^

To determine whether the levels A and 5, Figs. 161 and 163,
Plates LXXXV and LXXXVI, read zero when the cylindrical
film is vertical, and also to ascertain whether the indices H and H'^
Fig. 162, Plate LXXXVI, representing the horizon line are cor-
rectly placed, we may proceed as follows:

A theodolite. Fig. 163, Plate LXXXVI, is set up about 10
or 15 metres behind the cylindrograph (after the back of the
camera had been removed to bring the indices H and H' into
view) and both instruments are leveled. After bisecting the
upper edge of the cylindrograph and the telescope of the theo-
dolite is moved in azimuth the bisection should continue, and
the same should be the case for the lower surface edge of the
cylindrograph. If this does not take place, then the cylindrograph
should be adjusted by means of the leveling-screws until the
bisection does take place and the level A is then changed to
read zero. The theodolite is now set up in the direction of the
level A (at one side of the cylindrograph) and the level B is
adjusted in a similar manner as just described for A,

To adjust the indices H and H' into the horizontal plane
containing the optical axis of the adjusted cylindrograph a com-
parison may be made on a cylindrograph picture showing several
points of known elevations, the elevation of the cylindrograph
being also known, or the theodolite may be set up with the hori-
zontal telescope at the same elevation as the optical* axis of the
adjusted cylindrograph. The horizontal telescope of the theo-
dolite is now moved in azimuth until a well-defined point is

274 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

bisected, which point may be identified on the ground glass of
the cylindrograph. The image of this point on the ground
glass is marked and the cylindrograph is moved in azimuth,
marking the image on the ground glass in two more places. A
(horizontal) line passing through these marked points should
pass through H and H',

The objective is attached to a funnel-shaped box situated
within the camera (see Fig. i6o, Plate LXXXII) and permitting
the simultaneous exposure of a vertical strip of film of a width
of but 62 mm. Points of the film that would be pictured out-
side of this strip cannot be acted upon by the light-rays until
the objective be revolved (about the axis aa) sufficiently far
to expose them to the effects of the light. After the time needed
for the correct exposure of this strip (of 62 mm. width) has
been ascertained (by experiment or otherwise) the correct expos-
ure may be given the entire semi-cyUnder by moving the sight-
ruler 5, with a quick uniform motion, about aa, from one extreme
end of the film to the other. The semi-cylindrical film being
860 nmi. long, each strip of the film would then have been exposed
the ^^/seoth part of the time required to make one full revo-
lution of the objective. If one complete revolution required
10 seconds, and if the correct exposure for the strip was found
to be 5 seconds, each strip would have received an exposure of

- Q^. seconds =0.72 second. To give each strip the required

oOO

exposure of 5 seconds the entire revolution of the lens should

be repeated -^ times, or about seven times, each revolution
0.72

taking 10 seconds.

These panorama instruments are not made sufficiently pre-
cise, in their present form at least, to be reconmiended for topo-
graphic surveys. Moessard's cylindrograph, however, is well
conceived, and where the transportation is an easy one, the
topographic cylindrograph, in a more perfected form, may give
results sufficiently accurate for surveying purposes.

CHAPTER X:
ICONOMETERS AND PERSPECTOGRAPHS.

Under iconometers we comprise a series of instruments
which have been devised to simplify the constructions of photo-
topographic plotting or iconometry.

After two drawing-boards have been covered with paper
(gummed down on the edges), both sheets are provid-cd with a
chart projection upon which all trigonometric (triangdJaAion)
points are plotted and their elevations inscribed. n ui

The construction incidental to the iconometric plotting, of
the phototopographic survey may be divided into three classes:
First. The plotting of all horizontal directions that had
been observed, instrumentally, for the location of the
camera stations and for the orientation of the panorama
views.
Second. The determination of the horizontal projection of
points pictured on three or more photographs, taken
from different stations.
Third. The determination of the elevations of the various
camera stations and tertiary points that are located icono-
metrically to facilitate the plotting of the horizontal con-
tours of the terrene.

I. Graphic Protractor (of L. P. Paganini).

With the aid of a specially constructed protractor. Fig. 164,
Plate LXXXVII, and tracing-paper the directions obtained
with the theodolite or transit in the field can readily be plotted

27s

276 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

upon both the working- and chart-sheets. This protractor,
represented in Fig. 164, Plate LXXXVII, consists of two con-
centric rings AA and J55, the former being movable within the
latter about the common axis C, secured in the center of -4^4
by means of the plate aa. The rotary motion is applied to A A
by means of two projecting ribs as and sa on the plate aa.

The inner circle AA has a graduation divided into degrees
and half degrees, while the outer circle BB bears a vernier n>
reading to half minutes, the zero of which lies in the prolongation
of the fiducial edge of an arm ii, the latter being permanently
secured to the outer circle BB and in a radial position to the
same. The clamp-screw P serves to hold the two circles in
any position.

An alidade ruler, DD, the fiducial edge of which also passes
through the center C, common to both circles, is revolvable about
the axis C and it may be moved over the uppQr surfaces of the
two circles AA and BB. This ruler, DD^ bears a vernier n',
graduated like n to read to half-minutes and its zero coinciding
with the fiducial edge of DD. The clamp-screw P' serves to
clamp this movable arm DD to the outer circle BB.

The axis C has a conically shaped hollow interior, at the
bottom of which a thin pifece of isinglass or horn is secured in
such a manner that it may be removed for renewal whenever
the small puncture indicating the center of the circles and axis
of revolution be worn too large.

When using an ordinary protractor to lay ofif the various
directions (radials, that were observed with the transit in the field)
from one camera station, much valuable time will be absorbed
in making the additions and subtractions (which have to be
made in order to obtain the actual values for the successive angles
between such lines of direction), particularly when a series of
panorama stations are to be plotted.

The protractor, as shown in Fig. 164, Plate LXXXVII, may
be used not only as an ordinary protractor — by bringing the
zeros of both circles to coincide and clamping the two circles

L. P. PAGANINl'S GRAPHIC PROTBACTOR. 277

in that position, by means of the clamp-screw P — but it may
also be used to plot the directions upon the map or working-
sheet in the same manner as they were obtained in the field with
the transit; that is to say, they may be referred to zero or to
any other direction as the beginning, and then be plotted in
successive order.

To do this, the inner circle is revolved imtil the zero of BB
(vernier «) gives the same reading upon the graduation of the
movable circle -4^4 as the recorded reading on the transit for the
prime direction. Now both circles are clamped together by
means of the clamp-screw P. The line of prime direction is
drawn along the fiducial edge of the fixed ruler bb upon the work-
ing-sheet (or upon tracing-paper if the station is to be located
or fixed upon the tracing of the lines), the center C of the instru-
ment coinciding with the point representing the station upon
the paper.

The zero of the vernier n' of the alidade DD is then brought
successively (upon the inner-circle graduation) to the readings
of the other directions which radiate from the plotted station
point at C, each direction being plotted in successive order by
drawing a pencil line along the edge of the alidade DD. Care
must be exercised not to change the primary position of the
instrument as defined by the first line during all subsequent
motions of the aUdade ruler DD.

With the aid of this instrument the radials from the plotted
camera stations may be obtained with rapidity and accuracy.
If we have a sufficient number of directions to well-determined
points which are evenly distributed about the station, their
corresponding intersections upon both drawing-boards may be
plotted with as much rapidity and accuracy as a graphical plotting
will admit of.

This protractor may also serve to locate points on the con-
struction board that, on account of importance or for reasons
of control, had been bisected from several stations with the
transit, and also, as will be shown, to orient a perspective view

278 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

(the picture trace) upon the board, if such perspective contains
no triangulation point, or when the picture of such point is blurred
or not sufficiently well defined to be identified with precision. ;
After all stations, including such secondary and tertiary
points that were determined by transit observations from the
several camera stations, have been plotted upon the two boards,
the work of iconometrically determining, upon the working-
sheet, such points as seem needed to complete the map is taken
up. For this purpose the various elements of the perspectives
are tested and corrected, if needed, after the manner previously
described, and all tertiary points are selected, identified, and
markerf, searching for well-defined points common to two or
more plates, carefully selecting therefrom only such as appear
to be the most useful, either for drawing the contours or for
tracing the general trends of mountain ranges, torrents, and
streams, boundary lines of glaciers, etc., the number to be
selected depending greatly upon the character of the terrene,
upon the adopted scale, and upon the accuracy to be attained..
All tertiary points are marked upon the prints (perspectives)
by numerals, letters, or symbols in red ink.

n. L. p. Paganini's Graphic Sector (" Scttore Grafico ")•

Instead of actually drawing the horizontal projections of all
perspectives (the picture traces) upon the working-sheet, much
time may be saved by using the instrument represented in Fig. 166,
Plate LXXXVIII, devised by Paganini, who termed it '' settore
grapcOy^ or graphic sector. With this graphic sector the hori-
zontal directions to points marked upon the prints may be drawn
c irectly on the horizontal plan without first drawing the picture
traces.

In Fig. 165, Plate LXXXVII,

V represents the station, plotted on the working- sheet;
00' the horizontal projection of a perspective (the picture

L. P. PAGANINI'S GRAPHIC SECTOR. 279

trace, oriented with reference to the plotted triangulation
point 5);
/= focal length for the perspective 00';
P= principal point (of view) of the perspective;
Ps upon OO' is the measure of orientation of the perspective,

corresponding to the azimuthal angle w;
VP is perpendicular to 00' and =/.
We now prolong VP beyond V by VP^VP'^f and erect a
perpendicular to Vr=^0'''0" in F. Produce, likewise, VB,
VA, VS to their intersections with 0''[0''y which intersections
are marked V^ a\ and 5', respectively.

VP'^VP

and 00' parallel to 0'"a'\

hence the rectangular triangles VP'a'^ VP'V, and VP's' are
congruent with VPa, VPh^ and VPs respectively. Therefore

P'a'=Pa=Xy

and Fs'^^Ps,

giving also the measure of orientation (=a>) of the perspective
of the picture to the picture trace.

The screw e serves to clamp the screw m whenever the posi-
tion of T with reference to 7 is to be fixed, after it has been
brought to the desired distance from the center of rotation V.
Two thimib-screws W and W (with hollow centers into which
fine needles may be inserted to hold the sector in place after
having been adjusted over a plotted station) serve to secure the
metal sector in any desired position upon the drawing-board.

The arc SS' of the sector is graduated to ten minutes, and
the zero of this graduation coincides with the axis VP of the
instnunent, giving readings from 0° to 25° to either side of VP.

The ruler or alidade RR is provided with a vernier 7, by

28o PHOTOTOPOGRAPmC METHODS AND INSTRUMENTS.

means of which the arc readings may be obtained within 30
seconds. The thumb- or clamp-screw Z of the alidade has a
counter plate at its lower end to secure the end B! of the alidade
ruler upon the arc ssf of the sector and upon the steel ruler T.

In order to draw the lines of direction upon the construction
board to a point of the terrene (the picture of which had been
identified and marked upon the perspective) the instrument is
placed with its center of rotation, F, over the needle, marking
the camera station on the working-sheet, and the button r is
given a quarter-turn (care must be taken that the side bearings
of the button r of the instrument may have no loose play about
the needle), then T is moved by turning the screw m until OO^
is distant from the center Y by VP==f, whereupon the orientation
of the instrument is accomplished as follows:

RR' is to be directed to bisect a plotted triangulation point
the image of which appears on the perspective sufficiently dis«
tinct to serve as a reference point; its abscissa is taken from
the photograph by means of a pair of dividers and plotted, in
the inverse direction, upon the line 00' from the puncture,
marking P; the alidade ruler RR' is now gently brought into
contact with the other point of the dividers and it is secured in
this position by clamping Z.

Now the entire instrument is revolved about V imtil the
end R of the alidade bisects the plotted triangulation point, when
VP will indicate the direction of the principal line and OO' will
be parallel to the picture trace, which really would fall beyond V
at a distance from V^VP^f.

The instrument is secured in this position by gently turning
the screws. The construction of the graphic sector, Fig. 166,
Plate LXXXVIII, is based upon the preceding consideration,
and it serves to draw from the station point V, on the horizontal
plan, the various horizontal directions to points pictured on
the panorama views.

The metal plate YSS', shaped like a sector, may be revolved
on the surface of the working-sheet, about the center of a.

L. P. PAGANINI'S GRAPHIC SECTOR. 281

needle, puncturing the plotted station in the center r of the
sector.

This needle passes through an oblong opening (of the same
width as the diameter of the needle) of a revolvable button at r,
secured in F, and through a similar slot at 7 in the metal sector
plate VSS\ The metal ruler RR' is revolvable about V, gliding
with the end R' over the arc SS' of the sector plate. The fiducial
edge of the ruler J?J?' passes through the center of V or r, where
it is secured to the revolvable button r by means of a cylinder,
the bottom of which is provided with a slot similar to those in
the button r and sector plate just mentioned.

When the ruler RR' and the button r are in a certain position
these three slots (in sector plate, button, and ring of ruler) will
coincide, one falling above the other, and the needle may then
be inserted through the three superimposed slots into the sta-
tion point under 7, the center of rotation. By a quarter-turn
of the button r the needle will become inclosed in a square, of
which the needle circimiference will form the inscribed circle.
The entire instrument may now be revolved about the needle
center in V.

The lever-screw m serves to move the steel ruler T parallel
with itself and vertical to the axis «n' of the screw m. The axis
of the screw m coincides with the direction of the central axis
of the sector which passes through V and the middle of the arc SS\
When m is turned the ruler T glides up or down, its ends moving
along the grooves u and ,w', the inner edges of u and uf being
graduated, so that the distance of the edge OCy of the ruler T
!rom V may be read off to o.i nmu

If the edge OCy of the steel ruler T is brought to a distance
VP=l from the camera station in the center of F, by turning
the screw m it will represent the trace of a picture with the focal
length / (in inverse position, however, as it will correspond to
the horizon line as viewed upon the ground-glass plate of the
camera) (see Fig. 165, Plate LXXXVII).

The point P, intersection of the axis of the instrument with

282 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

OO'j will represent the principal point, plotted in horizontal
plan. It is marked on the edge of OCy by a small conical cavity
to receive the point of one arm of the dividers when the abscissae
of pictured points are transferred from the horizon line. W and
W are now pressed down, whereby the fine needles in the
centers of W and W are pressed into the working-sheet. The
end K of the alidade is now released and the abscissae of all
points, identified and marked on the perspective, are transferred
to the line 0(y from P, by means of the dividers, in their succes-
sive order but in inverse direction (the fiducial edge of the
alidade RR being gently brought into contact with the point
of the dividers each time), and the lines of direction are drawn
along the fiducial edge of the ruler end R with a well-pointed
hard pencil.

Should the image of the triangulation point be indistinct
or appear blurred upon the perspective, the instrument will
have to be oriented upon the drawing-board by means of the
angle of orientation ( = c«>) of the photograph, which angle had
been observed in the field (in the Italian survey that angle is
recorded in the field book, Model I, Chapter VIII).

The end R' of the alidade is placed and secured in such a
position that the fiducial edge of RR forms the angle (o with
the axis VP of the instrument, which angle is laid off (in the
inverse direction of the one observed) on the arc SS' of the sector
by means of the vernier v. The instrument is then revolved
about the needle in V the same as before, until the end R of
the ruler passes through the trigonometric point in question
marked on the plotting-sheet. The instrument having been
secured in this position, by turning the screws W and W, is
used in the same manner as just now described for drawing
the radials which served to locate the pictured points on the plan.

If a plate had been exposed while the vertical thread (prin-.
cipal line) bisected a triangulation point, the angle (o becomes
zero and the orientation of such photograph trace on the plot-
ting-sheet may be accomplished by bringing the zero of the

L. P. PAGANINl'S GRAPHIC SECTOR. 283

alidade vernier v to coincide with the zero of the arc gradua-
tion, 55', clamping RR' in this position and directing the end R
of the ruler to bisect the plotted triangulation point in question
and securing the sector upon the working-board in this position*

Should, finally, the perspective of which the trace in hori-
zontal plan is to 'be plotted contain no images of points pre-
viously located and plotted, then the zero of v is again made
to coincide with the zero of the arc SS' and the instrument is
revolved about the center of the needle V until the fiducial edge R
of the alidade coincides with a line that had been drawn from
the plotted station by means of the graphic protractor previously
described, which forms in V (station point) an angle with the
horizontal direction to some triangulation point observed in the
field and equal to the angle of orientation { = w) of the plate.
This angle is taken from the field note-book (Model I, Chapter
VI, I-C-6) and laid off on the sector in the inverse direction
and the sector is again oriented in the manner shown before.

After the horizontal directions to the^ different points, iden-
tified on the panorama pictures, have been drawn with the graphic
sector, they are provided with numerals or symbols to correspond
with the designations affixed to the points upon the panorariiic
views, in order to facilitate their identification when seeking
for the subsequent intersections with radials to the same points
from other camera stations. The positions of the secondary
and tertiary points on the plotting-plan are secured by inter-
sections, as has been described in the preceding chapters, and
they serve to make up the control of the map. It is weU to
transfer to the final drawing by means of tracings, which are
oriented with reference to the plotted triangulation points and
previously located camera stations, all the different points
obtained by intersections upon the construction board, in order
to erase therefrom all lead-pencil lines which served for their
determination, to obscure as little as possible subsequent con-
structions for the location of the positions of other points of the
terrene.

284 PHOTOTOPOGEAPHIC METHODS AND INSTRUMENTS.

in. L. p. Paganini's Graphic Hypsometer (<< Squadro grafico ").

After the plotting of the positions of the more important
secondary and tertiary points, in the horizontal sense, is well
under way, it remains to ascertain the elevations of the various
stations, mcluding the secondary and tertiary points of the per-
spectives, in order to enable the draughtsman to interpolate the
contours between the plotted points that control the terrene
forms of the area to be mapped.

The elevations of all plotted camera stations may be ascer-
tained by aid of Paganini's graphic hypsometer, Fig. 167, Plate
LXXXIX, using the plotted distances between the camera sta-
tions and surrounding triangulation points and the correspond-
ing angles of elevation, which had been observed to the latter
from the camera stations and which are recorded in the field
note-books (Model No. I, Chapter VIII).

The elevations of all secondary and tertiary points may be
determined with the same instrument by means of their graph-
ically measured distances from the camera stations and their
corresponding ordinates (y) measured on the perspectives.

Two rulers LL! and MM', Fig. 167, Plate LXXXIX, may
be made to glide with their ends L! and M' along a ruler AB,
always maintaining a position perpendicular to the latter, how-
ever, for which purpose their ends are secured to two sleighs L"
and M" which fit into two parallel grooves g and ^. The
motion of LL'y or rather L", is free, and it is accomplished by
pushing the button O up or down the ruler AB.

M" is provided with a ratchet and pinion P. By turning
the latter in one direction or in the other the ruler MM' will
be gradually moved up or down AB, as the latter is provided
with a row of teeth into which the ratchet-wheel of if" bites
while P is being revolved.

The alidade ruler d!d is secured with one end, (i, in F in such
a manner that M may be freely revolved about the axis of V

L. P. PAGANINl'S GRAPHIC HYPSOMETER. 285

as a center, while the other end, d\ passes over a graduated
arc Gg^. The plug in Y is similarly constructed as the one in V
of the graphical sector, Fig. 166, Plate LXXXVIII (it is pro-
vided with a revolvable button containing a slot, in such a man-
ner that the ruler AB may be revolved simultaneously with
the alidade d!i about a needle, marking the station point on the
construction board). In this instrument the plug, the revolvable
button, and the alidade ruler dd! have each a slot which inter-
sect each other in the center of rotation F, and through which the
needle marking V may be passed when they all have a certain
position, the needle being again secured in place by a quarter-
turn of the button. The entire instrument may be revolved
about the needle in F.. the center of which lies in the directions
of the fiducial edges of the ruler AB and alidade dd\

The alidade ruler dd' is provided with a vernier w, graduated
to read to 30 seconds on the graduation of the arc Gg^. This
vernier serves to lay off angles from V included between the
fiducial edges di AB and dd'. When dd' is brought close to
and in contact with ABy the zero of the vernier n and the zero
of the arc graduation Gg^ will coincide. The axis of the instru-
ment is represented by that edge of AB (facing dd') which passes
through the center of rotation 7, and which passes through
the zero of the graduated arc Gg^\ it also passes through the
point p of the line ^g, which is marked upon the ruler MM'y and
it is provided with a millimeter graduation. This line pq cor-
responds with the zero of the vernier n', which is attached to
MM' and which glides along AB with MM' when the latter
is moved iip or down the ruler -45. -45 is provided with a milli-
meter graduation also, and by means of the vernier n' the dis-
tance pV oi the line pq from the center of V may be read to o.i mm.

When the line pq is brought to the distance =/ from F, by
means of the fine ratchet movement at P, the line pq may be
regarded as the axis of abscissae drawn upon the perspective,
while the point p will then represent the principal point of the
perspective (see Fig. 168).

286 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

In this case the Ime pq may also be regarded as the axis of
ordinates of the perspective w«, Fig. i68, Plate LXXXX, pro-
vided the principal plane (containing VP and the axis of ordinates)
is supposed to have been rotated about VP until it coincides
with the horizontal plane VPCy.

The point p is permanently marked upon the line pq (in
the same way as described for the graphic sector) by a small
puncture, which likewise serves to receive one point of the dividers,
when such are used to lay oflf the coordinates, taken from the
perspective.

After Pq has been secured, at a distance =/, from the center V
and the abscissa :x; of a point a, taken from the perspective w»,
Fig. 1 68, Plate LXXXX, has been transferred to the line pq
from Py the second point of the dividers, upon pq, will represent
the horizontal projection a' of the point a. If we now move
the alidade ddf until its fiducial edge touches the second point
of the dividers, the triangle formed by the edge of the alidade
d'dj the edge of the ruler ABj and the line a'p will represent
the horizontal triangle VPaf of Fig. i68, Plate LXXXX.

The end df of the alidade is provided with a steel index mark f,
which may be moved along dd' by means of a revolvable but-
ton, £, ending in a ratchet-wheel below, which rotates in a row
of teeth attached to one side of the groove sfs. If this index
mark i is moved to a' (the intersection of the fiducial edge of
the alidade dd' and line pq)^ the distance Va' (cut oflf on dd')
will represent the horizontal distance of the point a' (of the per-
spective mn) from V (i.e., the value d in Fig. i68, Plate LXXXX)
measured on the scale with which the fiducial edge of dd' is pro-
vided. Maintaining the index mark i (Fig. 167, Plate LXXXIX)
in this position on dd' and revolving d'd about F, until its fiducial
edge coincides with the edge pV of ABy then moving the ruler MM'
away from V (by turning the button P) imtil the line pq coincides
with the index mark i, we will have transferred the distance d
(Fig. 168, Plate LXXXX) upon the axis of the hypsometer; we
will also have brought the line pq (engraved upon MM') to a dis-

L. P. PAGANINl'S GRAPHIC HYPSOMETER. 287

tance equal to d from the center of rotation in F, and by trans-
ferring the ordinate y (Fig. i68, Plate LXXXX), measured on
the perspective mn with a pair of dividers, upon the line pq (while
the latter is still in the position just described), by inserting one
point of the dividers into the cavity p and bringing the fiducial
edge of the alidade dd' gently into contact with the other point of
the dividers, restmg on the line ^jat a' (Fig. 167, Plate LXXXIX),
then the triangle Vpa' of the h3rpsometer will also represent the
vertical triangle Va'a of Fig. 168, Plate LXXXX, except that
now it has been revolved about Va' into the horizontal plan.

The movable ruler LU, which also remains always perpen-
dicular to the hypsometer axis {Vp) like MM\ consists of two
plates firmly joined together at their ends, between which the
alidade d'd may freely glide when revolved about V. The upper
plate of LL' is slotted like the handle of a penknife and the edges
LI and LP are beveled and provided with a millimeter gradu-
ation, the numerals of which correspond with a scale of

^ 50000

(50 m. = i mm.). A ratchet-screw c serves to move a plate
KOK'i with two index marks K and K' that may be adjusted
to coincide with the intersections of the fiducial edge of
the alidade dd' and the two graduated and beveled edges Li
and LV. The index plate KOK' has a double vernier, «",

on the opposite side of the ratchet-screw c, graduated to read —

millimeter (i.e., to read single meters for the scale) in

*=» 50000 ^

connection with the millimeter scales LI and LV.

When the zeros of the double vernier «" coincide with the

zeros of the graduated edges LI and i/', the marks K and K'

of the double index will coincide with the edge Vp of AB (i.e.,

with the axis of the instrument) and also with the fiducial edge

of the alidade d'd, the zero of the vernier n also coinciding with

the zero of the arc graduation Ggg^ (i.e., the fiducial edge of

dd' will fall together with the axis pV of the instrument).

388 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

In Fig. 1 68, Plate LXXXX, A may represent a point of the
terrene the image of which is designated in the perspective mn
by a. If i4' be the orthogonal projection of i4 in the horizontal
plane passing through the second nodal point F, then A A'
will represent the diflFerence of elevation =L between the points
A and V. A'V will be the horizontal distance =Z) of the point A

from the camera station F, which distance is represented by

for a scale of map of

'^ 50000

Returning to Fig. 167, Plate LXXXIX, we now imagine the
hypsometer revolved about the needle center in V until the
hypsometer axis pV passes through a plotted point i4' in the
drawing. If the ruler MM' had previously been secured in
such a position that the distance pVoi p from center of station V
is equal to d and if dd' had been set to lay off the ordinate y
upon pq from p, and if we now bring the index mark K in a,
position to mark the intersection of the fiducial edge of the ali-
dade dd' with the edge LI of LU, then the triangle VAA% Fig. 167,
Plate LXXXIX, will also represent (in the scale of i : 50000)
the triangle VA'A of Fig. 168, Plate LXXXX.

The index mark K indicating, on the beveled graduated

edge LI, the length , we find the difference of elevation

between the point A and camera station V by reading the cor-
responding vernier of the double vernier n".

The triangles Vpa' and VA'A (Fig. 167, Plate LXXXIX)
being similar, we will have

AA' Pa' y

We know that

VA' Vp d*

p. PAGANINI'S GRAPHIC HYPSOMETER.

289

hence

VA'^D^ and as F^'^^^,

we have

50000'
i = 50000 Xilil'.

The numerals of the graduation of the edges LI and LV and
of the double vernier n" give the value A A' already multiplied
by 50000, which is the true difference of elevation.

With reference to Fig. 168, Plate LXXXX, we have

L y AA'
tan a^^^=^-:= —

D d VA''

Hence if we know the angle of elevation of a point A of the
terrene we need only to lay off this an le upon the graduated
arc Ggg^ by means of the alidade vernier n, from g and place
the index mark K upon the intersection of the fiducial edge of
the alidade ddf and edge LI (the instrument having been placed
upon the plotting-sheet in such a position that the hypsometer
axis pV passes through the station V and the plotted point ^'),
and then read off on LI and corresponding vernier n" the differ-
ence of elevation between camera station and point A,

This -case becomes very much simplified when the image -4'
of A is bisected by the principal line of the perspective (axis
of ordinates), as then

x=o and J = /.

The alidade dd^ is placed so as to lay off the ordinate of the
point a upon pq from />, after the ruler MM' had . been secured
in a position at a distance = / from F; then the index mark K

290 PHOTOTOPOGRAPHIC METHODS AND tNSTKUMENTS.

or K' is brought into the point of intersection of the fiducial
edge of M' ^ with the edge LI or LV of the ruler LL' (the axis of
the hypsometer passing through the plotted point 4'), and the
difference of elevation between A and V is read off, either on
the vernier corresponding to the graduation L/, or on that cor-
responding to the graduation LV.

The corrections for curvature and refraction, to be applied
to these differences of elevation, are taken from the ordinary
field tables.

IV. The Centro-linead as Used by Capt. E. Deville.

Reference to this instrument has already been made under
the head of Photograph-board, Chapter VI; it is used in Canada
under Capt. E. Deville.

We had seen that the distance between the principal point
and the vanishing points of lines increases the nearer parallel
to the picture plane such lines would become.. Lines parallel
to the picture plane have their vanishing points at infinite dis-
tance from the principal point; practically they have no vanish-
ing points. Their perspectives are parallel with the original
lines.

In iconometric plotting it frequently occurs that the vanish-
ing points of some lines fall outside of the limits of the drawing
board, and in order to draw a line which if produced would
pass through the distant vanishing point, special constructions
would have to be made to locate the direction of such a line.

To avoid making such auxiliary constructions on the photo-
graph board Capt. Deville uses the so-called " centro-linead,"
with which a line vanishing at any distant point may be drawn
upon the picture plane no matter how far off from the principal
point of the perspective the vanishing point may be.

This instrument, represented in Fig. 169, Plate LXXXX,
is composed of a straight edge L (of wood) and two wooden
movable arms / and /'. The inclination of these arms / and /'

THE CENTRO-LINEAD AS USED BY CAPT. E. DEVILLE. 29!

against the straight edge may be varied to any angle. The arms
may also be permanently fixed in any position by means of the
clamp-screws r and r'.

We had seen that the photograph-board, Fig. 68, Plate XXXV,
was provided with four points ABC and £, indicating the centers
of the studs against which the arms / and /' of the centro-linead
play or rest when the same is used on the photograph-board.
The distance between these studs may vary. Each two forming
a pair are generally placed from six to eight inches apart, and
the arms of the centro-linead being held in contact with the
studs, the various directions of the ruler L will intersect each
other in one common point.

Referring to Fig. 170, Plate XCI,

A and B may represent one pair of studs, fixed upon the board ;
OA and OB the arms of the centro-linead, clamped in the posi-
tions indicated, and
OC the ruler of the centro-linead.

If we describe a circle through the three points A, O, and B —
the angle AOB remaining constant — the angle AOB will be
an angle of the periphery AB for any position given the ruler L
(or OC) as long as / and /' {OA and OB) remain in contact
with A and B (the two studs on the periphery of the circle).
When OC is changed to the position O'C thp intersection V
of the two lines OC and O^C will also be on the periphery of
the circle because the angle AOV {AO'V) remains the same
and must subtend the same* arc -47 as long as the position of
the studs remains unchanged. Hence for the assumed position
of the studs the directions of all lines drawn along the fiducial
edge of the ruler L (giving O all positions on the arc AOB)
will pass through the point V\ they will vanish at V.

In the iconometric work of the Canadian surveys the centro-
linead is used only for drawing the perspectives of horizontal
lines whose vanishing point is on the horizon line. The studs
A and B are placed on the photograph-board on a line AB per-

292 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

pendicular to the horizon line and at equal distances from the
latter. The horizon line {DD', Fig. 68, Plate XXXV) HH',
Fig. 170, Plate XCI, becomes a diameter of the circle AOBV
andFi4 = F5.

If the arms / and V of the centro-linead include the same
angles with the ruler i, the line OC^ bisecting the angle AOB^
must pass through V midway between A and B.

The distance of the vanishing point V from the principal
point P may be vjiried at pleasure by changing the inclination
of the arms / and /' against the ruler L. When the directions of the
arms / and /' fall together and are perpendicular to L, the vanish-
ing point will fall at infinite distance from the principal point Py
and the lines drawn along the ruler L will be parallel to the horizon
line.

The distance of the vanishing point V from P may also be
varied by changing the distance between the studs A and 5,
or C and £, Fig. 68, Plate XXXV; increasing this distance
enlarges the circle i4 05 F and V moves farther off from P\ reduc-
ing that distance decreases the diameter of the circle AOBV
and V will approach the principal point P, The practice in
Canada, however, is to retain the position of the studs unchanged
on the photograph-board and to change the inclination of the
arms / and /' of the centro-linead instead.

If we gradually close the arms / and /', V will approach the
line i45, and when the angle ^405 becomes equal to 90° the
arc AOB will have become a semicircle and the intersection of
AB with HH' will be the center of the circle AOBV^ the distance

of both O and V from AB will be equal to — ; continumg to

close the arms / and /', V will approach closer to AB without
ever reaching it.

THE CENTRO-LINEAD AS USED BY CAPT. E. DEVILLE. 293

A. To Set the Arms I and V of the Centro-linead if the Direction
to the Vanishing Point be given by a Line in the Ground
Plan.

In Fig. 171, Plate XCI,

P= principal point on the photograph-board;
A and 5= positions of the two studs;

5v= given direction of the line on the ground plan, when V will
be the vanishing point for that line.

If we revolve the picture plane about the horizon line as axis
into the horizontal plane the station may fall in 5, Fig. 171,
Plate XCI, when SP will represent the horizontal projection
of the principal ray or the distance line (focal length) of the
picture. If the point V would fall upon the drawing-board we
could describe a circle through AB and V and place the fiducial
edge of the ruler upon DP (the horizon line) with the axis of
rotation o of the arms / and /' in D upon the circle, bring the
arms / and /' into contact with the studs A and B and clamp
them in that position. In this case there would be no use for
the centro-linead, however, as F is accessible.

If we join VB, the angle VDB — the inclination of the lower
arm /' against the ruler L — is equal to VBA, both angles sub-
tending equal arcs of the same circle. Draw the lines CS and
BS. At any point c on CS draw cM and cv parallel to -45
and DP and join b and v. By reason of similarity of triangles
vb must be parallel to VB and the angle

vbc^VBC=BDV.

Hence the arms of the centro-linead may be set, in the case
imder consideration, by placing the ruler L on Mb, the axis of
notation, O, coinciding with b and adjusting the lower arm /'
of the centro-linead to coincide with bv. The other arm /, having
the same inclination against the ruler L as /', may be set by placing

294 PHOTOTOPOGRAPHIC METHODS AND INSTRUBfENTS.

the ruler L upon the horizon line DP and moving it along this
line until the lower adjusted arm /' comes mto contact with the
stud -B, then moving the other arm / about O until it comes into
contact with the stud A and clamping it also.

The lines 55, C5, Afc, and cv are drawn once for all upon
the photograph-board, Fig. 68, Plate XXXV. The only line
to be drawn for setting the centro-linead arms is 5v, which is
the direction of the given line on the groimd plan. The line bv
need not be drawn, the points h and v being located by draw-
ing cv parallel with the horizon line and cM or ch parallel to the
distance line SP.

B. To Set the Arms of the Centro-linead if the given Line (VE)
belongs to the Perspective.

Take any point F, Fig. 172, Plate XCI, on the horizon line,
join F with E and F with B, then draw cM parallel to AB.
Through e draw ev parallel to EV and join vb. Owing to the
similarity of triangles vb will be parallel to VB and the angle

vbc^VBA,

which is the inclination of the arm against the ruler L of the
centro-linead. The lines FB and cM are permanently laid
down on the photograph-board. Fig. 68, Plate XXXV, but FE
and ve will have to be drawn for every given line; in this case
two lines will have to be drawn instead of one as in the pre-
ceding case.

Centro-lineads are usually sold in pairs; one serves to work
on the left side of the principal point and the other on the right

V. The Perspectometer as Used by Capt. E. Deville.

The perspectometer serves to dispense with the con-
struction of the squares on the perspective, when using the
method of squares (Chapter IV), to draw a figure in the

THE PERSPECTOMETEK AS USED BY CAPT. E. DEVILLE. 295

ground plane by means of its perspective. On a thin trans-
parent film (glass, xylonite, isinglass, horn, etc.) two parallel
lines AB and J9Z)', Fig. 173, Plate XCII, are drawn intersecting
the common perpendicular pP, Make DP = Piy ^pA^pB =dis-
tance line (focal length) and from p lay oflf on AB (to both sides
of p) equal distances,

pm^mn^no . . . =^fm'^m'n'—n'o' ....

Join these points of division to P and through the corresponding
intersections of the radials from P with AD and BU draw lines
rr'yie . . . ^ which lines will be parallel to -45 and DD'.

Use of the Perspectomeler, — ^The instrument is placed upon a
perspective with P on the principal point and DD' coinciding
with the horizon line. The ground line of the perspective may
fall in XY] it is divided into equal parts by the radials from P,
and the trapezoids of the perspectometer represent the per-
spectives of the squares in the ground plane having the equal
parts of XF as sides.

By placing the perspectometer on the perspective in the
manner indicated above, the squares covering the perspective
of the figure, which is to be plotted iconometrically on the ground
plan, are at once apparent and only those needed are drawn
on the ground plan.

The sides of the squares to be drawn on the ground plan
(their side lengths are equal to the divisions on the ground line
between the radials trom P) are laid off from the trace of the prin-
cipal plane on the ground line, and the position of the front line
nearest the picture trace (or groimd line) is laid off on the ground
plan either by estimation or by construction. The estimation
of the position of this line (corresponding to if) on the ground
plane is made by noting the fraction of a square's side which
represents the distance (between if and XY, Fig. 173, Plate XCII)
from the ground line on the perspective.

The perspectometer serves only for perspectives which have

296 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

the same distance line (like photographs of distant objects taken
with the same lens), different distance lines requiring a new
perspectometer to be constructed for every one.

The width pP should be equal to the height of the horizon
line above the foot of the picture; the radials need not extend
beyond the width of the picture, the distance points D and Z>'
having been taken as the limit of the perspectometer in the
j&gure (Fig. 173, Plate XCII) merely to show more fully the
principles involved in its construction. The length of a single
division on the line AB should be selected with reference to the
resultant equal division lengths of the lowest ground line used
for the pictures, as the divisions on the latter give the measure
for the sides of the squares to be drawn on the ground plan.

These division lengths on the ground line should be in har-
mony with the scale of the plan and with the degree of accuracy
that may be required for the delineation of the topographic
features. The smaller the size of the squares on the ground
plan selected the more accurately the transfer to the ground
plan of the figure from its perspective may be made; the same
principles being involved in this method of iconometric plotting
as in the well-known method of reducing (or enlarging) draw-
ings by means of two sets of squares, the ratio of their sides
corresponding to the scale of the required reduction (or en-
largement).

Capt. Deville recommends the perspectometer to be made
by first drawing it on paper in a fairly large scale and then
making a negative of it, reduced photographically to the desired
size of the finished perspectometer. A positive copy of said
negative may then be made on a transparency plate, which,
if bleached in a solution of bichloride of mercury, will show
white lines (they were dark before the bleaching took effect)
on clear glass. For the sake of better preservation the per-
spectometer is varnished when completely dry and hard.

When using the perspectometer to transfer figures from
their perspectives to the ground plan, when such, figures are

THE PERSPECTOMETER AS USED BY CAPT. E. DEVILLE. 297

situated in planes perpendicular to the picture plane but in-
clined against the horizon plane, the center of the perspec-
tometer is placed upon the principal point P of the picture
plane the same as before, but the perspectometer is now re-
volved about P until the parallel lines of the same are parallel
to the trace on the picture plane of the inclined plane (contain-
ing the figure to be transferred). In this case the trapezoids
of the perspectometer represent the perspective of a net in the
inclined plane composed of squares which are now to be pro-
jected into the ground plane.

This net of squares in the inclined plane, when projected
on the ground plan, will be composed of rectangular figures
of equal size, their long sides being in a direction at right angles
to the picture trace (or ground line) ana of a length equal to
that which is intercepted between .two adjoining radials of the
perspectometer on the trace of the inclined plane (on the pic-
ture plane), while the short sides of those rectangles forming
the net of the ground plan will be equal to the lengths obtained
on the groimd line by projecting the points of intersection of
the radials of the perspectometer with the inclined plane's
trace on the picture plane upon the ground line of the picture
plane.

The construction of the rectangular net on the ground plan
may now be made in an analogous manner to that mentioned
for the squares, and the drawing-in of the figure on the ground
plan with reference to its position within the trapezoids of the
perspectometer is accomplished in the usual manner.

Should the figure be situated in planes that are inclined
to both the picture and the ground planes, then the figure is
first projected upon a plane perpendicular to the picture plane
and having the same trace in the latter as the inclined plane.

298 PHOTOTOPOGRAPHIC liETHODS AND INSTRUMENTS.

VI. The Peispectograph, Devised by H. Ritten

Numerous instruments have been devised for the mechanical
drawing of perspectives from plans (or from nature), or by
meajis of optical devices, some of which may, inversely, be-
come of use for transcribing perspectives of figures into orthog-
onal projections, and we have seen that Col. Laussedat as far
back as 1849 inade his first experimental perspective surs-eys
with the camera lucida or camera clafa, devised by Wallaston,
which in this case had been improved by Regnault. The per-
spectograph invented by H. Ritter serves to construct the
orthogonal projection of a plane figure from its perspective
or to draw the perspective from the plans of the object without
referring to the object itself.

Ritter's instrument, manufactured by C. Schroeder & Co.
in Frankfort on the Main, has been patented in Germany,
Oct. 13, 1883, under No. 29002. It was devised primarily
for architectural purposes. For the title of Ritter's descriptive
pamphlet, see Literature, paragraph 2, Chapter I.

This instrument in its present form, composed largely of
wood, is not well suited for surveying purposes, as it contains too
many sources of error, due to lost motion in its bearings; still,
its theory being a good one, there is no reason to doubt its ulti-
mate value, even for precise work, if it were carefully made by
an expert mechanician, excluding the use of wood and using
metal throughout, being guided in its construction by the de-
mands of the utmost attainable precision. Since a carefully
constructed instrument based on the present pattern may become
useful in plotting the data of a topographic reconnaissance;
where, in the nature of the work, rapidity in making the results
practically available will often be of greater value than a high
degree of accuracy, the following description of this instrument
may not be out of place here. For its methods of use in photo-

THE PERSPECTOGRAPH, DEVISED BY H. RTTTER. 299

topographic surveying we respectfully refer to Capt. Deville's
work on photographic surveying.

We have seen that the plotted position in the ground plan of
a point may be found from its perspective by locating the inter-
section of the horizontal projection of the ray: "station pic-
tured point" with the line of direction foimd by revolving this
ray with its vertical plane into the ground plane (about the trace
of the vertical plane in the ground plane as axis of revolution).
With reference to Fig. 174, Plate XCII,
S may represent the camera station;

M the position of a point plotted on the groimd plan GG;
fi its perspective in the vertical picture plane MN\
s the foot of the station S\
XY the ground line of the picture plane MN.

If we draw through the foot of the station a line parallel to
the ground line XY and make its length, 5(5), equal to sS, join
the plotted point M with (5), then it will follow, from the simi-
larity of the triangles OiiM and sSMy that

5S\Oii^M5:MO.

The triangles s{S)M and 0{p)M being also similar, we find

s{S):0(/i)=Ms:MO;
hence

• sS:Ofi^s{S):0(fi).

As we had made sS^s(S), the last equation can only prevail if

To find, therefore, the perspective, /£, of a point, Af, given on
the ground plan, we first draw through the plotted station, on
the ground plan, a line .5(5) parallel to the groimd line XY,
making 5(5)= height of the station 5 above the ground plane.
Draw the lines sM and (5)M, which will intersect the groimd
ine, XY, in O and (/£), Fig. 175, Plate XCIII. On the ground

300 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

line X'F', drawn in another place of the working-sheet, we assume
a point O', representing O of the ground plan, and erect o/i per-
pendicular to X'F' in O' and equal to 0(p)y when p. will be the
perspective of M in the reverse position of the perspective. The
perspective of any other point N on the ground plan may be
found in the same way, making 0'Q' = OQ and Q'i^ = Q(»>).

Ritter devised the perspectograph with reference to the pre-
ceding relation between the visual ray, 5Jf , Fig. 174, Plate XCII,
to a point Af , the horizontal projection of the ray, and the plotted
position of such point M, the perspectograph performing the
preceding construction. Fig. 175, Plate XCIII, mechanically.

The general arrangement of this instrument is shown in Fig.
176, Plate XCIII: sM and {S)M are two slotted wooden arms
carrying the tracer, M, at their point of intersection. The con-
nections at Sy Oy (s), and (/i) are such that the rulers sM and (S)M
may -slide through these points. The slide connections ^ and
(5) may be moved along the groove or slot of the wooden ruler
RT, The sliding piece O is secured to a rod which may slide in
the groove shown in the wooden ruler XF, being connected at
its other end D with a system of arms, joined together after the
manner of a pantograph. The distance OD is maintained
unchanged while the instrument is in use.

The center of 5 is placed over the point which marks the
plotted camera station on the ground plan, and the ruler RT is
placed parallel to the ground line of the picture plane, s and
RT are then secured in this position on the groimd plan.

When the arm sM is moved, 5 being held in a fixed position,
the point O will follow the motions of the arm sM, also applying
its motion directly to the arm OD (which slides in the groove of
XY) and indirectly to the arms of the pantograph system.

The fourth sliding piece (/z) is connected with the point A of
the pantograph system by means of a separate piece which insures
a permanent distance between (ft) and A while the instrument is
in use, and which may slide on the rod OD. The pantograph
system is composed of six pieces: four straight arms, AB,AC, Ffi^

THE PERSPECTOGRAPH, DEVISED BY H. RITTER. 30I

and F/i', and two double arms, CDE and BDG^ which arc bent at
right angles in their points of junction D, The sides of the two
parallelograms ABDC and DGFE are all of equal lengths, and
the six arms are joined in i4, 5, C, Z), £, F, and G. The lengths
of the arms F/z and F/z' are twice that of the side of the parallelo-
grams. The pencil which describes the perspective may be
attached to the free end of either arm F^i or Ffx\

The angles GDB and EDC being each equal to 90°, the sum
of the two other angles CDB and ODE must be equal to 180°.
The sum of two adjacent angles in a parallelogram being also
equal to 180°, it follows that

CDB +GDE=CDB +DCA,

or GDE==^DCA,

which shows that the two parallelograms are also equiangular,
and as their sides are equal in length it follows that the parallelo-
grams themselves must be equal, but they are placed in different
directions. The diagonals FD and GE of the one are equal to
BC and DA of the other, respectively. The two long arms
Ffj! and Ffi being of the same length, /z/z' will be parallel to G£,
both will be perpendicular to the direction of XY^ and //// will
pass through D. We have, therefore,

Dii'^Dfi^GE^DA.

Use 0} the Perspeciograph. — The sliding piece 5 is secured
to the working-board over the plotted position of the camera
station on the ground plan, still permitting a gliding movement
of the arm sM in the direction sM. The center line of i?r is
brought into a position parallel to the plotted ground line and
its position is also secured to the board. The sliding piece (5),
finally, is moved from s (in the groove of RT) until s{S) is equal
to the elevation of the station 5 above the ground plane, also
securing (5) in this position, when it will still permit a gliding

302 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

movement of the arm (S)M in the direction of (S)M. The
center line of tbe wooden ruler XF is placed upon the ground
line (picture trace) on the ground plan.

The manipulation of the instrument and its general working
will now readily be imderstood. For instance, when the tracer M
is moved in a direction parallel to RT or XY, the arm sM will
also move the slide OD in the same direction. The distance
0(pl) remaining unchanged as long as s(S) imdergoes no change,
(fi)A will also remain of a constant length. Hence, AD and
also GE as well as Dfi undergo, no changes, and the pencil in ft
or in / will trace a parallel line to XY, representing the perspec-
tive of a line of the ground plan (the one traced by M) and parallel
to the picture plane.

When M is moved in the direction of sM, away from XY,
the positions of O and D remain the same, but 0(/z) will be
lengthened, (/i) moves to the right — away from O — carrying the
point A with it {A{ii) being a constant length) and increasing
the length of the diagonal DA in proportion to the increase of
the length 0(p). DA, being equal to GE, equal to DpL{^Df/)y
the latter will also be lengthened and /i will move down — away
from XY — by the same amount as (/r) is moved to the right.
The relation between the construction made in Fig. 175, Plate
XCIII, and the mechanical plotting with the perspectograph,
Fig. 176, Plate XCIII, will now be evident.

VII. Prof. 6. Hauck's Trikolograph and its Use in Iconometric

Plotting.

This . instrument has been described by Dr. G. Hauck in a
memorial commemorating the opening of the new building of
the Royal Technical High School at Charlottenburg, near Ber-
lin, Nov. 2, 1884. It serves to reconstruct an object from two
perspectives obtained from two different points of view.

The principles which underlie the construction of this instru-
ment hold equally good for the construction of an instrument

THE HAUCK TRIKOLOGRAPH. 303

which could serve to plot mechanically the ground plan of any
object represented on two photographs obtained from diflFerent
stations.

Prof. F. Schififner, in 1887, suggested the changes to be made
to Dr. Hauck's instrument in order to render it available as an
instrument of precision for the use of the phototopographer; still,
it seems that mechanical difficulties in its manufacture are yet
to be overcome, as the writer has not met with any record of
such an instrument having been in use or even constructed.

In Chapter IV it has been shown that a point, A^ photo-
graphed from two stations, S and 5i, may be plotted in hori-
zontal plan, if the two picture traces gg and gigu and the two
camera stations 5 and 5i, are given on the horizontal plan, Fig.
177, Plate XCIV.

The two picture planes may be revolved about their groimd
lines, gg and ^1^1, into the horizontal or ground plan, when (a)
and {cLi) will be the two images of the point, A^ revolved into
the ground plane. If we draw lines through (a) and (ai) per-
pendicular to the corresponding ground lines gg and gigiy then
a' and a'l (Fig. 177, Plate XCIV) will be the projections of the
pictured points a and ai into the horizontal plan and the inter-
section of the radials drawn from 5 and 5i to a' and ai', re-
spectively, will locate the position A' of the point A pictured
on the two plates as a and ai.

This graphical determination of the plotted position A' of
the point A may be accomplished mechanically by placing
slotted rulers with their center lines upon gg and gtg^ Fig. 178,
Plate XCIV, and indicating the directions of the perpendiculars,
dropped from the pictured points (revolved into horizontal plan)
upon the ground lines, by two arms, {a)bc and a'6, of a panto,
graph combination, where

(a)6=k==a'&.

The points {a)a' and c will always be situated on the pe-
riphery of a semicircle described about h as the center, and as

304 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

the points c and a' are permanently held on the line gg^ the angle
aa'c (angle of the periphery subtending the semicircle) will be
equal to 90° for all inclinations that may be given {a)c against gg.

The directions of the radials So! are laid down mechan-
ically by means of two slotted rulers So! and S\a^y held in posi-
tion by the studs in 5 and o! (in 5i and ai'), both rulers being
revolvable about the fixed points 5 and S\,

This instrument, of which the characteristic features are
shown in Fig. 178, Plate XCIV, performs the constructions
mechanically which were made graphically or geometrically in
Fig. 177, Plate XCIV.

The slotted rulers gg and ^1^1 are secured to the plotting-
ioard (with their center lines on the picture traces) by means of
thumb-tacks T. The pantograph-arms {d)c{a^ c^ and a!h ai'6i
arc connected with these rulers by means of sliding joints c
(and Ci) and a! (and ai')j while the studs which mark the sta-
tions 5 and 5i end in cylindrical projections which fit into the
slots of the rulers So! and S\a\^ the latter fitting also over similar
cylindrical attachments to a! and ai', in such a way that the
rulers So! and S\a\ may freely glide over the points 5 and a' (or
S\ and ai')j and at the same time may revolve about the fixed
points S and 5i respectively.

The points (a) and (ai) are provided with tracers and a pencil-
slide is attached to the intersection of the rulers Sa' and S\ai
(in A') in such a way that the pencil point may freely slide either
way in the grooves of Sa! and S\a\,

A comparison between Figs. 177 and 178, Plate XCIV, will
plainly show that A' will always represent the plotted position
of two images (a) and (a\) (revolved into horizontal plan) of
the identical point A,

It may not always be possible to identify both images of the
same point A on the two pictures, and in order to apply Prof.
Hauck's method, to identify the second image (on the second
photograph) by means of the so-called '* kernel points'* the
instrument, shown in Fig. 178, Plate XCIV, must be modified

THE HAUCK TRIKOLOGRAPH. 305

in such a way that the point of the second tracer will always be
upon the image (on the second picture) which the point of the
first tracer designates on the first picture (revolved into the
ground plane).

We had seen in Chapter IV that the line connecting the
image of any point A on the first picture with the image of the
second station (kernel point (^i), Fig. 179, Plate XCV) and
the line connecting the image of the same point A on the se ond
picture with the image of the first station (kernel point (5), Fig.
179, Plate XCV) will bisect the same point a of the line of
intersection of the two picture planes. The picture planes being
vertical, this line of intersection will be the vertical line passing
through the point Q of the ground plane (point of intersection
of the two picture traces or groimd lines gg and g\g{)* The
picture planes having been revolved about their ground lines
as axes into the horizontal plan, this line of intersection aQ^ also
revolved into the ground plane (and about gg and again about
^i^i)> will appear twice, once as Q{a\ perpendicular to gg in Qy
and again as fi(<Ti), perpendicular to ^1^1 in C As the points
(<y) and (<yi) represent the same point <;, revolved into the hori-
zontal plane, once about gg and again about ^1^1 as axes, the
lengths (a)Q and {a{)Q must be equal.

In order, therefore, that this instrument (Fig. 178, Plate XCIV)
may work in harmony with the principles which underlie Prof.
Hauck's method, it will have to be modified to fulfill the follow-
ing conditions:

A line drawn through the kernel point 5i and any point pictured
on the first photograph, and a line drawn through the kernel
point 5 and the image on the second photograph of the same
point, are to intersect the line of intersection of both picture
planes in the same point <;, or, the two lines revolved into the
horizontal plan (with the picture planes) must bisect the re-
volved lines {a)Q and {ax)Q in points (a) and (<7i), which are
equidistant from Q.

The complete instrument is represented in a general way

3o6 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

in Fig. 179, Plate XCV. The two slotted rulers gg and gigu
of Fig. 178, Plate XCIV, have been supplied with additional
arms Q{o) and Q{ai)f each arm including an angle of 90^ with
its ruler. These rectangular elbow-pieces are secured to the
plotting-board by four thumb-tacks T after the rulers gQ and
giQ had been placed with their center lines upon the picture
traces gg and ^1^1, respectively, in such a way that the intersections
of the center lines of the elbow-rulers, at the rectangular elbow
end of the rulers, coincide with the intersection Q of the ground
lines or picture traces gg and ^1^1. The pantograph-arms, repre-
senting the ground lines of the pictures, are attached to the rulers
the same as in Fig. 178, Plate XCIV. Studs are inserted into
the kernel points (si) and (s), and the arms Q(a) and 0(ai) sup-
port a ruler (<t)(<ti), which may glide freely over these arms
of the elbow-pieces. To cut oflF equal lengths on the elbow-
arms Q((7) and fi(<Ti) by this ruler (<t)(<7i) the angle d{a)e is ad-
justable, and it should be regulated for each set of two picture
traces to make

When (a)d is moved along the slot of {a)Q the slide point
(<7i) will move along {oi)Q, Q{a) always being equal to Q{ai).
The screw d serves to clamp the angle d(a)e for any opening
corresponding to the angle gQgi included between the picture
traces. Slotted rulers are now placed over the studs marking
the kernel points (^i) and (5), the slots also receiving the cylin-
drical prolongations pf the tracers (a) and (ai) and those of the
slide points {a) and (<ti) respectively. Finally two slotted
rulers RS and R\S\ are placed over the studs 5 and S\ (they
mark the plotted positions of the two stations) and over the
sliding joints a' and ai' (which are the same as those in Fig. 178,
Plate XCIV). At their point of intersection, A'^ the sliding
pencil point is inserted into the slots, and this completes the
instrument. If we now move the tracer (a) on the first photo-
graph, the pantograph arms {a)c and ba' will change the position

THE HAUCK TRIEOLOGRAFH. 307

of the ruler SR into the direction of the radial from 5 to the hori-
zontal projection — on the picture trace — of the pictured point
designated by the tracer point (a) on the first photograph and
the ruler {a)(s) is moved, locating the point (<j). This change
in the position of (a) produces a corresponding change in the
sliding point (<7i), which in turn changes the position of the tracer
(ai), causing the pantograph-arms (ai)c and Jifli' to move, and
a change in the position of ai will cause the radial ruler RiSi
to assume a new position also and the intersection of RS with
the new position of RiSi locates the plotted position in hori-
zontal plan of the point under the tracer on the first photo-
graph without actually having identified the corresponding
image as the identical point under the tracer (ci) on the second
picture.

If a line on either photograph is followed out by one of the
tracers (a) or (ai), the pencil point A' will draw the horizontal
projection of the pictured line, the second tracer being watched
merely for the sake of obtaining a check or to aid its course,
if necessary, by a gentle tapping, when the movements of the
various parts of this instrument should retard its motion owing
to too much friction or lost motion.

Until now no perfect perspectograph has been constructed,
and no matter how accurately such instruments — like the one
just described — may be made by the mechanician, there will
always remain some unavoidable imperfections in the piaterial
or in the worknianship of the instrument, producing more or
less error in the results. For accurate and precise work, there-
fore, all iconometric plotting (when applying the radial or so-
called plane-table method) should be accomplished with the
aid of graphical or geometrical constructions, at least for all con-
trol points of the survey, relegating the use of perspective instru-
ments to the filling in of such details, which in an instrumental
survey of like character would be sketched by the topographer.

3o8 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

VIII. The Carl Zeiss Stereoscopic Telemeter and the Stereo*
comparator, including the Stereophotogranunetric Survey-
ing Method, Devised by Dr. C. Pulfrich.

Stereoscopic surveying, when employed for phototopography,
has many advantages, especially if the stereoscopic views of
the terrene may be transferred into the orthogonal horizontal
projection of the plan or map by means of stereoplanigraphs,
or stereoscopes that arc supplied with the necessary details
and means for adjustment that may be required for the semi-
mechanical plotting of topographic control points.

The idea of using two stereoscopic views of the ground, ob-
tained from two properly selected stations, in a specially devised
stereoscope and projecting the selected characteristic terrene
points of the stereoscopic image directly on the plotting-sheet,
by means of a movable projecting index mark, occurred to
Capt. Deville about ten years ago. Owing to the pressure of
other official duties, however, Capt. Deville had to suspend the
continuance of his experiments in this direction. This inter-
ruption is greatly to be regretted, as he had practically solved
the problem of stereoscopic plotting by using a modification
of the Wheatstone stereoscope. A description of Capt. Deville's
interesting instrument may be found in:

Transactions of the Royal Society of Canada, Second Series, 1902-1905,.
Vol. VIII, Section III, " On the Use of the Wheatstone Stereoscope in
Photographic Surveying." E. Deville.

Also in

A. Laussedat. "Recherches sur les Instruments, les M^thodes et le Dessin
topographiques." Tome II. Paris, 1903. "La St^rdoscopie appliqu^
k la Construction des Plans."

Dr. C. Pulfrich. "Ueber eine neue Art der Herstellung topographischer
Karten und ueber einen hierfuer bestimmten Stereoplanigraphen.*^
Zeitschrift fuer Instrument cnkunde, Heft V (Mai), 1903, XXIII Jahrg.

STEREOSCOPIC TELEMETER AND STEREOCOMPARATOR. 309

Dr. Pulfrich has devised a stereoplanigraph which is being
made by the Carl Zeiss firm in Jena, a description of which may
be found in the last-mentioned paper by Dr. Pulfrich. This
instrument seems to be planned on the lines suggested by Capt.
Deville.

A perfected stereoplanigraph would be the ideal instrument
for the rapid plotting of topographic features and details if the
terrene is controlled by a close network of triangulation.

A. The Stereoscopic Telemeter,. or Range-finder,

The stereoscopic telemeter, or aerial distance measure,
manufactured by the Carl Zeiss Optical Works in Jena, Ger-
many, was first brought to general notice in a lecture delivered
by Dr. C. Pulfrich before the Society for Natural Research,
Munich (Sept. 19, 1899).

This telemeter, devised by Dr. Pulfrich, is the outgrowth of
ideas that had been suggested in a measure by Prof. Porro to
break the straight course of the light-rays in a telescope, by means
of a series of prisms, into a zigzag path and thus reduce the length
of the ordinary telescope.

The Carl Zeiss Optical firm not only succeeded to improve
on the quality of the prism telescopes heretofore in use, but it
succeeded also to combine two such telescopes into a binocular
set. The relief effect produced by the Zeiss prism binoculars,
based on the difference between the two retinal images, is ac-
centuated by an optical increase of the interocular distance,
simply by setting the two objectives of the binoculars farther
apart. The ratio between the ocular and the objective distance
gives the "stereoscopic power" of these stereobinoculars.

The great practical success of this combination, however,,
is mainly due to the recent discoveries made in the optically
worked glass compositions produced by the now world-famed
Jena Optical Glass Works. Dr. Pulfrich could now realize
H. Grousillier's idea of the aerial distance scale, and aided by

3IO PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

the* excellence of the mechanical equipment of the Carl Zeiss
firm, the present form of the "stereotelemeter*' has been manu-
factured and placed on the market.

With this portable stereoscopic telemeter distances may be
read oflf directly, the degree of accuracy attainable in the meas-
ures being almost entirely independent of the sha,pes of the
objects determined, which, furthermore, may be stationary or in
motion. A special transverse scale is also provided for measur-
mg the width or length and the height of any distant object, for
making measurements in "frontal planes."

The Carl Zeiss firm has placed three distinct t)rpes or grades
of stereotelemeters on the market, differing in range, magnifi-
cation, and weight, and, of course, also in price.

The so-called " total relief effect " may be expressed by the

product — — ,

where £=distance between objectives ( = 510 mm.);
e« distance between eyepieces (=65 mm.);
G = magnification ( = 8. ).

The middle-size telemeter, to which the figures just given refer,
will have a total relief effect of 63. That is to say, if differences
in relief on the single plate are not observable beyond 450 meters,
the stereoscopic image, as it appears to the observer through
this stereotelemeter, will show differences in depth or relief
at 63X450 m. = 28.3 km. This, however, does not mean that
any such distances may be read with its aerial distance scale;
it simply gives the extreme limit for recognizing terrene forms,
all points beyond that distance appearing as infinitely far off.
If we direct the stereotelemeter to a point P at infinite dis-
tance (Plate CIX) the component images of the point P will
be at p and f. If we now consider a second point F, just in
front of P, its image will still coincide with p in the left image
plane, but in the image plane of the right binocular tube it will
appear at ^', to one side of p^.

STEREOSCOPIC TELEMETER AND STEREOCOMPARATOR. 31I

The distance //', spoken of as the linear parallax of the two
points P and P', is directly proportional to the distance between
the two points. The rays o'^ and </f include the angle of
parallax =*, and as the triangles (/ff' and P'0& are similar,
we will have the proportion

where /=» focal length of o\

£«interobjective distance, or telemeter base;
Z)= distance of point P from O, PF being negligible in com-
parison with OP.

Hence the linear parallax

JE and / being constants, we find by differentiation

dD-^ day

and substituting the above value for a we find

The error in linear parallax, (to, is directly proportional
to the product of the focal length and the angular parallax d,
and inversely proportional to the magnification G.

^ fXd , Gxda
da^-Q^^ f ^.

and we may now write

Z>2

312 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

If we now designate by r the range of stereoscopic vision
by unaided eyes — in other words, if r is that distance at which
an object must be placed to be seen under an angle of parallax =5
— we will have the relation

As d will always be a small angle we can substitute the tangent
for the angle.

If we designate by R the range of the field that is con-
trolled by the total effect of relief, we will have

E ^ EXG

e d

After substitution of this value in the above equation for dD we
finally find

Numerous experiments have shown that the angular parallax
(d)f the angle under which objects situated in different but very
distant frontal planes cease to appear to be at different depths
when examined under binocular vision, amounts to 30 seconds
for normal eyes (Helmholtz gives d — i'^).

Hence we find from

e=»r-*=rX3o".

For small angles we can substitute for one second the value
1:206265; hence for an interocular distance of 65 mm.,

^30 0.065 ^ ^

*'-°^5"=,-^6;6^. or r=— 20626s;

f «446.9 m., or in round numbers, 450 hl

STEREOSCOPIC TELEMETER AND STEREOCOMPARATOR. 313

The ^' hunting''^ or ^^sporting^* telemeter has a base of 32 cm.^
a telescopic magnification of 4, and a scale for reading distances
from 20 to 500 meters. Objects beyond 8000 meters appear as
infinitely far off. The weight of this instrument is about 2 J kgr.
Fig. I, Plate CVIII, shows a general view of the Zeiss sporting-
telemeter.

The ** infantry telemeter ^^ has a 51 -cm. base, a telescopic
magnification of 8, and a scale for distance reading from 90
to 3000 meters. Objects beyond 28 km. appear at infinite dis-
tance. This instrument has a weight of about 3I kg., and it
cannot well be manipulated without a support. The telemeter
with suitable (tubular) stand weighs 6J to 9J kg.

The so-called ^^ stand telemeters ^^ (the central part of one is
shown in Fig. 2, Plate CVIII) have a 1.44-m. base, a telescopic
magnification of 23, and a scale for reading distances ranging
from 500 to 8ooo> meters. Objects beyond 230 km. appear at
infinite distance. 'Jhese stereotelcmeters require a rigid sup-
port, and the Zeiss firm has devised a special tripod for them.
The weight without tripod is 15 J kg. The packing-case weighs
2oi kg. The tripod with fork- rest and tilting joint weighs 18^ kg.

The attachment marked B in Fig. 2, Plate CVIII, is to be
secured to the right eyepiece for illuminating the image planes
in the binocular microscope of the stand telemeter when ad-
justing the ocular scale in the stereoscopic image plane. It is
used in connection with a pair of Gautier-Prandl prisms that
may be adjusted over the objectives, as indicated by dotted lines
in the diagram, Plate CIX. When these Gautier-Prandl and
the eyepiece prisms are in the position shown, the light-rays
entering 0' through the prism B will pass from w' to O', thence
through the right Gautier-Prandl prism, through the left prism,
through O and m, emerging through 0, illuminating both image
planes in their course, as indicated by the dotted line, Plate CIX.

We will not refer here to the adjustments nor to detailed
directions for using the different types of stereotelcmeters, as
printed directions are sent out with every instrument.

314 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

The errors affecting the readings of these telemeters increase
with the square of the distance. There is a certain zone of un-
certainty for all points in a frontal plane. That is to say, that
for a certain reading made with the well-adjusted telemeter the
distance, as read oflf on the aerial scale, may be too long or too
short by a certain amount; each reading will be affected by a
positive or a negative error.

In the table opposite the probable errors,

iiffecting different distances, read with the three types of telem-
eters, have been tabulated for comparisoii.

To use a stereoscopic telemeter successfully the observer
must be able to see ** stereoscopically"; this, of course, ej^cludes
all persons with defective vision or who have developed the power
of vision in one eye at the expense of the other, or whose eyes
are abnormally spaced, less than 58 or more than 72 mm. apart.
Both eyes should be used simultaneously, and it will require quite
a little practice before the observer will become expert in dis-
tinguishing differences in the distance between objects appar-
ently close together in the stereoscopic field and yet in different
frontal planes.

To test an observer's ability to see stereoscopically. Dr.
Pulfrich has constructed a stereoscopic " test-plate " (" Prue-
fungstafel fuer stereoskopisches Sehen "), which is issued together
with his treatise, '* Ueber eine Pruefungstafel fuer stereoskop-
isches Sehen," published in the Zeitschrift fuer instrumenten-
kunde. Heft 9, 1901. The figures and diagrams shown in
that test-plate not only give the means for a quantitative test
but they are also designed for making a qualitative test of the
observer's stereoscopic vision.

Plates CII and CHI show roughly made diagrams for test-
ing stereoscopic vision in the quantitative sense only. The

STEREOSCOPIC TELEMETER AND STEREOCOMPARATOR. 315

Probable Error in Meters for the

Distonce-i>

•

in Meters.

SportinfiT Telemeter:

Infantry Telemeter:

Stand Telemeter:

Base -3a cni.,

Base — 5z cm.»

Base — 1.44 ra..

Magnification 4*

Magnification 8.

Magnification aj.

0.05

O.II

0.20

0.31

0.70

0.3

100

1. 25

0.3

0-5

150

2.8

0.8

17s

I.I

200

5-0

1.4

300

"•3

3-2

400

20.0

500

31.3

^:l

600

700

3.6

750

4.4

800

Objects beyond

6.7

900

8 km. appear as
at infinite distance

9-9

1000

1250

1500

1750

109

2000

140

2250

181

2500

223

2750

270

3000

320

3500

156

4000

215

5000

Objects beyond 28

278

6000

km. appear as at
,infinite distance

352

7000

440

8000

9000

lOOOO

Objects beyond 230
km. appear as in-
finite

circles, Plate CII, are numbered with their increase in distance,
No. I being nearest and No. 9 farthest. In Plate CIII the pyra-
mid is nearest the eyes; then follow the cross, concentric rings,
circle near the cross, large ring or circle (inclosing the figures),
central wheel with four spokes, cube, smaller circle (below the

3l6 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

cube), base of large cone, and finally the base of the small cone.
The axes of the two cones are not in Ime and the base of the
pyramid is not in a plane parallel with the large circle, its left
comer being tilted up a little. The Maltese cross, too, has its
upper two arms tilted forward toward the observer, the end
of the left upper arm being somewhat nearer than the upper
end of the right arm. A careful examination of these plates
will show whether the observer can see stereoscopically.

In looking into the stereotelemeter the aerial distance scale
should appear free in space, in a plane slightly raised toward
the distant end of the scale. As soon as the particular part
of the scale has been determined, by inspection, which coincides
with the object, the distance of the latter is obtained within a
certain- margin of limitation, as already referred to in the pre-
ceding paragraphs.

The more expert the observer, the smaller the limit of differ-
entiation will be, although there will always remain a certain
margin of uncertainty corresponding to the limit of power of
the stereoscopic definition, as noted in the table previously
cited.

To become efficient in the rapid use of the stereotelemeter,
it is essential that the observer make himself thoroughly familiar
with the divisions of the aerial scale of his instrument. It should
be noted that the subdivisions of the scale apparently grow
smaller with increasing distance, and the observer should not
only be able to read off each actual scale mark quickly and cor-
rectly, but he should also be trained to estimate fractions of the
subdivisional scale lengths accurately. (Attention may be called
to the fact that the nearest half-reading between two successive
scale marks falls a little beyond the space center, the quarter
a little beyond the geometrical quarter-space, etc.)

Plate CI represents the measuring-scale used in the Carl
Zeiss hunting stereotelemeter. The marks of this scale are
arranged in four sections, the scale appearing as a zigzag line.
The first section, from 20 to 25 m., apparently appears in front

STEREOSCOPIC TELEMETER AND STEREOCOMPARATOR. 317

of the diaphragm or circle which incloses the scale. The four
sections control distances as follows:

(i) 20 to 25 m., divided into i-meter spaces;

(2) 25 to So m., divided into i-meter spaces;

(3) 50 to 70 m., divided into 2, and 70 to 100 m., into 5 m.

spaces;

(4) 100 to 160 m., in 10, 160 to 300 m., in 20, and 300 to

500 m., divided into 50 m. spaces.

The reproduction of this scale on Plate CI is faulty; inasmuch
as the triangular division marks 4 and 6, representing the 40 and
60 m. divisions of the scale, are considerably out of line, some
of the other marks show similar imperfections, but less marked
than these two.

The glass plate having the aerial distance scale etched into its
surface is also provided with a transverse scale (divided into
twenty equal parts, Plate CI) for measuring widths and heights
of objects. In the stereoscopic fields of the telemeters both
scales stand out very clear and distinct, being photographic
reductions of the large-scale originals.

For the first practice work with the stereotelemeter, well-
<lefined objects, preferably those that are silhouetted against
the sky, should be selected. The instrument is first directed
toward the sky, the interocular distance adjusted, and the eye-
pieces focused. The instrument is now gently revolved down-
ward until the object ranged upon appears in the lower field,
when the aerial scale should appear free in space above the
object. It will now remain for the observer to find that place
of the scale which coincides with the object (which will be in
the same frontal plane with it) to estimate the fractional dis-
tance from nearest scale mark to object.

The scale divisions are indicated by small triangles, the acute
angle pointing downward, and to determine the position of an
object with reference to the scale, the highest point of the object

3l8 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

is brought as close as possible to the imaginary line, connecting
the scale marks near the object, when the mark to the near side
of the observer and close to the object is picked out, the observer
estimating the distance of the object beyond this mark to arrive
at the actual distance. Should the mark just beyond the object
be observed, instead the nearer one as just stated, the tendency
generally seems to be towards overestimating the distance.

The Carl Zeiss firm has also constructed a stereoscopic telem-
eter without an aerial scale. The stereoscopic field of this
telemeter shows an index mark which is movable by the aid of a
micrometer screw. With this instrument several independent
measurements of the distance between two objects may be made,
similar in manner to the method of repetitions.

The Carl Zeiss stand stereotelemeter may be used at night
for estimating or measuring the distances of lights (vessels, light-
houses, etc.), by illuminating the scale on the diaphragm plate,,
means for doing this being provided if a suitable lantern be at
hand.

The binocular microscope of the stereocomparator, of which
a description follows, is built after the model of the stereo-
telemeter, its telescope with prisms being here replaced by a
binocular microscope with reflecting mirrors, through which the
upright stereoscopic images are examined under enlargement.

B. The Stereocomparator and the Stereophotographic
Surveying Method.

The angle included at a distant point between the visual
rays from the eyes of the observer is known as the parallactic
angle, or parallax. The parallax increases when the point
approaches the observer and vice versa. For a point at in-
finite distance the two visual rays will be parallel; the parallax
will be ==o.

The estimation of the distance of a point, when observed
with both eyesj depends largely upon the subconscious gaug-
ing, or mental measurement of the parallax. The distance of

STEREOSCOPIC TELEMETER AND STEREOCOMPARATOR. 319

an object when viewed with one eye only may still be estimated,
but in monocular vision such estimates must be based mainly
upon the degree of diminution in the apparent size of the object
if a familiar one, or the reduction in size of closely neighboring
bodies of which the actual sizes are knpwn; for instance, a per-
son or an animal that may be standing near the distant object.

The stereophotogrammetric ' method is based on measure-
ments n^de simultaneously on two stereoscopic pictures show-
ing the same terrene and exposed from all the ends of a compara-
tively short base line. These simultaneous measurements, on
corresponding plate pairs, of the coordinates to locate identical
terrene points with reference to the horizon and principal lines
are made with the " stereocomparator," ingeniously devised by
Dr. C. Pulfrich, a member of the scientific staff of the Carl Zeiss
Optical Works in Jena.

The principle underlying the construction of the stereo-
comparator may be elucidated in the following manner, sug-
gested by P. Seliger of the Prussian Topographic Bureau. Two
pointers, made of black wire of equal thickness and of equal
lengths, when suspended over the face of two stereoscopic views
secured in a stereoscope will become superimposed and appear
as a single wire in the stereoscopic image of the two views.

If we now move one wire over the face of the picture, bring-
ing it a little closer to the wire over the other picture (we reduce
the parallax), the apparent position of the wire index in the
stereoscopic image will have become more distant, and as soon
as the distance between the two wires is made to coincide with
the interocular distance the superimposed images of the wires
wiU appear infinitely far off in the stereoscopic field. By thus
changing the relative positions of the two wires, the observer
can transfer their stereoscopic image to any point of the stereo-
scopic field.

All points of the stereoscopic image that are in a vertical plane
parallel to the stereoscopic base have the same vertical distance
from the base line. Since the vertical distance of such frontal

. 320 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

plane has the sam3 ratio to the base length as the focal length
of the camera has to the parallax, we can compute such distance
if we know the parallax, the base and the focal length being
constant for every stereoscopic picture pair.

The stereoscopic picture pairs are placed on the stereocom-
parator to be measured through a binocular microscope similar
in construction as the Zeiss "stereoscopic telemeter," by means
of which the two coordinates and the parallax of any pictured
point may be measured, after optical bisection, by reading the
corresponding verniers of the three scales that are connected
with the stereocomparator. The data thus obtained suffice for
the cartographic location of the point in both the horizontal and
vertical sense.

Referring to Plate CIV, which shows the general arrangement
of the stereocomparator, we designate by

Pi and P2 the left and the right pictures;
H the rack-and-pinion motion for moving both pictures together

from left to right and vice versa;
M' a screw for moving the right picture alone and in the same

sense as the motion imparted by -ff;
M a screw for turning the right picture;
N a screw for raising or lowering the right picture;
T the turntable for the right picture;
S a screw for moving the left picture independently from right

to left and vice versa;
R and h plates running in grooves, to be raised, lowered, or

moved transversely;
A and B scales for measuring the coordinates of points pictured

on both plates;
a scale for measuring the parallax of pictured points;
C binocular microscope; it may be raised or lowered by turning

the screw V, the amount of such change in altitude to be

read off within o.i mm. on the vernier of scale •£.

To be viewed stereoscopically the two pictures Pi and P2

STEREOSCOPIC TELEMETER AND STEREOCOMPARATOR. 32 1

are placed on their plates, R and T, in a position correspond-
ing to that they had when the exposure was made, with their
principal lines made parallel and vertical. If the base-line ends
are not of the same elevation, the right plate is raised or lowered
imtil corresponding points appear equally high when examined
through the binocular microscope.

The binocular microscope may be moved toward or away
from the negatives by turning E and each eyepiece is inde-
pendently adjustable to the eyes of the observer. The objec-
tives also are movable in the direction of the optical axes, to
give a range of magnification of 4 to 8 diameters. Two index
marks have been provided, one in each image plane of the micro-
scopes, for bisecting identical terrene points. By turning the
micrometer screw -F the index of the right microscope may
be moved, changing the apparent distance of its stereoscopic
image. For one turn of the micrometer F the index will
be moved 0.2 mm., which would correspond with a change of
the right plate of 0.3, 0.2, and 0.15 mm., using a magnification
of 4, 6, and 8, respectively.

To find the parallax of a pictured point the stereoscopic
image of the index mark is set at apparent infinite distance and
both plates are now moved and adjusted until the terrene point
to be measured coincides with this index mark. The motion
of the right plate, accomplished by turning the screw M\
to bring the mark and point into contact, is read off on the vernier
a, which reads to 0.02 mm. By estimation, however, the
value for the parallax may be obtained within o.oi mm. After
the index has been made to bisect the point the readings of
the verniers A and B will give the coordinates of the pictured
point with reference to the left picture (left base station).

The main advantage claimed for the use of the stereocom-
parator in . phototopography rests in the fact that one pointing
of the index on the point at once gives the elements for the car-
tographic location of the point, whereas with the plane-table or
radial method three distinct measurements would have to be

322 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

made before the pictured point may be plotted, involving the
separate measures of two abscissae and one ordinate. With
the stereocomparator the coordinates are measured directly on
the negatives with microscopes and verniers, and the accuracy
obtained should be greater than obtainable with the plane-table
or radial method, using paper prints, dividers, and scales.

The index mark bemg placed upon each terrene point that
is to be plotted from the pairs of pictures, it is evident that the
better the definition of such points the closer will be their sub-
sequent cartographic location.

The three readings made on the scales Ay By and a give the
data for locating any pictured point (bisected with the movable
index mark) in regard to direction, distance, and elevation, and
with reference to the left station.

The abscissa x (Plate CV, Fig. i), read oflf on scale A^ is
plotted in the usual manner by laying oflf the distance on ^the
picture trace T from the principal point O, a line 5-R, drawn
through the end of x from the left station S gives the line of
direction to the bisected point P.

The distance is ascertained from the vernier reading of scale
a which gives the linear parallax of the bisected point The
distance may be computed from the equation

il:B=/:a,

where B is the distance between the two stations,

/ the constant focal length of the camera, and
a the parallax as read oflf on the scale.

In Fig. I, Plate CV, TT represents the picture trace,
X the abscissa of the point P, the plotted position of which
will be on the radial SR. The value for Ay as computed
above, is laid oflf on the principal line from 5. A parallel

STEEEOSCOPIC TELEMETER AND STESEOCOMFARATOR. 323

to JT, drawn through the end of 4, will bisect SR in P,
which is the horizontal projection of the bisected point.

Dr. Pulfrich recommends a graphical solution of the equation

of which the product -B-/ is a constant for every pair of pictures.
Referring to Fig. 2, Plate CV,

SSi = 5= base line;
TT= picture trace of left picture;
a = vernier reading for parallax, laid off on TT from O.

If we now draw the radial 5r, the intersection of the latter
with Siby drawn parallel with 50, will cut off the distance
A on Sib, and the point to be plotted will be on the line
MNj drawn parallel to TT at the distance A from 5. The
plotted position may now readily be found by laying off the
abscissa x from O and drawing the radial SR; the intersection
of the latter with MN locates the plotted position of the point P.

Fig. 3, Plate CV, shows the simple device suggested by Dr.
Pulfrich for the graphical solution of the equation for A.
A drawing-board is covered with a tough paper and a line SO
is drawn parallel with its lower edge. On SO, at a distance
1.5 /. from 5, a vertical UT is erected and a scale of divisional
parts equal to 1.5 mm. is laid off on UT. The line SO
is provided with a looo-meter graduation in the plotting-scale
(say 1:25000). LL is a straight edge secured to the board
parallel with SO. G is a transparent film of celluloid
attached to a brass strip k in such a way that it may readily
be slid along LL over SO. This transparent plate G has two
graduation, U and f, in the reduced scale of the map. The
ruler 5-R, also provided with the reduced scale of the map
(1:25000), may be revolved about the pin in 5.

To use this device the base line for a pair of plates is laid

324 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

oflf on //, say in tenfold scale of the map. After the parallax
a has been read off on the scale of the stereocomparator,
the ruler SR is placed on UT to cut oflf the length loa, and G is
moved until the base end, marked off on //, coincides with the
fiducial edge of SR. The corresponding value for A may now
be read oflf on SO, using the scale if for the subdivisional parts
of SO.

To find the distance SP of the plotted point from the
left station 5, the position of G is maintained unchanged,
while the edge of SR is made to coincide with that division
mark of the scale UT which corresponds with the abscissa
X of the pictured point. The distance SP may now be read oflf
on SR to be transferred to the radial SR, Fig. i, Plate CV.

The diflference in elevation of pictured-point and left-base
station,

h- ^ ,

may also be foimd graphically. After the distance SP has
been read oflf, G is held in the same position and SR is
brought to coincide with that scale division on UT which
corresponds with the ordinate (read oflf on scale JB of the
stereocomparator), when the reading of the scale //, between
SO and SR, will give the value for h in meters. Instead
of moving SR to bisect the end of the ordinate y on UT
it is desirable to use a multiple part of y, say loy, and
divide the final reading by lo. The elevation of the plotted
point is now derived from the elevation of the left station in the
customary manner, referring h to the elevation of the horizon
line of the instrument at the station and allowing for cur\''ature
and refraction for points over 2000 m. distant from S.

The apparent length of the index mark in the stereoscopic
image plane of course corresponds to different heights, accord-
ing to the distance of the bisected object; the relation between

STEREOSCOPIC TELEMETER AND STEREOCOMPARATOR. 325

both, however, may readily be ascertained when the actual or
absolute length of the index mark be known. This length is best
found by bringing the upper end of the mark into contact with
a well-defined horizontal line in the picture and noting the read-
ing of the scale of ordinates (scale B), then bringing the lower
end of the mark into contact with the same horizontal line and
again noting the vernier reading of the scale B. The differ-
ence between the two readings will equal the absolute length
of the index mark (=w).

The comparative length value =Af of the index mark for a
distance«=il may be computed from the formula

If the value for m be found 0.75 nun. and the constant
focal length of the camera be 250 mm. the value for M would
be 0.003 ^' Now, say the lower end of the index mark coin-
cides with the base of an embankment at an apparent distance
from 5 = -4 « 4000 m., while the crown of the embankment may
bisect the iudex line at one third of its length, the absolute
height of the embankment would then be

4000X0.003

— i ^""^

It is very essential that each pair of plates be exposed in a
vertical plane containing the base line or being parallel with it.
If such be the case, points lying at infinite distance in the ver-
tical planes of the objectives shoidd appear pictured in the prin-
cipal lines of the plates. If either of the plates, say P, includes
an angle «=^ with the vertical plane of the other, the distant
point will be pictured to one side of the principal line (see Fig. i,
Plate CVI). The distant point A will be pictured at ai in
plate Piu but the principal point will be to one side, at an.

326

PHOTOTOPOGRAPEIC METHODS AND INSTRUMENTS.

The plates after being placed on the holders ot the stereocom-
parator are adjusted by means of the horizon and principal
lines, and in this case all parallax values will be measured too
small by

A correct measurement of the length of the base in rough
mountain regions often offers serious difficulties, telemeter read-
ings generally being the only available means for measuring these
base lines. Any error made in the base will affect all distances
determined from its left station, and such being the case it would
appear advisable to select relatively long base lines. The lengths
of the latter, however, are controlled by the fact that picture
pairs can no longer be viewed stereoscopically in their full extent
when the length of the base exceeds a certain limit. Pictures
obtained from the ends of too long a base will have but limited
distance zones that may be examined stereoscopically through the
binocular microscopes; areas outside of these, both near and far,
will appear blurred and indistinct. The examination of such
plates through the microscopes is not only very trying to the
eyes, but the observer also loses the general view of the terrene
and he will have to refocus the microscopes for every change in
distance.

For a constant focal length of 241.5 mm. and an error in parallax
of o.oi mm., errors in distances may be made, for base lengths
of 50, 100, 200, and 300 meters, as listed in the following table:

Distance of

T^nRth of Base Line in Meters.

Bisected Point

in Meters.

so m.

xoo m.

200 m.

300 m.

1000

0.8

0.4

0.2

O.I

2000

3-3

1-7

0.8

o.S

3000

7.4

i'l

1.9

1.2

4000

13.2

6.6

3-3

2.2

5000

^?-z

10.3

5-2

3-4

7500

46.8

234

II. 7

4:2

lOOOO

82.8

41.4

20.7

STEREOSCOPIC TELEMETER AND STEREOCOMPARATOR. 327

If errors in position of ±15 m. be permissible in rough moun-
tain work plotted in 1 125000 scale, a mean error of ±3 m. may
be accepted for the same tind of work plotted in 1:10000 scale.

For a parallax error not exceeding ±0.01 nmi. distances to
6000 meters from the base stations should be controlled, and
for the 1 125000 plotting-scale base lines of 100 meters preferably
should be selected. For the 1:10000 scale a 100-meter base
should be selected for distances up to 3000 meters and a 200-
meter base for 4000 meters, etc. If the objective has a focal
length shorter than 240 mm. the base should be made propor-
tionately longer. For instance, for a focal length of 180 mm.
the base lines as given above should be increased by one quarter.

The terrene pictured on a pair of stereoscopic plates, when
examined through the binocular microscopes, appears very much
like a relief model of the coimtry, the changes in the surface for-
mation being far more clearly shown than in the landscape itself
when viewed from either of the two base stations.

For the best iconometric results each plate should contain
from 6 to 12 control points of known elevations and positioas
(tertiary triangulation points). After the left base station and
all the control points have been plotted, the two stereoscopic
plates are placed on the comparator frame to be adjusted in
the manner already described. After the parallaxes, abscissae,
and ordinates of all the control points that are pictured on the
plates have been measured and tabulated, the picture trace of
the left picture is plotted, based on the computation of the radials
drawn to two control points. It is preferable to select two extra
axial points, one near the left and one near the right margin of
the plate, for plotting the picture trace. The position of the
latter is checked by means of the abscissae of the other pictured
control points. It would not be sufficiently accurate for our
purpose (" stereophotogrammetry ") to plot the picture trace
by means of a paper strip, as generally used in the plane-table
or intersection method.

Parallel with the picture trace and from ij to 2 times its

328 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

distance from the station point a scale is drawn having the
same graduation and numbering as the scale (of abscissae) A
of the comparator, the divisional parts of course being enlarged,
according to the selected distance, i J to 2 times. A ruler having
the plotting-scale along its fiducial edge may be secured to the
station point in such manner as to revolve about the station
with the zero mark of the scale as pivot. With these means
the pictured points may be quickly plotted without actually
drawing their lines of direction, which radiate from the station.

The next step is to check the position of the horizon line by
means of the ordinates of the pictured control points. Any cor-
rection affecting all points alike is made by changing the zero
mark of the scale B on the comparator. If the horizon line
has to be raised or lowered on one side, the plate will have to
be turned correspondingly on the holder of the comparator. In
the latter case the adjustments of both plates on the comparator
will have to be repeated to allow for the change just made.

With the measured parallax values a the distances A
of the control points are computed and compared with those
of the plotted points. Discrepancies J between these exceed-
ing the amount due to errors in parallax of ±o.or mm. would
point toward an error in " swing " (J«) during the exposure
of the plate, errors in base measure (J^), or toward errors
due to both. We may, therefore, express these discrepancies
by the equation

^ = ^. + ^6.

If the error in base measure equals b and the error in
parallax, due to the "swing of plate," d, equals s=}d, we
will have the equations

^,—W'h^

J[2

STEREOSCOPIC TELEMETER AND STEREOCOMPARATOR. 329

We can now compute the errors J and ^i (discrepancies
between the computed and plotted distances A) from the
parallaxes of two pictured control points and substitute these
values for J and Ji in the equations

^ A ^ A^

after which the values for the base-line error (b) and the
error in parallax (s) may be computed and applied to the
base-line and parallax values.

A better way would be to use all the control points, tabulate
the errors (J) graphically, and find the values for J^ and J, by
interpolation, as shown on Plate CVII.

The abscissae of the control points are plotted in their true
lengths and the corresponding errors J are plotted as ordinates,
giving the points 47, 48, 44, 28, 38, 32, 34. A curve passing
through the initial point O is laid through this series of points.
To separate the ordinates, i, of this curve, OCy into the
component parts, Jf, and i,, a tangent OG through O to the
curve OC is to be drayni in such manner that the upper sec-
tions w, w, 0, etc., increase in length with the squares of A
((?=4W, ^=4W, q=40, etc.), as the increase in the errors i, is
directly proportional to the squares of the distances A and
the errors Jb increase in the same ratio as the distances A.

With a pair of dividers and a ruler the position of OG

may be located tentatively. For an error in the base line, 6 = 0,

B' f
the curve OC will be a parabola, having the parameter = — -;

for an error in swing, s^ O, a straight line will replace the curve,

andi=il--^.

330 PHOTOXOPOGRAPHIC METHODS AND INSTRUMENTS.

The lower ordinate section serves for the determination of
the base-line correction, 6 = -t--J^, and the upper section gives

the correction for the parallaxes 5=-j2"^«.

The general course of the curve OC will be a criterion
of the errors affecting a pair of plates, showing whether they are
due to regular causes or whether errors of level adjustments,
errors in computation, etc., have crept in also. If no smooth
compensating curve may be drawn to harmonize with the series
of plotted points, errors outside of those referred to in the pre-
ceding paragraphs should be looked for.

A serious, error in the swing of the plate may affect the curve
in a marked manner. The correction applied to the parallax,
as referred to in the preceding, neutralizes only the constant
df. It corrects the position of the principal line with refer-
ence to the pictured points, but when there is a decided swing
in the plate the parallax, for points to either side of the principal
line, will be in error, even after the correction ^ for the paral-
laxes has been applied.

After the correction df has been applied, the plates may

yet have the relative positions indicated in Fig. i, Plate CVI,

where points at infinite distance and situated in the principal

plane wiU be pictured in the principal lines of both plates, whereas

the images a of a distant point A, lying to one side of the

principal plane, will be pictured at a and an, instead of at a

and ai. In lieu of the correct parallax (xi—x) we obtain

the smaller value (xn—x), referring to Fig. 2, Plate CVI. The

x^
error thus remaining, which may be expressed as J«= -j*, increases

rapidly with an increase in the length of absdssa; it is positive
on one side of the principal line and negative on the other, being
±0 for points on the principal line. It is a prime requisite,
therefore, to expose pairs of plates as near as possible in a ver-
tical plane parallel with the base.

STEREOSCOPIC TELEMETER AND STEREOCOMPARATOR. 33I

After a plate pair has been tested and after the corrections
found necessary have been applied the iconometric mensuration
may be commenced. The pictured points may be plotted by
means of their lines of direction, based on the abscissa values,
recorded on the scale -4, and its horizontal distance from the
left station, based on the measured parallax as given on scale a
of the stereocomparator. The difference in elevation between
the station and the plotted point may be computed from the
reading of scale B. To ascertain the parallaxes of the pictured
points the index mark is moved from point to point in the stereo-
scopic image field, very much in the same way as the telemeter
is caried from point to point in the field when reading distances.
It is evident that the index mark may readily be moved to bisect
points in the image field that would be inaccessible for the ordinary
telemeter in the field. The distances, obtainable by moving
the index mark in the stereoscopic field, considerably exceed
those measured with the telemeter and the time required for
obtaining these distances stereoscopicaUy is so short that the
advantages of the stereoscopic method over the plane-table and
tachymetric methods are out of question for topographic recon-
naissance work in rough mountains.

The stereocomparator, furthermore, is peculiarly well fitted
for a quick location of points having the same elevation; the
index mark may be used to trace out the contours in the stereo-
scopic field. Points may also be readily located that are in
the same frontal plane, in the same plane parallel with the base
line. Actual profiles parallel with the picture traces may thus
be run out and by locating points to either side of the profile,
using the micrometer screw of the binocular microscope for this
purpose, terrene strips of 150 to 250 meters width (scale i : 25000)
may be developed, which will form the base for the subsequent
orographic development of the topography.

The positions of points that have been plotted by the usual
method of direction and distance may be checked by referring
them to the plotted positions of near-by pictured control points.

332 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

The stereoscopic photogrammetric methods evidently oflFer a
wide field for application to ascertain changes that may have
occurred during periods of time that are allowed to elapse before
taking a new set of pictures from the same base line (or at least
from the same vicinity). The examination of two stereoscopic
plate pairs of a glacier, for instance, would at once show any
change in form or location if the left plate of one pair be examined
in the stereocomparator with the right plate of the second pair,
both pairs being obtained at different times from the same base
line.

Good results may be expected from this method, if applied
by the navy, for mapping coast lines without making a landing,
by taking simultaneous views of the coast, fortifications, etc.,
from a vessel (a base line being measured on the deck between
the camera stations), noting the position of the vessel on the
chart at the time of exposure.

The use of the stereocomparator may also be recommended
for recording the positions of moving bodies (army corps, fleets
during maneuvers or in time of war), making plans of inaccessible
objects, for the mapping of cities, for making profiles, plans, and
relief models of areas to be studied for comparative locations of
roads, railroads, irrigation plants, etc.

To recapitulate, the actual mapping of the terrene details,
based on the examination of stereoscopic picture pairs, may be
made:

(i) With relation to a series of control points plotted from
data obtained directly with the stereocomparator;

(2) With relation to a series of contours obtained directly

from the picture pairs, or

(3) By means of profiles composed of points having the same

parallax, i.e., points in frontal planes.

Of the many-fold uses to which the stereocomparator is adapted

we may mention stellar surveys, the testing of banknotes, the

comparison of scales and their prototypes, comparing facsimiles

and replica of various kinds, the study of animals in motion^

STEREOSCOPIC TELEMETER AND STEREOCOMPARATOR. 333

migratory-bird flights, changes in the northern lights, cloud ele-
vations, terrene changes due to landslides, volcanic eruptions,
inundations, forest fires, etc., for the study of eflFects produced
by bombardments and explosives, changes in sand dunes, etc.

In the preceding paragraphs it was assumed that each pair
of plates was not only exposed in the same plane (the plane con-
taining the base line or laid parallel with it), but this plane was
also supposed to be vertical. The verticality, of course, greatly
facilitates and simplifies the iconometric constructions, yet it is
not a sine qua non. If a plate pair be exposed in the same inclined
plane, all that has been said about the stereotopographic method
still holds good if the angle of inclination of the plane containing
both plates during their exposures be measured and taken into
accoimt.

If the landscape pictures on the inclined plates could be trans-
ferred to vertical plates by photography, the latter could be used
on the stereocomparator just as if the plate pair had been exposed
in the vertical plane originally.

The inclined-plate position will often be imavoidable in making
stereophototopographic surveys from the decks of vessels, and
even in mountain work a suitable location for the base line will
sometimes necessitate exposures to be made on inclined plates
in order to control deep valleys or high elevations from the two
base stations.

CHAPTER XT.
PHOTOGRAPHIC OPERATIONS IN THE FIELD.

Until now the principal operations have been considered
for obtaining the so-called " latent " or invisible image on the
exposed plate, which is still to be converted into the " negative,"
in which form the terrene image is used, either directly or indi-
rectly, for the iconometric plotting of the pictured topographic
features.

The work of developmg»and fixing the negatives of an exten-
sive photographic survey is best done by a photographic expert
who has made special studies and experiments for this purpose.
He should be thoroughly familiar with the laws that control the
changes, both chemical and physical, which take place in the
compositions of the sensitized coatings of the photographic plates,
when they are exposed to the action of light, as well as those
which control the changes in the chemical compositions of the
sensitized films when the .plates are immersed in the developing,
toning, and fixing baths.

Still, every phototopographer should be sufficiently familiar
with the general routine practice of photography to develop
some ** trial " or " test " plates understanding^ and successfully
while he is yet in the field.

At least a few plates taken at random from every batch
originally packed together and which are likely to have passed
through the same conditions during transportation should be
developed, while still in the locality where the exposures were
made, to feel satisfied that no plates were spoiled and also to
feel reasonably assured that the exposures were correctly timed.

334

EXPOSURE OF A PHOTOGRAPHIC DRY-PLATE. 335

The wisdom of developing test-plates, to avoid loss of valuable
time and material by incorrect exposures or by the use of spoiled
plates, is beyond dispute. If all development of plates be post-
poned until after the return of the expedition, defective plates
cannot be replaced without expending large siuns of money, and
the results of the expedition may be robbed of much, if not of
all, practical value.

Whenever there is danger of losing undeveloped plates through
careless and ruthless inspection of baggage on frontiers, or through
the inquisitiveness of packers, to whom the transportation of the
plates must be intrusted, it is of course advisable to develop all
the plates of the survey in the field, pari passu with the progress
of the survey.

The principal records of the season's work, regarding the
topography at least, consist in a series of undeveloped plates,,
and the phototopographer should feel reasonably certain that
they are of as good a quality as could be obtained under the con-
ditions of climate and surroundings prevailing at the time of
their exposures.

We will give in the following a cursory review of those opera-
tions to which an exposed plate is to be subjected before it is
converted into the permanent negative, and with which the
phototopographer should be familiar to enable him, for the rea-
sons just stated, to develop some test-plates while he is still
in the locality where the exposures were made.

L General Remarks on the Exposure of a Photographic

Dry-plate.

When the sensitized coating of a photographic plate is exposed
to the action of the rays of so-called white light — solar light —
certain effects upon the chemical composition of the coating
will be produced, consisting primarily in a reduction of the silver
haloids that are embodied in the gelatine coating of the dry-plate
mto an unstable condition, permitting a deposit of metallic (black)

336 PHOTOTOPOGRAPHIC MLETHODS AND INSTRUMENTS.

silver to be readily made upon the plate when the latter is immersed
in the so-called " developer '* (reducing bath), which converts
the light-sensitive " latent image " of the exposed plate into the
" negative '' of a permanent and stable character.

The greater the intensity of the light that reaches the plate
in the camera, or the longer the exposure of the plate to the action
of the light- rays, the greater will be the amount of reduced silver.

The quantitative effect, in a given time period, of white
light upon the sensitized coating of a photographic plate may
differ perceptibly from the quantitative effect of chromatic or
color rays, although their qualitative effect upon the silver haloid
(bromide of silver) is essentially the same. For short exposures
the quantity of reduced silver may be regarded as directly pro-
portional to the duration of the exposure. The " density " of a
negative is more or less great according to the larger or smaller
amount of reduced silver that has been deposited; density increases
directly with the length of exposure.

Photographic dry-plates differ materially regarding their
" speed," or their sensitiveness to light action. The speed is
generally indicated by the so-called " sensitometer number,"
ascribed to each emulsion. The same density for two different
plates, when photographing the same object under identical con-
ditions, may be attained by giving each plate a different length
of exposure, corresponding to its sensitometer number, the less
sensitive plate, of course, being given the longer exposure.

Under ordinary conditions, three different stages of exposure
may be considered in practical photography:

1. Underexposure;

2. Correct exposure;

3. Overexposure.

A fourth stage, the so-called " period of reversal," may pos-
sibly be reached, but this requires so lengthy an exposure that
it will rarely be attained, inadvertently, when exposing plates
for phototopographic purposes.

An underexposed plate may be recognized by the marked

EXPOSURE OF A PHOTOGRAPHIC DRY-PLATE. 337

contrast in the negative between the lights and shadows and a
general deficiency in details. Such plates will be of little or no
value for iconometric plotting.

An overexposed plate shows little contrast between the lights
and shadows and the general details will be weak and flat.

When a plate had been exposed correctly, its scale of grada-
tion in tint, after proper development, will embrace the widest
range possible, from pure transparency (white) to black. A
negative appears transparent where the photographed object
was dark and vice versa. The negative should be a true inverse
of the original regarding the light gradations.

The source of th^ light-rays which are emanated by any
object in nature may be a threefold one, comprising:

1. Rays of direct sunlight;

2. The less intense rays of diffused skylight;

3. Rays originating from the foregoing two sources, but

reaching the subject indirectly after having been re-
flected from surrounding objects.

The intensity of the light-rays, generally summarized as
daylight, is subject to many variations. The sunlight alone will
have a variable intensity at different altitudes and under differ-
ent atmospheric conditions, irrespective of the geographic latitude.

The tendency of aerial perspective is in the direction of diffu-
sion of sharp outlines of distant objects and toward obliteration
of details. The skyline of distant mountains becomes merged
into the so-called " blue haze." The nearer sea-level the observer
is stationed the more indistinct will distant objects become,
while in high altitudes, with a relatively dry atmosphere, objects
will be discernible, as to form and color, at far greater distances.

Some of the polychromic rays of sunlight, on their passage
through the atmosphere, intervening between the observer and
the object, will become diffused or absorbed, while others will
transverse the same without suffering any perceptible modi-
fications. Color rays near the violet end of the solar spectrxmi,
rays of short wave-lengths, are more largely absorbed by the

338 P^OTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

atmosphere than those of longer wave-lengths near the red
end of the spectrum.

We had seen (Chapter VII) that the component rays of so-
called white light after transmission through a lens will be difFer-
ehtly refracted and the actinic effects of such refracted rays
upon the sensitized films will differ according to their colors
or wave-lengths. Those of short wave-lengths, the ultra violet
to blue, between the Fraunhofer lines Hi and F, have by far a
more pronounced chemical action upon the silver haloids of the
plate coating than light- rays with longer wave-lengths, the green,
yellow, orange, and red rays, between and beyond the Fraun-
hofer lines E and A,

The luminous (optical) effects of the component colors of a
landscape upon the eye are not identical with the actinic (chem-
ical) effects upon the photographic plate. The optical effect is
governed by the various degrees of tint, hue, or shade that the
several parts of the landscape convey to the eye, some parts
appearing as light, others in half-light, still others in middle
tint, half dark, and dark.

Practical experience, on the other hand, teaches that the
various blues and dark greens of the chromatic scale appear darker
to the eye than the yellows, the reds, and the lighter shades of
green, yet, when a photographic plate is exposed in the camera
to both, the combined action of the luminous and actinic light-
rays of an illuminated landscape, the actinic action of the blue
rays will be more intense on the sensitized film than that of the
light-green, the yellow, orange, and red rays. In order to obtain,
therefore, a clear and well-defined picture of the violet and blue-
colored parts, the exposure will have to be stopped long before
the parts having shades of light green, yellow, orange, and red
have been reproduced on the negative. The resulting mono-
chrome picture will have a scale of but three-tone gradations —
light, half-light, and dark — instead of the scale of five gradations
of tone mentioned above.

Thus it may happen, when an ordinary dry-plate is exposed

ORTHOCHROBIATIC DRY-PLATES AND RAY-FILTERS. 339

in the camera, the sky and blue-tinted parts in general will be
overexposed if the exposure had been timed correctly for the
.green, yellow, orange, and red- tinted parts. For the use of the
phototopographer who desires negatives showing also the dis-
tant details of the landscape clearly and well defined the ordinary
dry-plate is inadequate.

n. Orthochromatic Dry-plates and Ray-filters.

In the preparation of the sensitive film of the " isochromatic *
{rendering all color values evenly well), or " orthochromatic **
plates (rendering the color values correctly), it has been the
aim to make them equally sensitive to the actinic action of all
color-rays, so that during a properly timed exposure all colors
of the subject may be represented upon the finished negative
equally correct regarding their respective tints and light values.
As yet, attempts in this direction have been only partially suc-
cessful, however. Orthochromatic plates are indeed made
more sensitive to the actinic effects of red, yellow, and light-green
rays, but the blue rays remain, even with these plates, consider-
ably more active than the reds and yellows, and to retard their
chemical action still more, a so-called " color-screen,' ' or " ray-
filter," is interposed between the plate apd the subject. A suit-
able combination of orthochromatic plate and color-screen makes
it possible to reproduce landscapes (and colored objects) in
better harmony regarding chromatic values, reducing the actinic
power of the rays of long wave-lengths and increasing it for the
rays of the less refractive end of the solar spectrum.

At present the orthochromatic plates are prepared by impart-
ing color sensitiveness to the gelatino-bromide-silver emulsion
of the ordinary dry-plates by the addition of certain color ingrfj-
dients or " optical sensitizers," like erythrosine, cyanine, rhoda-
mine, eosine, etc. The nearer these optical sensitizers approach
a blue shade of color, the more sensitive the plate will become
ior light-rays of the less refractive end of the solar spectrum.

340 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

The addition of erythrosine is said to increase the sensitizing
action of the emukion for light reds, while rhodamine increases
the same for light greens, extending well toward the yellow and
light orange. Tetrachlor-tetraethyl-rhodamine-chlorhydrate im-
parts a more powerful sensitizing action for the orange yellow,
and green tints. Cyanine has a greater orange sensitiveness than
either of the ingredients named, excepting, .perhaps, the last-
mentioned dye, but the others have the advantage of not materi-
ally reducing the general speed or sensitiveness of the plate.
Valuable experiments in this direction establishing the fore-
going facts have been made by Dr. Eder, Valenta, Mallmann,
Scolik, Schumann, Obemetter, and others, who have published
formulas for the various optical sensitizers that they recommend
individually.

-4, Color-screens y or Ray- fillers.

The general introduction of color-sensitive plates has been
somewhat retarded on accoimt of the necessity of a materially
increased length of exposure when using a color-screen, pre-
cluding the use of this combination for all instantaneous work.

For phototopographic purposes ray-filters are used from a
bright-yellow to a deep-orange tint, varying with the character
of the plates and lenses used. In the Canadian surveys, for
instance, an orange-colored filter was used with the Zeiss Anas-
tigmat Lens No. 3, Series V, together with Edwards' Iso-
chromatic Medium Plate, while a light-yellow screen (Car-
butt's) gave good results in connection with Dallmeyer's W. A.
Lens and Carbutt's Orthochromatic Plates (sensitometer No. 23),
for the topographic reconnaissance, made by the U. S. Coast
and Geodetic Survey, in S.E. Alaska.

Carbutt's (pale-yellow) screens are composed of two thin
piano-parallel crystal-glass plates cemented together with bal-
sam and having the color matter between the plates. They are
2 J or 3 J inches square and can be placed in grooved pieces of
wood suitably attached to the back of the lens board. The

ORTHOCHROMATIC DRY-PLATES AND RAY-FILTERS. 341

screen should always be in position when focusing; when not
in use it should be kept in a box protected against light. With
Carbutt's light color- screen (yellow) the action of the chromatic
rays begins between the Fraunhofer lines C and 2> and it ends
between the lines E and F.

Dallmeyer's yellow screens are fitted into metal settings
which may be attached to the lens mount, close to one side of
the diaphragm when a lens doublet is used.

The Bausch and Lomb filter is in the form of a hollow glass
cylinder that may be filled with variously colored liquids to suit
different optical demands. The piano-parallel ends of these
cylinders are made of optically worked glass and the whole is
incased in a metal ring that fits over the lens mount.

Theoretically, the color-shade of the screen should decrease
in intensity, from the center toward the edge, in the same ratio
as the intensity of the illumination of the plate in the camera
decreases from the center toward the margin and its form should
be spherical, its center of curvature being in the second nodal
point of the camera-lens. For all practical purposes, however,
distortion, due to the use of a piano-parallel screen, placed at
right angles to the optical axis, is imperceptible, particularly
when the screen is placed in the nodal plane of the lens and when
using a relatively small stop.

Terrene points in the shadows of a landscape receive but
partial illumination from the sky and atmosphere and only reflected
light from the surfaces of surrounding bodies; the rays repro-
ducing such shadows on the plate will principally belong to
the violet end of the spectrum. Hence negatives obtained
behind yellow color-screens give the shadows in exaggerated
intensity, particularly when photographing moimtain views of
an Alpine character, since the rarefied air in high altitudes absorbs
less light than the air in lower altitudes. The exaggerated con-
trast between the high lights and shadows in such views makes
it desirable to employ specially prepared plates of a thick emul-
sion coating, which have the further advantage to widen the

342 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

range of correct exposure. Thinly coated plates require accu-
rately timed exposures to avoid a characteristic flatness in their
negatives.

B. Halation.

The naturally sharp outlines defining dark sections in Alpine
views, or objects with marked contrasts, frequently appear blurred
and imdefined in negatives obtained after a rather lengthy expos-
ure, such condition being caused by a reflex action of rays that
have deeply penetrated the emulsion and have been reflected
from the glass surface immediately below the plate coating.
This effect, known as " halation of the plate," may be greatly
reduced by covering the rear surface of the plate with an opaque
coating of the same refractive index as that of the glass used for
the plate. Plates provided with such protective backings are
called " non-halation " or " anti-halation " plates. The back-
ing of non-halation plates should be removed before these plates
are subjected to the developing process.

Besides the increase in range of the gradation of tints in a
monochrome reproduction of a landscape or multi-colored object,
color-screens also materially aid in the prevention of halation
when the necessity arises of having to expose a panorama plate
directly toward the sun.

To prevent possible side reflection and permit only such
rays to reach the plate which are conducive to the production
of the image, it is recommended to insert one or more diaphragms
in the camera-box and to paint these, like all other interior
surfaces of the camera-box, a dull black. Rays reflected by
the lens surfaces should likewise be excluded from the interior
of the camera. This is effected, in a measure, in the Zeiss Anastig-
mat Lens by giving the surface of the back lens a strong curva-
ture. For similar reasons certain surveying-cameras (Deville's
and that of the U. S. Coast and Geodetic Survey) are provided
with a " hood " or " lens shade."

Plates exposed in the field, when well protected against heat

COMPARATIVE LIGHT VALUES AND EXPOSURES. 343

dampness, injurious gases, and, of course, against light, both
before and after exposure, will preserve the undeveloped image
in the latent stage almost indefinitely.

m. Comparative Light Values and Exposures.

To secure as much detail in the shadows as possible, the
plate should be given as lengthy an exposure as it will bear with-
out becoming overexposed. This length depends upon a series
of circumstances and conditions; the more important ones are:

1. The intensity of the light that reaches the plate;

2. The sensitiveness of the plate;

3. The speed of the lens;

4. The size of stop used;

5. The color and illumination of the object;

6. The character of .the color-screen;

7. The distance of the object from the second nodal plane

of the lens.

Success in obtaining clear and well-defined negatives depends
largely upon properly timed exposures, demanding care, judg-
ment, and much experience, if the results are to be xmiformly
successful. Various tables (and diagrams) of comparative light
values and comparative exposures have been computed from
which much information may be gained simply by inspection,
to obtain which without such aid would require much experi-
ence, time, and trouble. Such tables and diagrams, of course,
vary with the latitude of the place and its altitude above sea-
level; both, however, may be neglected for exposures made while
the Sim is relatively high, say not below 45°. Under this proviso
we would have to consider only the conditions of the atmosphere,
including the illumination, the hour of day, the season of the
year, the rapidity of the plate, the character of the screen, and
the lens stop, to ascertain the time for correct exposure.

Any one who has experimented with a certain brand of plate
under certain atmospheric conditions, in a known latitude, and

344

PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

at a certain elevation above sea-level to ascertain the time required
for the correct exposure for a certain subject with a certain lens
and diaphragm, taking recourse to a table of comparative light
values, can readily decide what time should be given a similar
plate under the same conditions, with the same lens and dia-
phragm, at any other hour of the day and on any other day of
the year.

For a correct exposure the time given the light for action
upon the film should be inversely proportional to the intensity
of the light emanated from the subject. A subject, for instance,
requiring six seconds' exposure when the intensity of the light
be one, would, under identical conditions, require an exposure
of two seconds for a light value of three.

The following table contains the comparative exposures for
different lens stops and for " open '' and " dark " landscapes.

Comparative Exposures for Different Lens Stops.

Number of Lens StOQ

F/5

8
F/11

16
F/16

F/22

. 64
^/32

128

^/45

256

Open landscape (seconds). . . .

Average landscape with fore
and background of average
color (seconds)

1/32
1/6

1/16
1/3

1/8
3/4

1/4

1/2
3

I
6

2
12

4
24

The upper line of this table gives the so-called " Uniform
System" numbers of the lens stops or diaphragm apertures,
which numbers have the same ratio to each other as the areas
of their corresponding stops. The second line expresses the
ratio which the stop diameter bears to the focal length (F) of
the camera.

This table is based on a plate requiring i second's exposure
when using stop -F/32 (the diameter of this stop is A of the focal
length of the lens) for an open landscape. The same brand of
plate at the same date and hour and under the same conditions

COMPARATIVE LIGHT VALUES AND EXPOSURES.

345

of illumination would require ij seconds' exposure for an aver-
age landscape when using stop F/16.

Taking the length of exposing at noon, from the middle of
April to the middle of September, as unit, the corresponding
lengths of exposures for a plate at other homrs and at different
seasons of the year are given in Scott's table:

Hours

A.M.

January
i-iS 1S-31

February
i-iS is-a»

March
I-I5 15-31

April
i-iS IS-30

May

I-IS 15-31

Jtme
1-15 iS-30

HOUIB
P.M.

7
8

10
II
Noon

30 IS
10 6

5 4
4 3-5
3'S 3

30 IS
10 6

4 4
3 1.8
2.5 1.8
2.5 1.8

— 30
12 7

* ^
3.1 1.8

1.8 1.6

1.7 1.5
1.6 1.4

15 12
6 4
2.5 2
1.7 1.6
i.S 1-4

1.3 1-2
1.2 I.I

'I 1

1.8 1.7
1.5 1-4

1.3 1-2
I.I I.I
I I

— 30
14 10

5 4
2.3 2
1.5 1-6
1.3 1.3
I.I I.I
I I
I I

4
3

I
Noon

15-31 i-is
December

November

October

1J5-30 1-15
September

15-30 I -I 5
Augtist

'^iiy-'*

We see from this table that the brightest hours of the day
are between 11 A.M. and 2 p.m., the light during this time interval
having greater actinic power than at any other hour of the day.
Views taken between May and August at 5 A.M, or at 7 p.m.,
for instance, should be given exposures from 10 to 30 times longer
than the same subjects would require between the hours 10.30
A.M. and 1.30 P.M. for the same months.

The diagram shown on Plate XCVI represents the com-
parative lengths of exposure at different hours of the day for
the entire year. The abscissae represent the days of the year
from January ist to December 31st, while the ordinates give
the comparative lengths of exposure. This diagram has been
constructed for so-called daylight, sunli^t, and skylight com-
bined, for a station elevation of 500 feet above sea-level and at a
northern latitude of 40°. The lower line, marked " noon curve,"
gives the comparative lengths of exposure (as ordinates) for all
days of the year at noon. The second curve gives similar values
for the hours 11 a.m. or i p.m., one hour from noon, etc.

346

PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

Plate XCVII shows similar curves of comparative exposures
at a northern latitude of 50°. The full-line curves correspond to
an elevation of 5000 feet above sea-level and the dotted cur\'es
correspond for the same latitude, but at sea-level.

Experience teaches that the actinic power of light-rays ema-
nating from objects in the shade on a bright day with a deep-blue
sky is about ten times as great as at the same hour on the same
date, but in dark and threatening weather. A smoky atmosphere
reduces the actinic power of light still more; it may then be from
twenty to thirty times less than it would be on a bright clear
day at the corresponding hour and date. On a bright day with
thin fleecy white clouds in the sky it is even more intense, from
two to three times greater, than on a cloudless diy at the same
hour and date.

The following table, by Bunsen and Roscoe, gives the gen-
eral change in the light intensity as it increases with the altitude
of the sun above the horizon:

Altitude of the Sun (above
the Homon in Degrees).

Actinic Power of the

Light-rays of Sun and

Atmosphere (Combined).

Actinic Power of the

Light-rays, Diflfusod by

the Atmosphere (without

Sunlight).

17. 1

52.6

91.9

122.8

H5-S

38-

160.7

170.8

176.4

178. 1

39.7

Test Exposures and Trial Plates.

Having selected an orthochromatic plate suitable for the
work in view, the observer should make some test exposures
to ascertain the speed of the plate combined with the color-
screen under known conditions. The exposed test-plates should

COMPARATIVE LIGHT VALUES AND EXPOSUKES. 347

be developed with the developer that is to be used for all sub-
sequent exposures. A very satisfactory way to expose these
plates is to give one plate several exposures, say four, by with-
drawing the slide from the plate-holder a quarter of its length
for each successive exposure. By allowing a quarter of a second
for each exposure, the four zones exposed on the plate will have
received exposures from one to one quarter of a second. A
second plate may be similarly expK)sed, only increasing the final
exposure, say to one second, when the four strips will have received
the following exposures: ij, i^, ij, and i second respectively.
All exposures should be made with the same lens stop.

After development of the two trial-plates, it may be found
that the second zone of the second test-plate, the one having
received i J seconds' exposure, may have been the correctly timed
strip. Having noted the conditions of light and atmosphere,
the hour, date, stop, subject, and whether the time of i^ seconds
was given with or without the color-screen, we can, with refer-
ence to comparative exposure and comparative light- value tables
or idagrams, ascertain the time for correctly exposing a similar
plate under other conditions of illumination and atmosphere
at other hours and dates, and, if need be, using different stops.

It may have been foimd by experiment .that a certain cor-
rectly timed plate strip required i J seconds' exposure with stop
F/ii for a dark landscape at 2.30 p.m. on June 8th, and we want
to ascertain the correct exposure time for the same plate brand>
but using stop -F/32 and photographing an open landscape at
3 P.M. on August 2oth.

By inspection we find under stop F/11 (page 344), in the
table of comparative exposures, for the average or dark landscape
the exposure value }, and the corresponding exposure for stop -F/32
under open landscape, i. Now, as our plate required ij seconds*
exposure for stop F/11 and dark landscape it will require 2
seconds for stop F/32 and open landscape. From the diagram^
Plate XCVII, showing the comparative exposures at different
hours and dates, we find that if on June 8th at 1.30 p.m.

348 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS

the illumination required i second's exposure, the same illumina-
tion on August 2oth at 3 p.m. would require an exposure of ij
seconds, hence the time required for our plate and subject, on
August 20th at 3 P.M., would be 2X1^=3 seconds.

For phototopographic purposes a rather slow, double-coated
orthochromatic plate is preferable, as it 'gives a wide range for
correct exposure, is less subject to halation, and the negative will
have the strength requisite for making good prints.

IV? Development of Orthochromatie Dry-plates.

All photographic plates should be carefully dusted with a
soft camePs-hair brush, both after insertion in the plate-holders
and again just before immersion in the developing-bath, to remove
all dust and foreign matter from the film surface, thus preventing
the formation of transparent irregularly shaped spots on the
negative. Immediately after immersion in the developer all
air-bells or bubbles should be removed from the film surface
by gently swabbing the submerged plate with a small tuft of
cotton.

The plate is placed, film side up, in the developing-tray and
the developer is at once poured over the plate from a gradu-
ate or pouring- vessel, with a sweeping motion to cover the entire
plate surface simultaneously, and the tray is kept gently rock-
ing to flow the liquid back and forth in a wave-like motion.

AX^en orthochromatic, double-coated non-halation plates are
used, development should be prolonged to allow the developer
to penetrate the several layers of the film. The high lights in
negatives of the ordinary single-cpated orthochromatic plates
should appear after an immersion of about 30 seconds; for double-
and triple-coated plates the developing process should be con-
tinued up to 6 and 10 minutes.

If the image flashes up too quickly, the plate no doubt has
been overexposed, and it should at once be removed from the
developer and rinsed in clear water; development may now be

DEVELOPMENT OF ORTHOCHROMATIC DRY-PLATES. 349

continued in a diluted developer to which a few drops of a lo per
cent solution of bromide of potassium have been added as a
restraining agent. An old developer may be used to advantage
for developing overexposed plates.

When the image is rather slow in making its appearance the
exposure probably was undertimed, in which case the nega-
tive will develop up too strong with clear shadows and no details.
The latter may be brought out, in a measure, by a lengthy immer-
sion of the plate in a diluted or old developer, the bath being
kept at a temperature of not over 60®. Experience and obser-
vation will best teach when the proper stage in the development
may have been reached and when the plate should be removed
from the bath. The old thumb-rule, to continue development
until the image may be dimly outlined on the back of the plat^,
would lead to overdevelopment if observed for heavily coated
plates. The operator may be better guided by stopping develop-
ment as soon a? the " white portions " (the shadows of the orig-
inal) of the negative begin to change and darken. Weak nega-
tives with clear shadows are generally due to underdevelopment.
Too much density often results from development in a developing
solution too concentrated or too warm.

Since ortho and isochromatic dry-plates are extremely sen-
sitive to yellow, orange, and, to an appreciable extent, to red
light, it becomes necessary to exercise great care in using the
ordinary dark-room light in their manipulation. These plates
should be exposed only to dark (" Venetian '') ** ruby light,"
covered with one or two layers of *" non-actinic paper '* .(Deni-
son's Orange Tissue or Gold Bank Envelope Paper).
Even under the exercise of these precautions, the plate in the
developin^-tray should be kept covered with a cardboard or
slab of hard rubber, except when necessary to examine the progress
of development. Only enough illumination should be permitted
to enter the dark-room or dark-tent as is absolutely necessarj'
to conduct the operations incident to change of plates and their
development.

350 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

Nearly all manufacturers of photographic dry-plates and
printing-papers put up special developers, recommended for use
with their films, in the form of powders, tabloids and concentrated
stock solutions, with full directions for their application.
Amateurs preferring to make up the developing solutions directly
from the chemicals had best prepare the same in the form of
saturated stock solutions, to be mixed and diluted just before
use as needed. In this connection it is well to bear in mind
that dried granulated chemicals are far more active than equal
weights of the same in crystallized form, the weight of the latter
being partly made up of ** crystallization water." For instance,
dried granular sulphite of sodium has double the strength of the
saiyie chemical in crystals, and five parts of carbonate of sodium
in the dried state have the same chemical strength as twelve
parts carbonate of sodium crystals.

If photochemical solutions are prepared by weights and
measures, careful attention should be given to the relative strength
of their component parts. Trouble in this respect, however,
may be avoided by using a " hydrometer " (the single-degree
hydrometer generally used for testing silver solutions is best)
for testing the concentration of the solutions, as dried or crys-
tallized chemicals may then be used indiscriminately.

Stock solutions are best put up in limited quantities, as most
photo-chemical solutions deteriorate with age. It may be noted
here that chemical action may be increased considerably by con-
ducting development under relatively high temperatures, and
decreased when working under a temperature lower than 70°,
the latter being generally recommended for the best results.

Nearly all developing compounds are composed of two parts,
the developing agent proper and the alkaline solution. Among
the developing agents more generally in use we have: Ferrous
oxalate, pyrogallic acid, hydrochinon, eikonogen, metol, rodinal,
their various combinations, and many others, increasing in num-
ber from year to year. They differ in speed, action, keeping
qualities, density imparted to the negative, latitude permissible

DEVELOPMENT OF ORTHOCHROMATIC DRY- PLATES. 35 1

in the exposure of the plate, etc. After having become familiar
with the action of any one developer it is recommended to adhere
to its use to insure uniformity in the resulting negatives.

If, in a given developer, the developing agent be used in
excess of the correct proportion, too great a contrast between
the lights and shadows will result; the development will not be
.under control, it will progress too fast. An insufficient amount
of the developing agent will produce a negative deficient in strength
and lacking the qualities essential for good printing. The gen-
eral development will be slow with the concomitant danger of
** frilling," which is the separation of the film from the edges-
of the glass plate.

The alkaline ingredients of the developing-solution, prin-
cipally carbonates of sodium (" sal soda ") and potassium, bicar-
bonate of soda, and sulphite of soda, serve to open the pores of
the gelatino-bromide of silver emulsion, permitting a free entrance
of the developing agent into the upper layers of the softened
plate film, thus producing a more prompt and effective action
on the embedded particles of the silver haloids. An excess of
the alkaline solution tends to produce a dense negative, imparts
a tendency to fog the plate, and often converts the film of the
latter into a granular condition.

The progress of development will be materially retarded if
an insufficient quantity of the alkaline solution be used. Bicar-
bonate of soda, being less active than carbonate of soda and
carbonate of potassium, is often used, in combination with sul-
phite of sodium, for developing thinly coated plates to reduce
their tendency toward fog formation and to prevent injury to
the film. The alkaline solutions are preferably kept in hermet-
ically closed bottles to prevent decomposition, which would soon
take place on exposure to the air. Old alkaline solutions, or such
containing impure sulphite of sodium, are apt to produce yellow
stains on the negative and feshly prepared solutions of pure
sulphite of sodium should be used whenever possible.

All chemicals for photographic use should be pure, and it is

352 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

recommended to purchase those especially manufactured for
photochemical purposes. Some manufacturers of compressed
pharmaceutical preparations have extended the tabloid system
to photographic preparations. The advantages of the tabloid
form to the traveller, explorer, and to the novice in the practice
of photography are apparent. Tabloids when prepared by dry
compression do not readily decompose; they retain in their full-
est energy the qualities of the various ingredients of which they
are compounded, and they have reliability, uniformity, and
portability in their favor.

A. Water and Water Tests.

The water used in photographic operations should be dis-
tilled or pure and free from foreign matter. By reason of its
great dissolving power, ordinary water, in the absence of the
distilled article, should be boiled for some time and then allowed
to cool before decanting it for making photochemical solu-
tions. The following simple tests may be applied to discover
the presence of iron, magnesia, lime, etc., in ordinary water:
For Iron: The addition of an infusion of nutgalls to water
will show the presence of iron by imparting a grayish
color to the mixture. If the liquid turns blue, after the
addition of a pinch of prussiate of potash, the presence
of iron is unmistakable.
Por Magnesia: Reduce a certain amount of water by boiling
to V2oth of its original weight, then dissolve a few
grains of neutral carbonate of ammonia in this liquid.
If a whitish precipitate is formed, after the addition of a
few drops of phosphate of soda, magnesia will be present.
For Lime: If two drops of a concentrated solution of oxalic
acid be added to a glass of water, the latter will contain lime
if a milky appearance be thus produced.
For Alkalies: Water that will change red litmus paper, on
immersion in the same, to blue may be considered alkaline.

DEVELOPMENT OF ORTHOCHROMATIC DRY-PLATES. 353

For Organic Water: Water becoming turbid, after the addi-
tion of one tablespoonful of tannin solution (i part
tannin, 4 parts water, and i part alcohol) to a tumblerful,
will contain organic matter. Such water is unfit for drink-
ing purposes, particularly if the impurities are of animal
origin.

For Hardness: If no change be noted after adding a few
drops of a solution of good soap in alcohol the water is
soft; if it becomes milky in appearance it may be con-
sidered hard.

For Carbonic Acid: To half a tumblerful of water add an
equal amount of lime-water; if carbonic acid be present
a precipitate will be formed and the addition of muriatic
acid will cause effervescence.

B. Developers,

There is a distinct relation between the intensity of the light
that has acted upon the sensitized emulsion coating of a plate
and the actual amount of silver that is deposited upon the plate
under the action of the developer and which defines the density
of the negative. The laws which control the combined effects
of light and developer upon a photographic plate have been
studied by many photochemists. The results of the researches
made by F. Hurter and V. C. Driffield in England are generally
accepted as representing the best discussion on the subject.
Capt. E. Deville, in his work on "Photographic Surveying",
gives an abstract of the principal papers published by Messrs.
Hurter and Driffield, with which every photographer should
familiarize himself if he is desirous to obtain a knowledge of
the laws of correct exposure and development.

I. Developing wtth Ferrous Oxalate.

Ferrous oxalate is the best developing agent for phototopo-
graphic purposes if the exposed plates are packed away in the
field to be developed at some later period. For the develop-

354 PHOTOTOPOGRAPHIC IfETHODS AND INSTRUMENTS.

ment of test-plates in the field where the means of transpor-
tation are limited, and where, owing to the numerous other duties
to be performed, dark-room operations must be reduced to a mini-
mum, developers in the form of dry powders or tabloids will
generally be preferred.

Good results are obtainable with the iron developer, even
after the exposed plate has been stored away for a long time,
before the actual development of the same is undertaken. Fer-
rous oxalate may furthermore be recommended, because it affects
only those particles of the silver haloids that had previously
been acted upon by solar light, and because the final metallic
silver deposit on the plate shows great uniformity in color. For
these reasons negatives developed with the iron developer are
particularly well suited for making enlargements by " optical
projection."

The products resulting from the oxidation of ferrous oxalate,
after an exposed plate has been in the developer a short whfle,
exercise a restraining influence over the progress of development,
without, however, stopping it altogether. The plate continues
to gain density under the prolonged action of the developer,
but the energy of the latter is held under control and the progress
of development becomes more and more retarded under the
gradual advance of the process of oxidation. The details of
the image slowly become visible on the plate under the restrained
action of the developer, gradually gaining strength and density,
and at fuU development, when the plate is removed from this
bath, the plate coating will have imdei^one a permanent change,
inasmuch as particles of the silver haloid that have not been
acted upon by the solar rays practically will have remained
unchanged, while those that had been acted upon will become
reduced to free silver. The final image on the negative is formed
by a more or less gradated series of tones, conditioned by the
various thicknesses of metallic black silver deposits that have been
formed on the different parts of the plate.

For the development of the plates obtained in connection

DEVELOPMENT OF ORTHOCHROMATIC DRY- PLATES. 355

with the Canadian surveys, Captain E. Deville uses freshly pre-
pared ferrous oxalate compounded in two stock solutions after
the following formulas:

Metric Weight. Apothecaries' Weight.

30 grammes. . . Oxalate of potash i oz.

90 c.c Distilled water (hot) 3 oz.

I gramme. . . . Bromide of potassium 15 grains

J c.c Acetic acid 10 minims

30 grammes. . . Sulphate of iron i oz.

60 c.c Distilled water (hot) 2 oz.

i c.c Acetic acid 2 minims

These stock solutions, A and B, keep well if bottled sepa-
rately; they should be mixed for immediate use only. For the
normal developer take to each ounce of solution A two drachms
of solution B. The plates are developed, a dozen at a time, in
grooved hard-rubber boxes, in which they are placed in an upright
position.

The formulae for Dr. Eder's ferrous oxalate developing- bath
are as follows:

A.
Metric Weight. Apothecaries' Weight.

200 grammes . . Neutral oxalate of potassium 63 oz.

800 c.c Distilled water (hot). 26J oz.

This stock solution should be acidulated with oxalic acid,
adding one gramme for every 30 c.c. of the solution.

Metric Weight. Apothecaries' Weight.

100 grammes. . Protosulphate of iron (crystals) 3} oz.

300 c.c / . Distilled water (hot) 10 oz.

J c.c Sulphuric acid 5 minims

Mix in the order given, adding the acid last. These solutions
are good keepers when bottled separately, and they should be
mixed (cold) for immediate use only. For the normal de-

356 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

veloper and for correctly timed exposures take volumes of
A and i volume of B and mix in a graduate.

(a) Restraining the Ferrous Oxalate Development.

Metric Weight. Apothecaries' Weight.

10 grammes. . . Bromide of potassium 2) drachms

100 c.c Distilled water si oz.

By adding a few drops of this solution C, termed a re-
strainer, to the normal iron developer, given above, the
development of the latent image will be kept under control. A
moderate overexposure of a plate may thus be neutralized by
adding from five to ten drops of solution C to the ferrous
oxalate developer, which will check the general progress of develop-
ment sufficiently to impart density to the plate and to allow the
details to appear in the high lights before the shadows become
overdeveloped.

The simultaneous appearance of lights and shadows on the
plate (" the flashing- up " of the image) immersed in the develop-
ing-bath would be indicative of overexposure, and the plate
should be immediately removed from the developing- tray, be
well rinsed in soft running water and be subjected to one of the
following means of " retarding " development:

1. Reducing the sulphate of iron solution (against the " nor-

mal " amount of oxalate of potash solution);

2. Reducing the temperature of the "normal developer";

3. Increasing the bromide of potassium (or other bromide

salt used) ;
For the development of overexposed plates it is advisable
to withhold about 30 c.c. (i oz.) of solution B in a separate
graduate and add from 2 to 4 c.c. (30 to 60 minims) of solu-
tion C gradually pouring enough of this mixture to the de-
veloper in the tray to produce the desired density in the plate.
Additions to the developer are preferably made after the latter

DEVELOPMENT OF ORTHOCHROMATIC DRY-PLATES. 357

has been poured off the plate into a pouring vessel, flooding the
plate with the modified developer homogeneously mixed.

4. Diluting the " normal developer " with water;

5. Using an old (already used) developer.

(b) Accelerating the Ferrous Oxalate Deyelopment.

The application of so-called " accelerators '' overcomes, in
a measure, the effects of underexposure. They may also b2 of
value when developing plates representing subjects of great
contrasts, or in all cases where the normal developer would
produce too harsh and too dense a negative. The following
means for accelerating development may be employed.

1. Increasing the sulphate of iron solution (against the

"normal" amount of oxalate of potash solution);

2. Increasing the temperature of the "normal developer";

3. Using a freshly prepared and slightly concentrated solution

of the " normal developer ";

4. Adding a very little hyposulphite of sodium to the " normal

developer."

About three ounces of a developer, when mixed ready for
use, will suffice for the development of a 4X5, and four ounces
will be required for a 5X8 in. plate. When several plates are
to be developed it is best to prepare a larger quantity of the nor-
mal developing mixture at one time and develop a dozen plates
at once.

After the proper stage in the development has been reached,
the plate should be well rinsed in clear running water, then to
be placed in the following so-called " clearing solution," which
serves to prevent the precipitation of iron from the developer
into the upper layer of the plate film.

Clearing Solution.

Metric Weight. Apothecaries* Weight.

150 c.c Saturated solution of alum 5 oz.

4 C.C Citric acid z drachm

150 c.c Distilled water 5 oz.

358 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

The negative is submerged in this bath from three to five
minutes, after which it is again rinsed in clear running water to
remove any deposit that may still adhere to the film surface,
to be finally placed in the " fixing-bath."

2. Pyro Developer.
A good " pyro " developer may be made in two solutions:

Alkaldw: Solution.
Metric Weight. Apothecaries* Weight.
360 c.c Distilled water 60 oz.

30 c.c Carbonate of sodium (crystals) 5 oz.

60 c.c Sulphite of sodium (crystals) 10 oz.

To prepare this solution with the ** hydrometer," mix equal
parts of:

Carbonate of sodium solution, testing 40 degrees

Sulphite of sodium solution, testing 80 degrees

B.
Pyro Solution.
Metric Weight. Apothecaries' Weight.

4 c.c Sulphite of sodium (crystals) i drachm

180 C.C Distilled water 6 oz.

After the sulphite of sodium has been dissolved in the 6 02.
of water, add acetic acid to this solution until the liquid turns
blue litmus paper red, then add:

Metric Weight. Apothecaries' Weight.
30 C.C Pyrogallic acid i oz.

For the " normal developer " mix:

Metric Weight. Apothecaries' Weight.

4 c.c of B (pyro solution) i drachm

60 c.c of A (alkaline solution) 2 oz.

For winter use, dilute this with 60 c.c. (2 oz.) of tepid dis-
tilled water, whereas for summer use, dilute with 90 to 150 c.c.
(3 to 5 oz.) of cold distilled water.

DEVELOPMENT OF ORTHOCHROMATIC DRY-PLATES. 359

Both solutions should be kept in well-stoppered bottles. If
the negatives show yellow stain a new solution A should be made
or a freshly prepared sulphite of sodium should be used.

A smaller quantity of sulphite of sodium in solution A will
produce a warmer tone, a larger quantity a grayish-blue to bluish-
black tone. An increase of A in the normal developer mixture
may bring out detail in an underexposed negative. If the high
lights in the negative are flat more of the pyro solution (B) may
be used, if they are too intense less may be used.

If too little of solution B be used the alkali will be in excess
and a foggy negative may be the result.

Pyrogallic acid, being a strong poison, should be carefully
handled, clearly labeled, and securely stored.

3. Metol Developer

may be made in either one or two solutions, both keeping well.
The two-solution developer, however, is preferable, as it not only
gives a better control over the progress of development but it
also gives the means for developing overexposed plates by
adding a little of solution B to the " normal developer," or by
using solution A alone (diluted) if the plate was greatly over-
exposed.

Metric Weight. Apothecaries' Wei^t.

1000 c.c Distilled water 10 oz.

15 grammes.. . . Metol 75 grains

120 grammes.. . . Sulphite of sodium (crystals) z} oz.

Dissolve the metol in water before adding the sulphite of
sodiuHL

Metric Weight. Apothecaries' Weight.

1000 c.c Distilled water 10 oz.

150 grammes. . Carbonate of soda (crystals) i . 75 oz.

1.5 grammes. . Bromide of potassium 8 grains

360 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

For the normal developer take:

Solution A, I volume

B, I '*
Water, i **

Metol developer may be used repeatedly. An old developer
is to be recommended for overexposures. If the plate shows
a tendency to fog add from 10 to 20 drops of a 10 per cent solu-
tion of bromide of potassium to the developer.

For correct exposures the image should appear in detail within
from 4 to 10 seconds and development should be complete in
4 or 5 minutes.

As the density of the plate is somewhat reduced in the fixing-
bath development should be carried on a little further than one
would otherwise do.

For underexposed plates the normal developer should be
diluted.

4. Metol Bicarbonate Developer

is to be recommended for its excellent keeping qualities and uni-
form results. It may be used repeatedly without materially
aflfecting the general progress of the development. The bicar-
bonate of soda makes this developer very safe in action, prevent-
ing injury to the film and fogging of the plate.

Metric Weight. Apothecaries' Weight.

10 gp-ammes. . . Metol i oz.

600 c.c Distilled water. 60 oz.

Thoroughly dissolve the metol in the water and then add:

Metric Weight. Apothecaries' Weight.

60 grammes. . . Sulphite of soda (crystals) 6 oz.

30 gp'ammes. . . Bicarbonate of soda 3 oz.

To prepare this developer with the hydrometer, mix:

Metric Weight.

300 c.c (30 oz.) Sulphite of soda solution testing. . • . 75 deg.

300 c.c (30 oz.) Bicarbonate of soda solution testing. . 50 deg.

10 c.c ( I oz.) Metol dissolved in 120 c.c. (12 oz.). . . Water

DEVELOPMENT OF ORTHOCHROMATIC DRY-PLATES. 361

5. Hydrochinon Developer. •
A.

Metric Weight. Apothecaries' Weight.

600 c.c Distilled water (hot) 20 oz.

120 grammes. . . Sulphite of soda (crystals) 4 oz.

4 grammes. . . Sulphuric acid i drachm

23§ grammes. . . Hydrochinon 360 grains

2 grammes. . . Bromide of potassium 30 grains

Diluted with enough water to make up to

Metric Weight. Apothecaries' Weight,
960 c.c 32 oz.

60 grammes. . . Carbonate of potash 20 oz.

60 grammes. . Carbonate of soda (crystals) 2 oz.

With enough water to make up to

Metric Weight. Apothecaries' Weight.
960 c.c 32 oz.

C ("Accelerator").

30 grammes. . . Caustic soda i oz.

300 gp-ammes. . . Water 10 oz.

D ("Restrainer").

14 grammes. . . Bromide of potassium } oz.

150 c.c Water 5 oz.

For the normal developer take

Metric Weight. Apothecaries' Weight.

30 c.c Solution A i oz.

25 c.c *' B }oz.

120 c.c Water 4 oz.

The working temperature of this developer should not vary
miich between 65° and 75°.

For underexposure add a few drops of solution C to the normal developer
" overexposure '* *' ** '* '* '* D *' " " "

362 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

More of scflution A (as given for the " normal developer '')
will increase the density and more of solution B produces an
increase in detail.

Should the negative, after development with hydrochinon,
show yellow stain, it may be cleared and intensified, if need be,
by immersion in the following bath:

Bichromate of potassium 10 parts

Hydrochloric acid 10 * *

Water 1000 * *

The stained negative is kept in this solution until it appears
completely bleached, when it should be well rinsed in running
water. If the bleached negative be now developed anew no
trace of fog will appear; redevelopment should be carried on
until the desired strength and density may be attained.

The following hydrochinon developer is recommended by
L. E. Jewell for photographing clouds with a ray-filter:

Metric Weight. Apothecaries* Weight.

30 c.c Hydrochinon i oz.

150 c.c Sulphite of sodium (crystals) 5 oz.

750 c.c Distilled water (hot) 25 oz.

7 c.c Alcohol (95%) J oz.

After the sulphite of sodium has been dissolved in hot water
add the hydrochinon and shake well. Filter the solution, add
the alcohol, and again shake well

Metric Weight. Apothecaries' Weight.

30 c.c Carbonate of potassium i oz.

30 c.c Ferrocyanide of potassium i oz.

360 c.c Distilled water 12 oz.

The ferrocyanide of potassium acts as an accelerator for the
hydrochinon. For the normal developer take

Metric Weight. Apothecaries' Weight.

90 c.c Solution A 3 oz.

30 c.c * * B I oz.

DEVELOPMENT OF ORTHOCHROMATIC DRY-PLATES. 363

adding from 6 to 10 drops of a 10 per cent solution of bromide
of potassium to this mixture.

Development may be begun with the "normal developer"
and if signs of over or underexposure are noted the developing-
bath should be modified to the following mixtures:

For overexposure: 3 J volumes sol. A;

I volume sol. B;

J volume of a ten per cent bromide of potassium solution.
For underexposure: 3 volumes sol. A;

I volume sol. B, omitting the bromide of potassium sol.

In changing from one of these developers to the other the
plate had best be rinsed in clear water, although this would
not be necessary when changing the plate from the underexpos-
ure bath to the normal developer, nor when transferring the
plate from the normal to the overexposure bath.

6. Metol Hydrochinon Developer.
For use in winter, dissolve in the order given:

Metric Weight. Apothecaries' Weight.

7 grms Metol J oz.

7 grms Hydrochinon J oz. in

2400 c.c Distilled water 80 oz., then add

120 grms Sulphite of soda (crystals) 4 oz.

75 grms Carbonate of soda (crystals) 3} oz.

To prepare this solution with the hydrometer, mix in the
order given:

Metric Weight.

6co c.c (20 oz.) Sulphite of soda solution testing. . . .60 deg.

600 C.C (20 oz.) Carbonate of soda solution testing. .30 deg. with

7 grammes (} oz.) metol and 7 granmies (} oz). hydro-
chinon dissolved in
1200 c.c (40 oz.) Water.

For summer use this normal developer should be diluted with
an equal quantity of water to keep the development imder good
control. If the negatives show too much contrast less hydro-
chinon and more metol may be taken.

364 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

7. Bromo-Hydrochinon Developer.

Bromo-Hydrochinon Developer recommended for developing
overexposed plates and for producing density in the negative.

Metric Weight. Aix)thecaries' Weight.
750 c.c Distilled water (hot) 25 oz.

90 grammes. . . Sulphite of soda (crystals) 3 oz.

15 grammes. . . Hydrochinon J oz.

7 grammes. . . Bromide of potassium } oz.

750 c.c Distilled water 25 oz.

180 grammes. . . Carbonate of soda (crystals) 6 oz.

For the normal developer take equal volumes of A and B.
If the plate shows signs of underexposure it should be immersed
in a freshly prepared and diluted developer, and if sufficient
detail does not appear in this bath, the plate should be removed
to another tray containing water to which a little of the alkaline
solution (solution B) has been added, leaving the plate in
this bath as long as an increase in detail may be noted. If
still weak, development may be finished in a fresh developer.

8. EiKONOGEN Developer.
A.

Metric Weight. Apothecaries' Weight.

15 grammes. Eikonogcn i oz.

60 grammes. Sulphite of sodium (crystals) 4 oz.

0.3 gramme. . Bromide of potassium 10 grains

900 c.c Distilled water 60 oz.

45 grammes. . . Carbonate of soda 3 oz.

300 c.c Distilled water 20 oz.

For the normal developer take 3 parts of solution A and i
part of solution B, adding i drop of a 10 per cent solution of

DEVELOPMENT OF ORTHOCHROBiATIC DRY- PLATES. 365

bromide of potassium to each 30 c.c. (each oz.) of mixed
developer.*

If the " acid fixing-bath " be used after development with
eikonogen the negatives are apt to be marred by semi-transparent
streaks, t

9. "Eiko-Cum-Hydro" Developer.
A.

Metric Weight. Apothecaries' Weight.

600 c.c Distilled water (hot) 20 oz.

1 20 grammes. . . Sulphite of soda (cr>'Stals) 4 oz.

22 grammes. . . Eikonogen 330 grains

10.5 grammes. Hydrochinon 160 grains

adding enough water to make up to

Metric Weight. Apothecaries' Weight.
960 c.c 32 oz.

600 c.c Distilled water 20 oz.

60 grammes. . . Carbonate of potash 2 oz.

60 grammes. . . Carbonate of soda (crystals) 2 oz.

adding enough water to make up to

Metric Weight. Apothecaries' Weight.
960 c.c 32 oz.

* For developing bromide paper prints add 2 parts distilled water to the normal
developer as given here. The developer should be renewed after each 4 to 6
prints have been developed.

t The following "fixing-solution" should be used with this developer for both
plates and bromide prints, as it prevents- all possibility of the developer staining
the negative:

Metric Weight. Apothecaries' Weight.

60 grammes Hyposulphite of soda 4 oz.

15 grammes. Bisulphite of sodium x oz.

300 c.c Distilled water 30 oz.

This fixing-solution remains colorless after repeated use.

366 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

For the normal developer take

Metric Weight. Apothecaries' Weight.

30 c.c Solution A i oz.

15 c.c Solution B 4 drachms

90 C.C Distilled water 3 oz.

More of solution A would increase the density of the nega-
tive and more of solution B would tend toward an increase in
detail.

10. Amidol Developer.

Metric Weight. Apothecaries* Weight.

600 c.c Distilled water 20 oz.

4 c.c Sulphurous acid i drachm

15 grammes. . . Sulphite of soda, granular, dry 4 drachms

3 grammes. . . Amidol 46 grains

Dissolve the chemicals in the water in the order given. With
this developer the image should appear rather quickly with full
intensity and a wide gradation of tones.

C. Fixing the Negative.

After removal from the " clearing-bath," the negative is
placed in the " fixing-bath " to preserve the developed image,
to render the negative light-proof.

After the proper stage in the development of the plate has
been reached, by immersion in one of the preceding developers,
in the dark-room, the negative should be well rinsed in clear
running water to remove all traces of the developing and clearing
solutions. It may next be placed in the " clearing-bath " given
under " iron developer " (ferrous oxalate), solution D. This
bath should not be omitted after development with ferrous
oxalate, but it may be omitted when using most of the other
developers. After removal from the clearing-bath the negative
should again be rinsed in clear water before it is subjected to the
'' fixing-bath."

DEVELOPMENT OF ORTHOCHROMATIC DRY-PLATES. 367

Metric Weight. Apothecaries' Weight.

4 c.c *. . . Sulphuric acid i drachm

480 grammes. . . Hyposulphite of soda 16 oz.

60 grammes . . . Sulphite of sodium (crystals) 2 oz.

30 grammes . . . Chrome alum * i oz.

1920 c.c Distilled water (warm) 64 oz.

This acid fixing-bath should be mixed in the following order:
Dissolve the hyposulphite of soda (16 oz.) in 1440 c.c. (48 oz.)
of warm distilled water, the sulphite of sodium crystals (2 oz.)
in 180 c.c. (6 oz.) water. Next dilute the sulphuric acid
(i drachm) with 60 c.c. (2 oz.) water and pour this slowly
into the sulphite of sodium solution and add this to the hypo-
sulphite of soda solution. Now dissolve the chrome alum (i oz.
in summer and J oz. in winter) in 240 c.c. (8 oz.) of warm
distilled water and add this solution to the bulk of the mixture,
when the fixing-bath (after cooling) will be ready for use.

This fixing-solution is a good keeper and will not discolor
until after repeated use. It clears the shadows of the negative
and hardens the film, thus materially preventing " frilling "
(separation of film edge from glass surface) in the final washing.

The negative should be kept in this bath until the last trace
of the milky-white appearance of silver bromide, when examined
from the back of the negative, has entirely disappeared and the
shadows have become perfectly transparent. This should require
an immersion of about 5 minutes. The negative has now become
light-proof and it should be thoroughly rinsed in clear running
water for at least one half hour, or when the water is cold, for
fully one hour, to free the film from any trace of the hyposul-
phite. Before removal from the water the film surface should
be swabbed with a wad of cotton, again rinsed, and finally be
placed in rack to dry spontaneously. If no running water is
available the washing may be done in ten to fifteen changes

♦ The given amount is for use in summer. In winter only 15 grammes
(4 drachms) chrome alum should be taken. An excess of alum may cause a
precipitate to form on the negative, imparting a mottled appearance to the latter.

368 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

of water, transferring the negative from one tray to the other
and refilling each tray with fresh water after passing the nega-
tive from tray to tray at intervals of five to ten minutes.

It is most important that every trace of hyposulphite of soda
be eliminated in the final washing to impart keeping qualities
to the negative. Any subsequent formation of a crystallized
coating of the film, followed by a gradual fading of the image,
may generally be regarded as the direct result of an imperfect
removal of the hyposulphite of soda in the final washing.

I. Tests por Presence of HvposuLprnTE of Soda.

To ascertain whether the hyposulphite of soda may have been
thoroughly removed from the film of the negative the following
tests may be applied.

The simplest test for presence of " hypo " in the last washing
is made by adding a few drops of the following solution (Prof.
Boettcher's test) to a little of the water drained from the negative:

Metric Weight. Apothecaries' Weight.

0.2 gramme .... Permanganate of potash 3 grains

I gramme Caustic soda 15 grains

460 c.c Water 16 oz.

If " hypo " be still present the red color of the mixture will
be changed to green.

This solution should be kept in a well-stoppered bottle incased
in a light-proof wrapper or box. This test is generally con-
sidered very satisfactory, as the quantity of " hypo " left in the
pores of the film must be very small indeed if the pink or red
colored solution does not change color within a few minutes after
mixing the drainings from the plate with the permanganate solu-
tion.

Another test is as follows:

Beat up a piece of starch, about the size of a pea, with J oz.
of water and boil in a test-tube to a clear jelly. To this add

DEVELOPMENT OF ORTHOCHROMATIC DRY- PLATES. 369

one drop of tincture of iodine, which will produce a dark-blue
color. Now fill another test-tube with the drainings from the
negative and add one drop of this blue solution, stirring the
mixture well. If " hypo " be present the blue color will be dis-
charged.

Should the final washing of the negative have to be cut short,
on account of " frilling of the film," during warm weather and
in the absence of ice, or should it have to be interrupted for any
other reason, it is recommended to place the negative in a freshly
prepared " clearing-bath " (solution D, given under ferrous oxa-
late developer), or in a 1:30 solution of bromine in water, to
neutralize and destroy any "hypo" that may still be retained
in the surface layer of the film.

2. Drying the Finished Negative.

Negatives are best dried in a cool, dry atmosphere, prefer-
ably under a mild draft. In a warm, damp, or wet climate the
drying of the finished negative would proceed too slowly, greatly
increasing the density of the film, particularly toward the center
of the plate, which generally dries last. Under such adverse
conditions the drying of the films may be accelerated by flowing
proof alcohol over the film a few times before placing the nega-
tive in the rack to dry. Heat, however, should never be applied
to the negatives for purposes of drying, as it liquidizes the soft
gelatine coating. Great care should be exercised not to inter-
change the trays and vessels used for the various photochemical
solutions.

3. Intensification of a Negative wrrn the Aid of Metallic Salts.

With correct exposure and development intensification need
not be resorted to. Light somewhat retards the process of inten-
sifying, and it is advisable to conduct the operation in the dark-
room, or at least in subdued light. A negative that had been

37© PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

dried after fixing is more readily acted upon by the intensifier
than one that has just been removed from the washing-tank.

Should the final negative be too thin for good printing it
may be intensified, after thoroughly washing to eliminate all
traces of " hypo," as previously noted, by subjecting it to the
following treatment :

Pour a sufficient quantity of a saturated solution of bichloride
of mercury in water into a solution of 37 granunes (i J oz.) iodide
of potassium in 180 c.c. (6 oz.) water until the point be reached
when the forming red precipitate can no longer be dissolved by
shaking. No more bichloride of mercury solution should be
added than just enough to malce the solution a slight shade turbid.

Now add 28 c.c. (i oz.) hyposulphite of soda, dissolve, and
add enough water to make up 600 c.c. (20 oz.) of this inten-
sifier. Just before use dilute one part of this intensifier with
three i)arts of water. The weak negative is submerged in this
diluted solution, exercising care not to carry the intensification
too far, although the negative may be reduced in a measure
by again placing it in the fixing-bath for a short while. If the
negative was not perfectly ** fixed " before subjecting it to the
action of the intensifier it will be marred by a yellow stain.

4. Intensification with Silver Cyanide.

After having washed the weak negative thoroughly for half
an hour in clear running water it should be immersed for ten
minutes in a five per cent solution of alum and again thoroughly
washed before applying this intensifier.

Metric Weight, Apothecaries' WcUfht.

16 grammes. . . . Bichloride of mercury 240 grains

16 grammes, . . . Chloride of ammonia 240 grains

600 c.c Distilled water 20 oz.

16 grammes. . . . Chloride of ammonia 240 grains

600 c.c nistilkni water 20 07.

DEVELOPMENT OF ORTHOCHROBiATIC DRY- PLATES. 37 1

Metric Weight. Apothecaries' Weight.

4 grammes .... Nitrate of silver 60 grains

60 c.c Distilled water 2 oz.

This nitrate of silver solution is poured, while stirring, into
the following solution:

Metric Weight. Apothecaries' Weight.

4 grammes .... Cyanide of potassium, C.P 60 grains

180 c.c Distilled water. 6 oz.

The cyanide of potassium serves as a fixing-agent; it is a
very strong poison and the solution C should be labelled " poison *'
and carefully stored.

To intensify the negative enough of solution A is flowed
over it to completely submerge the film. It is subjected to this
bath sufficiently long to either partially or completely whiten
the film, according to the degree of density desired. After
removal from this bath the plate should be carefully rinsed and
immersed in solution B for one minute, again be rinsed and
then be placed in the cyanide solution (C), where it is kept until
the color of the film is changed to a dark brown or black, when
it should be removed, thoroughly washed in running water, and
placed in rack to dry. The solutions A and B should be thrown
away when once used, while the cyanide solution (C) may be
returned to its bottle to be used again.

5. Prof. R. E. Liesegang's Intensifer.

Prof. Liesegang recommends the following solution for the
intensification of underdeveloped negatives:

Apothecaries' Weight.

r Sulphate of copper 75 grains

Sol. I < Bromide of poteissium 75 "

I Water 6i oz.

Sol. ir

f Nitrate of silver 91 grains

I Water 4 oz.

372 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

After the negative, which may be found too thin to print
well, has been washed from three to five minutes in running
water to remove every trace of hyposulphite of soda (see Prof.
Boettcher's " hypo " test) it is placed for ten minutes in solution
I until thoroughly bleached. The longer the negative remains in
this bath the greater the final density will be; therefore this part
of the process should be carefully watched. Still, should the
intensification be too marked, the density may again be reduced
with the usual fixing-bath of " hypo."

After removal from solution I the negative is again rinsed in
running water, or washed in five changes of water for ten minutes,
and it is then placed in the nitrate of silver solution until it becomes
thoroughly blackened (no white spots should be apparent when
viewed from the back). The intensified negative is now washed
for at least one hour in frequent changes of water; in running
water half an hour will probably suflSce.

A negative that had already been dried after fixing . is more
i;eadily acted upon than one that has just left the washing-tank.
Light retards the process materially and intensification is best
conducted in the dark-room, or at least in subdued light.

6. Intensifying Negatives without the Use of Metallic Salts.

After a thorough washing in running water the negative is
immersed in the following solution:

Metric Weight. Apothecaries' Weight.

0.3 gramme Potassium bichromate 5 grains

0.6 gramme Potassium chloride 10 grains

0.25 c.c Hydrochloric acid 4 minims

30 c.c Distilled water i oz.

Under the action of this solution the silver deposit on the
negative is converted into chloride of silver. The plate is.retained
in this bath until the image appears well bleached, when it is
removed and thoroughly washed in clear running water to elimi-
nate the chromium salts from the film. The negative may

DEVELOPMENT OF ORTHOCHROMATIC DRY- PLATES. 373

now be redeveloped in any developer. Some operators recom-
mend soaking the plate in a dilute solution of sulphurous acid
or acid metobisulphite to facilitate the elimination of the chro-
mium salts in the washing between the bleaching and redeveloping.
Pyro-soda, pyro-anmionia, metol, and pyro-metol developers
give a considerable increase in density when used for redevelop-
ment with this method of bleaching the image of a weak negative

7. Reducing the Density of a Negative.

Brown and yellowish stains and also a certain iridescence
of the film surface may all be caused by having the developing,
bath too warm, too strong in alkali for the plate, or by having
used the plain " hypo '' solution in fixing the negative. The
same defects may also be caused by using too old a solution in
the fixing-bath, or when the latter has been used too often, and
sometimes, too, by an insuflSicient fixing of the negative. A weak
solution of perchloride of iron will remove the yellow stains
. when this bath is followed by an immersion in the acid fixing-
bath.

Density in a negative, brown stains, and the metallic irides-
cence may all be removed by applying the following " reducing
solution." Dissolve

I part of red prussiate of potash in
15 parts of water.

Wrap the bottle containing this solution in yellow paper, to
delay decomposition of the solution by the effects of the lights
then dissolve

I oz. hyposulphite of soda in
15 oz. water.

Add from one half to one ounce of the red prussiate solution
to the above hyposulphite of soda solution immediately before
use and place the negative in this bath directly after fixing.

374 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

A dry negative should first be soaked in water for a few minutes.
Watch the negative carefully while it is in this reducing solution,
rocking the tray and avoiding strong light during the immersion,
and remove it to running water at once when it has been suffi-
ciently reduced or cleared.

Great care should be exercised to keep the trays and the
vessels as clean as possible and never to interchange them when
developing and fixing plates. All vessels used for developing
should never be used for any other purpose and the fixing-tray
should never be used for developing.

8. COOUNG-SOLUTIONS.

To accelerate the drying of the films in a damp or wet
climate the plates may be immersed, just after the final wash-
ing, in a bath composed of equal parts of water and alcohol
before they are placed in the rack to dry. All negatives, when
dried slowly in a damp and warm atmosphere, will become more
intense than when dried spontaneously in a draught or in a cool
current of air.

Should the conditions be unfavorable enough to require
artificial heat for drj'ing the films, it is recommended to immerse
the negatives from six to ten minutes in a solution of one part
of Woodbury Antipyr in ten parts of water, after which they
are rinsed and placed in the rack to dry. This solution may
be used repeatedly.

When the development of plates has to be carried on during
very warm weather in localities where neither ice nor cold water
is obtainable, the several baths may be kept cooled by setting
their trays in shallow vessels that are filled with some cooling-
solution, of which we may enumerate the following:

1. One part of nitrate of sodium and four parts of water.

2. One part nitrate of ammonia and one part water.

3. One part sulphocyanate of potassium and one part water.

4. Three parts nitrate of sodium and four parts water.

DEVELOPMENT OF ORTHOCHROMATIC DRY-PLATES. 375

5. One part chloride of potassium and four parts water.

6. Three parts sulphate of sodium and two parts diluted

nitric acid.

7. Nine parts phosphate of sodium and' four parts dilute

nitric acid.

8. One part sal ammoniac, one part saltpeter, and one part

water.

9. Five parts sal ammoniac, five parts saltpeter, and sixteen

parts water.

10. Eight parts sulphate, of sodium and five parts concen-

trated sulphuric acid-

D. Negative Varnish.

For a better preservation of the negative, its film may be pro-
tected by a thin and uniform coating of varnish, to be applied
after the film has become thoroughly dried and hardened. The
dry negative should be carefully dusted and held near a fire until ,
it is uniformly warm. It is now balanced on the finger-tips of
the left hand, film side up, and a small portion of the varnish
is poured on the film surface, gradually turning and tipping the
plate to cause the varnish to flow to each comer, covering the
entire plate but not going over the same place twice. The
plate is now kept warm, still holding it in a horizontal position,
under a gently rocking motion, until the varnish has dried with-
out leaving lines or ridges.

A colorless and transparent varnish may be made by dis-
solving one ounce soft copal in ten ounces benzine.

A good negative varnish that will permit of retouching the
negative may be prepared after the following formula:

Metric Weight. Apothecaries' Weight.

10 grammes Amber powder (melted) 150 grains

6 grammes Un vulcanized rubber 90 grains

1750 c.c Chloroform 50 drachms

1750 C.C Benzole 50 drachms

376 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

The proportion of benzol added determines the degree of
'* mat '* that may be imparted to the dry coating of this varnish.

V. Photographic Printing.

Photographic printing is the process of obtaining positive
copies from a negative, on specially prepared paper (or other
sensitized material), by means of light transmitted through the
negative.

The printing process is as follows:

Place the printing-frame, face downward, on a table and
remove the backboard. Lay the negative, which should be
perfectly dry, film side up, in the frame and on this place a
sheet of sensitized paper, face down, in contact with the film.
This paper may be covered with a blotting-paper or piece of
thin felt, after which the back of the frame is replaced and clamped
in position. The frame should now be carefully turned over,
and any spots that may be on the glass surface should be removed
before exposing the negative to the light. These operations may
be conducted in the subdued light of a room when using so-
called '* printing-out " papers, but even this paper should not
be exposed too long to such light and not at all to the direct rays
of any light.

The progress of the printing should be examined occasion-
ally, in subdued light, by opening one side of the backboard
of the frame and raising one end of the paper. The printing
should be a trifle darker than is desired for the final picture,
and darker when toning for blue-black tones than when toning
for warm-brown tones. If not sufficiently printed the back-
board is again closed and carefully latched. Only one side of
the back of the frame should be loosened for examination of
the print to maintain perfect registry between the paper and
the negative.

Weak negatives should be printed in diffused light by covering
the printing-frame with a sheet of tissue-paper (or with several

PHOTOGRAPHIC PRINTING. 377

thicknesses if required). Strong and dense negatives are best
exposed to direct sunlight. It is important to print a shade
deeper than required for the finished picture, as the print will
always bleach somewhat in the toning and fixing process.

Should the whites darken before the shadows become bronzed,
when printing in direct sunlight, it may be taken as an indication
that the negative is too weak for printing in intense light and its
prints should be made in the shade or in diffused light.

Should the shadows be fully bronzed before details in the
high lights appear, when printing in diffused light, it is a sign
that the negative is sufficiently dense to require printing in sun-
light.

To reduce overprinted pictures an old fixing-solution to
which a few drops of a saturated solution of ferrocyanide and
ammonia have been added may be recommended.

Prints sufficiently exposed may be collected in a light-proof
drawer or in a dark receptacle until a number are ready for toning.
There is a large variety of printing-papers in the market for
obtaining positive copies from negatives, and special directions
for their use and manipulation accompany each brand. They
may be divided into two groups. The first, comprising all papers
requiring special developing to bring out the latent image, are
known as develo ping-out papers. These undergo the same
process as a photographic plate after exposure to bring out and
fix the image.

The second group comprises all so-called printing-out
papers (mostly gelatine or collodion papers), the image becoming
visible during the printing.

The papers of the first group (" developing-out " papers)
are mostly very sensitive and the actinic action of the light on
their sensitive silver salt is so rapid that these papers should
be manipulated in the dark-room only. The printing, too, is
best done in the dark-room with artificial light. These papers
have the advantage that they give ready means for obtaining
prints at any time irrespective of the weather, and most of them

378 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

are invaluable for enlarging purposes, enabling the operator to
make prints of almost any size from relatively small negatives.
The so-called bromide, platinum, and platino-bromide papers
are well suited for making direct enlargements from photo-
topographic negatives. Such enlargements are preferably
used in iconometric plotting in place of the small contact
prints.

The papers of the second group are of special value for mak-
ing contact prints in the field; they are less sensitive. to the light
than the " developing-out " papers and only require toning
and fixing after removal from the printing-frame. We find a
large variety of these papers in the market, with special direc-
tions for their manipulation and use.

The older and still popular " printing-out " paper known
as " silver-printing," " albumenized " or " sensitized albumen '*
paper is not as sensitive as most of the modern " printing-out '*
papers. It is coated with albumen and sensitized with a solu-
tion of nitrate of silver.

Shortly before placing this paper in the printing-frame it
should be fumigated with ammonia vapors, thus increasing the
brilliancy of the prints and the sensitiveness of the paper as well
as reducing the time required for the actual printing and facili-
tating the subsequent process of toning by reducing any ten-
dency toward ** blistering " (separation of the sensitized film
from the paper).

The fuming should be done only for immediate use of the
paper, as it impairs the keeping qualities of the sensitized coat-
ing before fixing. Fuming is best done in a wooden box about
six inches or more deep and having a wooden grating supported
about three inches above the bottom of the box. A saucer con-
taining some " stronger water of ammonia " is placed on the
bottom of the box, the grating is placed in position with the albu-
menized paper laid flat upon it, and the box is now closed, expos-
ing the paper for fifteen to thirty minutes to the ammonia vapor.
This operation should be conducted under exclusion of light, and

PHOTOGRAPHIC PRINTING. 379

it is recommended to remove the paper from the box at least five
minutes before it is placed in the printing-frame.

As most prints have an unpleasant reddish tint when they
leave the printing-frame they are generally subjected to the
toning process, which converts the reddish tint into a warm
sepia, a brown or a dark-purple tint, approaching a black color,
according to the formula used for preparing the toning- bath
and dependent on the length of time they were exposed in the
printing-frame.

A. Toning Photographic Prints.

Papers requiring development of the latent image, of course
need no special toning-bath, their pictures appearing under the
action of the developer in soft and warm effects, either in black
and gray or in black and brown tones.

There are many formulas available and many preparations
in the market, both for making separate toning-solutions and
"combined toning-baths," which tone and fix the print at one
immersion. Nearly every brand of printing-out paper is fur-
nished with special directions and formulas for toning, fixing,
and hardening.

All toning-solutions contain besides gold (or platinum) an
alkali (bichromate of soda, borax, carbonate of soda, etc.) to
retard the action of the bath. The more gold the print may
be made to take up, the more the gold deposit will partake of a
ruby color and the more permanent becomes the picture. The
final tone of the picture is conditioned by both the character
and the quantity of the alkali used to neutralize the acidity of
the goJd solution, some alkalies (acetate of soda) giving a brown
to purplish tone, while others (carbonate of soda) produce
t©nes closely approaching a soft black in the deeper shadows
of the picture. For producing good brown to black tones the
following plain gold "separate toning-bath " may be prepared:

Dissolve 1.3 grammes or 20 grains chloride of gold in 570

380 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

c.c. or 20 ounces distilled water and label the bottle " Gold
solution."

As this solution contains one grain of gold per ounce of liquid
one may substitute one ounce of the solution in all formulas for
toning-baths for every grain of gold given in the formula.

Next dissolve one ounce each of acetate of soda and car-
bonate of soda in twenty ounces distilled water, shake the mix-
ture well and filter into a bottle.

To prepare the toning-bath one ounce (30 c.c.) of the " gold
solution " is added to forty-eight ounces (1440 c.c.) distilled
water and this solution is neutralized by gradually adding of
the acetate and carbonate of soda solution till litmus paper no
longer changes color when dipped into this mixture. When
cold tones are desired in the picture just enough of the alkaline
solution should be added that red litmus paper turns blue when
dipped into the liquid. An excess of alkali, however, has a ten-
dency to make the prints appear more toned than they really
are, and such prints undergo a decided bleaching in their sub-
sequent immersion in the fixing-bath.

Ten ounces of this toning mixture will suffice to tone about
eight 5X8 prints. To tone more, either a larger quantity may
be made up at once or more of the gold and alkali solutions
may be added to the old bath. The latter method has the advan-
tage that this toning solution may be used at once, whereas the
freshly prepared bath should be made up about twenty-four
hours before it is really wanted, the freshly prepared solution
working less uniform than an older one.

Should this bath tone unevenly or should the prints come
out streaky, it is advised to make the bath slightly alkaline and
diluted with water. The desired tone should be produced in
six to ten minutes.

The temperature of this bath should be kept rather low,
not to exceed 60° F.

The toning process proper is conducted as follows:

After the final washing of the prints in running water suffi-

PHOTOGRAPHIC PRINTING. 381

ciently long to remove all free silver, of in five or six changes
of water, they are transferred, one at a time and face down-
ward, to the toning-bath. The tray meanwhile should be gently
rocked and the prints kept in motion by transferring the lower
ones to the top singly, keeping this process up to maintain a
layer of liquid between the prints and to remove at the §ame
time any air-bubbles adhering to the film surface, thus assuring
an even toning for all.

Prints first begin to tone on their surface, and if not toned
sufficiently deep, they will turn a reddish brown later in the
fixing-bath. If the original red color appears to have disappeared,
on examining the print through transmitted light, toning may
be stopped.

Prints cannot be toned dark if the printing was not carried
sufliciently far, and it should always be remembered that the
original tone of the print will somewhat fade in the toning- and
fixing-baths. Soon after immersion in the toning-bath, of which
the composition has been given, the prints will change color to
a dark brown, then to purple, and finally to a soft black. As
soon as the prints may have been toned to the desired shade
they are to be removed to clear running water, where they may
remain until enough are ready for the fixing-bath.

If no running water be available the toned prints should
first be placed for about one minute in a saline bath of one
oimce of chloride of sodium (common salt) to sixteen ounces
of water (to stop continued action of toning), to be followed
by a thorough washing in several changes of water before remov-
ing the prints to the fixing-bath.

B. Fixing Photographic Prints.

After the washing following the saline bath the prints are
immersed for fifteen to twenty minutes in the fixing-bath, keep-
ing the prints in motion, the same as described for the toning
process. A good plain fixing-bath may be made up as follows:

382 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

Metric Weight. Apothecaries' Weight.

180 grammes Hyposulphite of soda 6 oz.

75 grammes Alum (powdered crystals) 2.5 oz.

II grammes Sulphite of soda (powered crystals). 3 drachms

2000 c.c Distilled water 70 oz.

When all these ingredients have been dissolved, add to the
solution 25 grammes (6 drachms) borax, dissolved in 300 c.c.
(10 oz.) hot water.

This fixing-bath keeps indefinitely and may be made up
in large quantity. It should be prepared fully a day before use.

Directly after fixing, the picture should be washed, for one
hour in clear running water or in ten to fifteen changes of water,
at intervals of fifteen minutes, using a large tray or tank and
keeping the prints separated so the water may have full access
to the film and leach out all unconverted salts. If there is a ten-
dency toward blistering the first change of water may be made
saline.

To make prints flexible and to rob them of a tendency to
roll up it is recommended to immerse them in the following
solution for a minute or two after removal from the final wash-
ing:

Metric Weight. Apothecaries' Weight.

90 c.c. Glycerine 3 oz.

120 c.c Alcohol 4 oz.

30 c.c Distilled water i oz.

It is advisable to drain the print well, by drawing its back
over the edge of the tray, to remove as much of the surplus liquid
as possible, before placing the print between blotters to dry
under a light pressure.

C. Formulas for Plain Toning-balhs.

For producing deep-purple or bluish-black tones in the final
picture the following plain toning-bath is recommended:

Metric Weight. * Apothecaries' Weight.

65 milligrammes. . . Pure chloride of gold i grain

5 grammes Sulphocyanide of ammonia 80 grains

315 c.c Distilled water 11 oz.

PHOTOGRAPHIC PRINTING. 383

For toning gelatine or collodion prints with platinum in
place of gold, the following bath may be given:

Metric Weight. Apothecaries' Weight.

65 milligrammes. . . Chloroplatinite of potassium i grain

0.5 gramme Chloride of sodium (salt) 8 grains

0.5 gramme Citric acid ^ 8 grains

115 c.c Distilled water 4 oz.

Z>. Combined Toning- and Fixing-baths.

When a " combined toning- and fixing-bath " is used, the
prints, after removal from the printing-frame, require no pre-
vious washing before inmiersion.

A one-solution toning- and fixing-bath may be made up as
follows:

Metric Weight. Apothecaries' Weight. I

65 milligrammes. . . Pure chloride of gold i grain

145 c.c Distilled water 5 oz.

30 grammes Hyix>sulphite of soda i oz.

4 grammes Sulphocyanide of ammonium. ... i drachm

I gramme Acetate of lead 15 grains

I gramme Nitrate of lead 15 grains

This solution should be well shaken before use, and it is best
prepared a day before wanted.

The following two-solution combined bath keeps better (in
separate bottles) than the one-solution bath:

Metric Weight. Apothecaries' Weight.

30 grammes Hyposulphite of soda i oz.

24 gjrammes Alum (powdered crystals) 6 drachms

8 grammes Sugar (granulated) 2 drachms

300 c.c Distilled water (cold) 10 oz.

After these chemicals have, all been dissolved in the cold water,
15 grammes (4 drachms) borax dissolved in 60 c.c. (2 oz.) hot
water are added and the mixture is well shaken. After allow-

384 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

ing it to stand for twelve hours the clear liquid may be siphoned
into a bottle marked " Solution A" or " Fixing-solution."

The stock solution for toning is made by dissolving chloride
of gold and sugar of lead in water.

Metric Weight. Apothecaries' Weight.

65 milligrammes. . . . Pure chloride of gold i grain

0.5 gramme Acetate of lead 8 grains

30 c.c Distilled water i oz.

This '* gold solution " should be well shaken, but not filtered,
before use.

For the " combined toning- and fixing- bath " mix in the pro-
portion of one part " gold solution " (sol. B) to eight parts " fixing-
solution " (solution A).

Half an ounce of stock solution B (" gold solution ") mixed
with four ounces of stock solution A (" fixing-solution ") will
tone about one dozen 5X8 prints.

The double salt " chloride of gold and sodium," which is
a mixture of chloride of gold and chloride of sodium, can be
more easily handled than the pure chloride of gold in making
the toning-solution, since it contains no free acid. If this crys-
tallizable double salt be used in place of the pure chloride of
gold in preparing the " gold solution " of a toning-bath, double
the quantity will be required of the amount given for the pure
chloride of gold in the preceding formulas.

After the desired tone has been attained for the prints in
the combined bath they should be removed to a saline solution
(i teaspoonful of salt to sixteen ounces water) and immersed
five minutes, after which they are washed in ten to twelve changes
of water at interv^als of fifteen minutes.

To insure thorough fixing the prints may be immersed for
ten minutes in the fixing-bath previously given, or in the follow-
ing one, immediately after the first change of water following
the saline bath.

PHOTOGRAPHIC PRINTING. 385

Metric Weight. Apothecaries' Weight.

240 grammes Hyposulphite of soda 8 oz. *

30 grammes Sulphite of soda (granulated, dry) . . i oz.

3.5 c.c Sulphuric acid i drachm

960 c.c Distilled water 32 oz.

After removal from this bath the prints should be thoroughly-
washed in clear running water for at least one hour or in ten
to fifteen changes of water as previously noted. If the prints
be now immersed in the following bath for five minutes any
remaining trace of " hypo " will be removed and the film will
become hard when dried:

Metric Weight. Apothecaries Weight.

960 c.c Distilled water 32 oz.

30 grammes Powdered alum i oz.

30 grammes Powdered chloride of sodium (salt) . i oz.

After removal from this " hardener and short stop " the
prints are again washed and dried.

Combined baths should be used but once and they should be
kept at a rather low temperature, not much over 50® F. ; if the
temperature is allowed to rise much above this the prints will
become stained yellow and the darker tones will be tinged with a
greenish tint. Prints should not be retained in the water over
two hours. All trays should be kept scrupulously clean and
not interchanged. Whenever prints come out " splotchy " it is
recommended to clean the trays, swabbing them out with diluted
sulphuric acid.

Those who prefer to *' cut '' their own gold for the " toning-
solution " can make up a stock solution of the " gold solution B "
in the above combined bath as follows:

Metric Weight. Apothecaries' Weight.

156 centigrammes . . Pure metallic gold 24 grains

3.5 c.c Nitric acid i drachm

10.5 c.c Muriatic acid 3 drachms

386 PHOTOTOPOGRAFHIC METHODS AND mSTRUMENTS.

After the gold has been fully dissolved, add 1440 c.c. (48
ounces) distilled water and then add enough bicarbonate of soda
to leave the solution slightly acid, just enough to turn blue litmus
paper red. After shaking well l&ltcr into a bottle, add 25 grammes
(384 grains) acetate of lead, and label this stock solution " Gold
solution," or " Solution B " of the combined bath.

CHAPTER XII.

CONCLUSION AND REMARKS ON THE PRECISION OF THE
"POLAR-ICONOMETRIC" METHOD AND GENERAL RE-
MARKS ON TELEPHOTOGRAPHY.

L General Remarks on Phototopography.

The main disadvantage in connection with phototopography,
resting principally in the great consumption of time in the pro-
duction of the maps in the office, promises soon to be overcome
through the perfections that are being made in the stereoscopic
methods and instruments. The plotting of from fifteen to thirty
control points, by means of the " polar-iconometric *' method
(by the intersections of at least three radials or horizontal direc-
tions for each control point), including the plotting (or the " ori-
entation ") of the necessary picture traces, together with the
verification of the focal lengths of the photographs, may be regarded
as a good day's work.

The main advantage in phototopography, on the other hand,
rests in the rapidity with which the field work may be done.
The phototopographer, spending most of his time in traversing
the country, stopping only long enough at the stations to photo-
graph the panorama, to make sketches, and to observe a few
sets of angles with the transit, can in a few good da3rs cover a
larger territory than is possible with any other surveying method.

A photo topographic party is essentially an economic one,
inasmuch as it comprises but one topographer, assisted by as
many packers or hands as may be needed to transport the party
outfit over the region that is to be surveyed. The time-con-

337

388 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

sumlng part of the work flhe iconometric plotting) is independent
of weather conditions and may be accomplished at any time
by one or two iconometric draughtsmen in the office.

The ready identification of points on the photographs is a
matter of practice and wiU be found far less difficult than would
appear at a first attempt. All apparent difficulties in this respect
may soon be overcome by a comparative study of several pic-
tures held side by side, and also by making use«of the numerous
tests and constuctions'^fhat are available for this purpose, the
most important of which ha\Tng been given under Prof. G. Hauck's
method. We have also seen that this difficulty disappears alto-
gether when applying the stereophotographic methods.

To economize in time, the general progress of the field work
should, as far as possible, be regulated by the weather and cli-
matic conditions of the region to be sur\eyed. Elevated sta-
tions should be occupied during good weather, as the lower sta-
tions, being more readily accessible and less often obscured by
clouds, may be successfully occupied at almost any time. Good
work may often be done at the lower stations when work on
the mountain peaks i^. impossible, owing to misty weather, snow,
or strong winds prevailing here, whUe the lower altitudes may
be free from either during the same time period.

Special attention should be given to a good selection of the
camera stations, with reference to the elevations and the dis-
tances of the terrene points that are to be determined, to the
focal length of the camera, the desirable degree of accu-
racy, the scale of the map, and the general character of the
country. A diversified and broken terrene will require more
stations to obtain a good topographic development and repre-
sentation on the map than a more regular section; the camera
stations, however, should be selected to obtain a full control
of all depressions, valle\'s, and general topographic features
from the smallest number of camera stations. Every feature
that is to find a representation on the map should have been
photographed from at least two, better three, stations. If a

GENERAL REMARKS ON PHOTOTOPOGRAPHY. 389

part of the terrene be visible from two stations only its icono-
metric location in horizontal plan may be accepted if its con-
trol points on the plan have been determined by good intersec-
tions (if the horizontal lines of direction intersect each other
at angles of 40 to 90 degrees), otherwise " vertical " intersections
or other means for checking the location of these points will
have to be adopted unless additional stations may be occupied,
while the party is still in the field, to obtain lines for a third inter-
section.

On the other hand, to reduce the number of photographic
plates that are to be transported, to simplify the iconometric
office work, and with due regard to the limited length of the
working-season in mountainous regions, it will be advisable
not to occupy more stations than are actually required for the
proper development of the terrene.

To secure the proper control for the location of the camera
stations oh the map, at least three, better four, lines of direc-
tion to surrounding geodetic (triangulation) points should be
observed from each camera station. If that many triangula-
tion points be not visible from the station, that number of direc-
tions should be observed anyway, pointing on other well-defined
points (to supply the deficiency in triangulation points) that
may have been located before (as other camera stations), or
which may be located by later observations to be made at sta-
tions still to be occupied. Every station should be marked
with a signal before leaving, and such signal is to be observed
upon from stations subsequently occupied, observing both hori-
zontal and vertical angles.

Regarding the selection of the hours that are most favorable
for photographing the panorama views, one should be guided
principally by local conditions. Generally speaking, views of
identical regions should, if possible, be taken at the same time
•of day and under similar atmospheric conditions, to facilitate
:lie recognition of identical points on the different views; the
actual shadows will then be alike in the different pictures. Pho-

390 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

tographing slopes altogether in shadow and exposing plates when
the sun is low should be avoided as far as possible. In the latter
case additional trouble may arise from the fact that one or more
pictures taken in the direction toward the sun may be affected
by halation; they will at best be more or less flat and will always
be deficient in details. Still, the phototopographer is seldom
privileged to select the most favorable time for making the expos-
ares, being governed by many considerations, having but a limited
time at his disposal, having to contend with moving cloud-masses,
inaccessibility of points, etc. Sometimes views will have to be
taken toward the sun if they are not to be dispensed with alto-
gether, but with the exercise of care and judgment photographs
may be obtained that will be of value for the iconometric plotting,
even when taken under such adverse conditions. The camera-
lens, however, should always be carefully shaded when taking
pictures under such untoward conditions.

It may generally be stated that the best results in mountainous
countries are obtained when the plates are exposed in the latter
part of the forenoon, the elevated peaks being mostly " hooded "
in clouds during the afternoon. Although these clouds fre-
quently disappear again late in the afternoon or toward evening,
still at this late hour all details of the valleys are obscured if
not perfectly hidden in a misty darkness.

When everything is favorable, the entire work at a camera
station may be finished within an hour and a half, or two hours
at the longest, and as three well-placed stations will control the
horizontal and vertical representation of an extended area, a
large territory may be reconnoitered phototopographically in a
comparatively short time.

The time consumption for accomplishing the field work of a
detailed phototopographic survey will be about the same as
for a more generalized survey, as about the same number of
photographs will be required in both cases. The difference,*
however, appears at once during the execution of the office work.
In the first case the number of points to be plotted iconometrically

^ PRECISION OF THE " POLAR- ICONOMETRIC " METHOD. 391

may be very large, while in the latter case it will naturally be
very small, comprising points which characterize and control
the main features and forms of the terrene only.

We j&nd, therefore, that outside of topographic reconnaissance
surveys in mountainous districts the phototopographic methods
are particularly well adapted for executing topographic pre-
liminary surveys made for that class of engineering works in
which the final and best location of the enterprise depends upon a
comparative study of the different sites as represented on the
topographic maps. Only a limited number of points would
have to be determined iconometrically to reach a decision whether
the site under consideration fulfills the required conditions. After
the best site has been determined upon, a more detailed and
accurate map may be constructed from the same field data
without having to supplement the original survey, either by
additional observations or photographs, every panorama view
giving the means to plot therefrom (iconometrically) almost an
unlimited number of terrene points.

n. Precision of the ** Polar-iconometric " Method.

The desired degree of accuracy in a survey will generally
determine the class of instruments and the methods to be used
in its execution^ To ascribe, therefore, the various surveying-
cameras and phototheodolites their proper places among survey-
ing-instruments, it will be of importance to know, or to ascertain,
what degree of precision may be obtainable with each repre-
sentative type of a special class. This has been done for some
of the special types that have been described in the preceding
chapters, and we will here enter upon a more general considera-
tion of the precision attainable in the so-called " polar " or
" radial " method of iconometric plotting.

We have seen that the graphic methods of phototopography

392 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

are very similar to those of the plane table. It is generally
accepted that azimuthal errors in the directions of the " radials,"
drawn with the plane-table alidade on the plane-table sheet>
should not exceed 1.5 minutes in arc, and we shall see that when
the principal focal length of the canj^era does not fall below 1 50 mm.
this same degree of accuracy in the angiilar values of the lines of
direction may be obtained iconometrically from the panorama
views.

The plotting from photographic perspectives being dependent
on the measurements of coordinates made directly on the pho-
tographic perspectives, the attainable degree of accuracy will
greatly depend both upon a good definition and upon the mathe-
matically correct representation in perspective of the landscape
upon the flat field of the perspective (negative).

The precision in the mechanical determination of the coor-
dinates of any point pictured in the photographic perspective
depends not only upon the more or less good definition of the pic-
tured point, but also upon the means used for measuring these
coordinates. According to Dr. Meydenbaur, the definition of a
photograph obtained with a suitable lens . will be sufficiently
good for phototopographic purposes if a point, or rather its
" phase,'' or the circle of diffused light that represents the point on
the picture, does not exceed o.i mm. in diameter; hence all pic-
tured lengths should be obtainable within a limit of dx^dy^o.i
nun. Unless special measuring devices are employed, this
value will also represent the attainable degree of accuracy in
making direct measurements on clear and well-defined nega-
tives with ordinary drawing instruments (dividers and trans-
verse scale). It is evident from the foregoing that a computa-
tion carried out to several places of decimals (analytic method)
cannot increase the accuracy of the resulting map as long as the
elements of the perspectives upon which such computations
are based have been obtained with a degree of accuracy not
closer than o.i mm. All iconometric plotting being dependent
on direct measurements executed on the photographic plates,

PRECISION OF THE " POLAR-ICONOMETRIC " METHOD. 393

the transcription of the pictured data into the horizontal pro-
jection plane is best done graphically.

Numerous experiments have shown that 0.14 mm. is the
smallest discernible difference in length that the average eye
may distinguish without optical aids. With a beveled scale
graduated to 0.5 mm. a fairly well-trained eye can determine
lengths correctly within o.i nun., while a well-trained eye reaches
the limit at 0.06 mm. By the use of special scales fitted with
verniers and microscopes the attainable degree of accuracy in
the measured lengths may be increased to reach 0.03 mm.

The attainable degree of precision in the angles may be found
from the equations

tan oc=^jz (for horizontal angles),

tan/? =—7^= (for vertical angles),

where :v= abscissa of the pictured point;
y= ordinate of the pictured point;
Z>= distance line of the perspective*

By differentiation we find

dtdLna=j:dx,

dy ^-y

(/tan p=--^^d===-—^==dx.

If we express x by an aliquot part of Z>, :x7=— , and disregard

-=:) ="2 and introduce the arcs in place of the tangents of the

394 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

angles, we will find

dx
da (in seconds of arc) « 206265-^;

dy
dp (in seconds of arc) =206265^7-.

These equations show that the angular errors are directly
proportional to the degree of accuracy attainable in the measured
lengths (on the photographs) and indirectly proportional to the
focal lengths of the lenses that are used. Assuming the attaina-
ble degree of accuracy in the measured lengths to be (te=rfy=o.i
mm., we will find the attainable degree of accuracy for the
angles for the following five focal lengths to be:

For a focal length = 20 cm. the angular accuracy is i' 43"

ti II n _2C ** " '* *' '* l' 2C"

c< (( tt — ^Q <« «< << «< ** l' 00"

ti ti it —^c *' ** ** ** *' o' ^o"

a it a s=4o *' ** ** ** ** o' C^"

The longer the focal length of the lens the smaller the angu-
lar error wiU be, and although it is desirable to have the photo-
graphic details as large as possible for a better identification
of the terrene points on different photographs and to increase
the attainable degree of accuracy, still, to reduce the weight of
the instrument as much as possible the focal length will natu-
rally be circumscribed for instruments to be used in mountainous
regions, where portability and compactness are among the prime
factors to be considered in their construction.

The attainable degree of accuracy for any particular camera
may be ascertained experimentally, after the methods of Dr. W.
Jordan and Capt. E. Devillc, by observing a series of horizontal
and vertical angles, included between lines of directions, to a
sencs of well-defined points of known positions and elevations.

METHOD. 395

taking also a photograph of the same points (in vertical plane)
from the same station and from the same elevation. The hori-
zontal optical axis of the camera should have the same height
as the horizontal optical axis of the transit telescope when the
angles were measured, or the difference in the elevation between
both should be taken into account.

The focal length of the photographic perspective and the
correct positions of its horizon line and principal point may now
be determined from the requisite number of observed directions
to the knpwn and plotted points. The remaining angles, meas-
ured in excess of the required number just referred to, may
well be used for comparison with the corresponding values,
obtained iconometrically from the oriented picture trace, to
arrive at a knowledge of the attainable degree of accuracy of
the camera in question.

If the camera-lens was of good quality (for surveying pur-
poses), if the selected points were well defined (both in nature
and on the negative), if the sensitized surface of the plate con-
tained no gross inequalities or irregularities, and finally, if the
measurements of the coordinates were carefully made on the
negative, the lengths obtained iconometrically on the plotting-
sheet, compared with those obtained trigonometrically, should
differ by no more than o.i mm. in actual length, and the icono-
metric angles should differ from those that were observed by
no more than from i to 2 minutes in arc for objectives with
focal lengths from 350 to 150 mm. and commanding a hori-
zontal field of view from 50 to 60 degrees.

This degree of precision may be increased by a reduction
in the field of view of the lens by using plate glass for the nega-
tives and by making microscopical measurements of the coor-
dinates of the pictured points. Still, when photography is applied
in this way, including precise computations (analytic method),
for surveying purposes, one of its main advantages is lost sight
of and sacrificed, since one of the chief advantages in photo-
topography rests in the numerous and varied constructions (based

39^ PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

on the laws of perspective) that are available in transfemng
the photographically recorded data to the plotting-sheet.

The attainable degree of accuracy in the elevations obtained
iconometrically may also be ascertained in the manner indicated,
but the results will only hold good for points having the same
distances as those observed upon with the transit from the camera
station.

To find the limit in the distances that are to be included
for a certain lens when a given degree of accuracy in the ele-
vations is to be maintained, we may proceed in the following
manner:

We have the known relation

a a—/' a—/'

where a = distance between object and second nodal point of
the lens;
ft = distance between image and second nodal plane of

the lens;
/= constant focal length of the camera-lens.
We had seen .that the elevation (5) of any pictured point
above the horizon line stands in the same relation to the ele-
tion {A) of the point itself, above the horizontal plane that passes
through the horizon line, as the horizontal distance (6) between
lens and image is to the horizontal distance {a) between the
lens and the horizontal projection of the point itself.

A a-f

PRECISION OF THE " POLAR- ICONOMETRIC " METHOD. 397

In topographic surveys the distance a will be so great compared
with / that the latter may be neglected in comparison with a,
and we will have, with close approximation,

^ = -, or a (approx.) = -g- .

Hence, whenever the considered distances a are large com-
pared with the focal length, we may place 6=/, which means
that the phototopographic cameras may be constructed with
constant focal lengths, which in point of fact is the general
practice.

If differences in the elevations of points of the terrene are
to be deduced photogrammetrically within a limit of error not
exceeding one meter, the pictured length (B) of a meter in nature
(A) should not appear shorter than o.i mm., and we find for
the following four typical values of focal lengths the correspond-
ing values for a, representing the extreme distance limit between
the camera station and an object one meter high that is to have
a pictured height of at least o.i mm. as follows:

a= 750 m. 1000 m. 2000 m. 2500 m.

for )= 75 mm. 100 mm. 2cx> mm. 250 mm.

It would require the exercise of special care and the measure-
ments of the coordinates would have to be made directly on
the negatives if this limit of error is to be maintained. Care-
fully made contact prints permit measurements to be taken
(with the ordinary instruments of the draughtsman) correctly,
with an average error of not less than 0.25 mm., and taking
this limit as commensurate with good work, we will now have:

a=»30om. 4cx> m. 800 m. 1000 m.

for /« 75 mm. . 100 mm. 200 mm. 250 mm. .

These values clearly demonstrate the necessity for a close
disDOsition of the camera stations over the area to be surveyed

398 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

(from 600 to 2000 m. apart) for detailed work, thus materially
circumscribing the advantages of phototopography, practically
excluding its application to the topographic surveys of rugged
mountains, a terrene for which the phototopographic methods
are best suited and to which, as a matter of fact, they have pri-
marily been applied with the greatest success as long as larger
errors (exceeding one meter) in the elevations were permissible.
Thus, if the error in the iconometrically determined elevations
may attain twenty meters, and if the coordinate measures may
be correctly obtained on the prints within an error of 0.25 mm.,
we will find the following values for the effective range for the
same four focal lengths to be:

a» 6000 m. 8000 m. 16000 m. 20000 m.
for /= 75 mm. 100 mm. 200 mm. 250 mm.

For a permissible error in the elevations of twenty meters
the camera stations may be located at intervals of from 10,000
to 40,000 meters, for cameras with focal lengths from 75 to 250
mm., provided the character of the terrene does not require a
closer disposition of the stations to obtain a better development
of the intervening terrene forms.

Attempts have been made to increase the effective range of
the surveying-cameras by constructing them with variable or
adjustable focal lengths; this, however, produces complications
and opens additional sources of error. Better success in this
direction has been obtained indirectly by using photographic
enlargements (" optical projections ") of the original negatives
in the iconometric plotting. This method is successfully pursued
in Canada under Capt. E. Deville, Surveyor-general of Dominion
Lands, who advises the use of enlarged positives on glass for work
requiring a high degree of accuracy.

If the reconnaissance of a given terrene has established the
greatest limit between the camera stations to be 2a, we can,

PRECISION OF THE " POLAR- ICONOMETRIC " METHOD. 399

for a required degree of accuracy, find the proper focal length
of the camera from the equation

, B

The accuracy attainable in iconometric plotting depends
greatly upon the distances between the several camera stations.
We had seen that the precision of a plotted line of direction
depends upon the accuracy with which the abscissa x may
be transferred from the photographic perspective to the plotted
picture trace. The camera station being fixed, the accuracy
in the direction of the ray will depend solely upon the value
or the amount of the error dx with which the measured abscissa
X may be affected. With a given limit dx for a constant
focal length /, the error in position of a plotted point will increase
w^ith the distance of the latter from the camera station. All
plotted points falling between the picture trace and the plotted
camera station are affected by no errors larger than dXy and if
the permissible limit of error in the map is not to exceed dx,
we will have to select the camera stations sufficiently close together
that no points are determined iconometrically which fall beyond
the traces of the pictures from stations whence the latter were
obtained. The base-line lengths are plotted to scale, while the
constant focal length of the camera enters into the iconometric
construction in its original length; hence the reduced lengths
separating successive camera stations on the plan should not
surpass the true focal length /. For a scale of map of i/n
the largest base line should be Sf'^f measured in millimeters.
For a focal length of / = 20o mm. and a scale of map of 1/20000
the base line should not exceed 200 X 20000 mm. or 4000 meters,
measured in the plotting-scale.

All elevations determined from the photographs should be
corrected for curvature and refraction. Refraction apparently
affects directions more in the vertical than in the horizontal

400 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

sense. Prof. S. Finsterwalder found in his phototopographic
surveys made in 1888 and 1889 that the accuracy in the ele-
vations increases directly with the distance of the observed point.
The elevation of a point 500 m. or less distant from the camera
station was three times less accurately obtainable, iconometric-
ally, than the elevations of points between 2500 to 5000 m. dis-
tant.

Generally speaking, terrene points determined iconometrically
will not be provided with signals, and the identification of a well-
defined point on several photographs may be affected by an
error of from one to two minutes in arc. Even points that may
have been supplied with signal poles of the ordinary size and
length, when five hundred meters and more distant from the
camera stations, will appear on the photographs as having no
signals, yet when viewed through the telescope of the ordinary
field transit, the same poles may appear very clearly and well
defined, even at distances up to several kilometers. Artificial
signals will, therefore, be of little use in general iconometric
plotting; still, as well-defined points are a necessity to insure
good results, the camera stations should be located not too far
apart, and thp selected reference points of the pictures should
not only be sharply defined, but the instrumental measurements
to the same objects (to provide the needed data for the orienta-
tion of the picture traces and for the control) should be made
as nearly as possible at the time of the exposure of the plates,
that such points may be seen under the same conditions of illu-
mination that prevailed when they were photographed.

Regarding the expeditiousness of the phototopograghic
methods considered in the preceding chapters, it may be stated,
from the experience of Dr. S. Finsterwalder, that so-called
topographic surveys of mountain regions, for which an artistic
representation of the terrene, in conformity to its natural appear-
ance, may be claimed rather than accuracy, may be made by
an expert plane-tabler, combining liberal sketching with the
instrumental survey, on 1/25000 scale in less time than it would

PRECISION OF THE " POLAR- ICONOMETRIC " METHOD. 4OI

take the phototopographer to select, locate, and occupy the
camera stations required for an " accurate " phototopographic
survey to be plotted on i/ioooo scale. This holds good for
surveys in mountains of an Alpine character aijd sparse vegetation.

Besides the errors considered in the preceding paragraphs,
there still remains another source of error to be considered in
the photographic developing and fixing process of both the nega-
tives and their positives. Distortion in the sensitized gelatine
coating of the modem dry-plate during the process of develop-
ment is rarely perceptible if the work is carefully done to avoid
so-called " frilling " of the film. The mean value of such dis-
tortion, according to Dr. H. C. Vogel, amounts to o.oi per centum
of the length, and as the plates used in phototopography are
never large, the errors due to this cause may be disregarded
altogether.

The distortion in the positives (particularly if made on paper
that requires subsequent development), however, is mostly so
large that it must be considered when using such prints
iconometrically. This distortion, moreover, is irregular, being
smaller* in the direction with the grain or fibers of the paper,
where it may amount to 0.5 per centum of the length, and larger
in the direction across the fibers, where it will amount to about
I per centum. The constants of the camera, therefore, and
the coordinates of the principal points of control should pref-
erably be obtained from the negatives. For the iconometric
plotting of the topographic details enlarged projections on bromide
paper may be used. Capt. Deville has recently substituted a
heavily coated " platino-bromide " paper for the ordinary silver
bromide paper heretofore in use. The length of exposure for
the enlargement is made directly dependent upon the density
of the original negative.

To give ready means for controlling or correcting the dis-
tortion affecting the paper prints nearly all modern surveying-
cameras are provided with a metal frame permanently fixed in
the image plane of the lens with constant focal length, the inner

402 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

margins of the frame having a graduation that is photographically
reproduced on the outer margins of the negative; thus the amount
of distortion that may aflfect the positive can readily be ascer-
tained in the directions of both the horizon and the principal
line of the photographic perspective. " Backing " the paper
prints would open another source of error even greater than
those just referred to. Dr. Stolze observed a permanent expan-
sion of five per centum for prints that had been mounted while
in a damp condition.

ni. General Remarks on Telephotography or Long-distance

Photography.

The range in the field of application of photography to sur-
veying and military reconnaissance has been considerably enlarged
during the past few years by the invention of the telephoto-
lens combination, used for obtaining well-defined photographic
views of objects at long distances, sacrificing or reducing the
angular value that the plain lens commands for the sake of
enlargement of the view, producing thereby the same effect as
if the view had been taken from a point of view much nearer
to the distant object.

Long-distance photography (" telephotography ") was proba-
bly first studied in France, principally by Matthieu and Lacombe,
and more recently by Guillemont and Jarret. This subject
continues to receive much attention in France, particularly among
the army officers stationed at Grenoble, as has been mentioned
in Chapter I. Quite recently telephoto instruments have been
devised and placed upon the general market by Hondaide
and Derogy in Paris. The lunette d'Etat-Major, one of the
smaller types of telephoto instruments, manufactured by Arizard
in Paris, controls distances up to 5 km. and weighs only about
8 kg.

The Intelligence Office of the British War Department is
also doing a great deal towards promoting the efficiency of the

GENERAL REMARKS ON TELEPHOTOGRAPHY. 403

telephoto instruments and towards familiarizing British officers
with the telephotographic reconnaissance methods. The inven-
tion of the telephotographic lens combination is ascribed by
Th. R. Dallmeyer to Peter Bariow, who combined a negative
lens with the astronomical telescope as early as 1834.

Researches in long-distance photography have notably been
made by T. R. Dallmeyer, London; Dr. A. Miethe, Potsdam;
Dr. Steinheil, Munich; Prof. R. Spitaler, Vienna, and others.

One of the main defects in phototopography rests in the small
scale to which the distant landscape features are reduced on
the negative, requiring precise and minute measurements to be
made on the negative in connection with the iconometric con-
structions. This defect is primarily conditioned by reducing
the weight of the surveying-cameras to a minimum. Cameras
with constant focal lengths are principally in use for topographic
sur\Tys in mountainous regions, where the reduction in weight,
as previously stated, means a great deal toward ultimate success.
For this class of work the use of objectives of long focal
lengths is precluded and we find topographic-surveying cameras
supplied with lenses having constant focal lengths from 75 to
350 mm.

In the preceding chapter the effective ranges of four-lens
tj'pes have been discussed, fully demonstrating the desirability
of providing means for obtaining special perspective views of
terrene sections lying beyond the reach of the ordinary camera-
lens, for certain inaccessible localities and particularly for mili-
tary reconnoitering purposes.

By adding the so-called telephoto attachment to the original
camera-lens an enlarged image of the view is photographed
directly on the plate (in the field). . The use of the telephoto
attachment (it may easily be removed) has the advantage that
the selection of the distant views rests entirely with the topog-
rapher in the field, as he can best decide whether by taking
such a telephotographic view from one of the ordinary camera
stations a lengthy trip of the party in that direction may be saved.

404 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

or whether a special advantage may be derived from a series
of such views taken during the occupation of some prominent
or isolated peak. In short, the phototopographer can best tell
whether time may be saved by. supplementing the ordinary pano-
rama views with some special long-distance views.

In high altitudes mists and clouds frequently hide the higher
peaks from view for weeks at a time, and it may often save many
days of waiting if the phototopographer be provided with a
telephoto attachment for his camera-lens, to enable him to photo-
graph distant terrene features that may casually be visible on
a clear day while taking the panorama views for the develop-
ment of the topography in the immediate neighborhood of the
station.

The topographer may not see the same peaks free from " cloud -
hoods " again during the rest of the season, at least not from
that particular direction. It will seldom require more than one
such telephoto view in several panorama sets, the critical points
pictured on the other plates being near enough to the station
not to need special enlarging.

Little regarding the telephotographic results, obtained prin-
cipally under military auspices, reached the general public
until Dr. A. Miethe, in Germany, and T. R. Dallmeyer, in Eng-
land, each apparently independent of the other, published de-
scriptions of their telcphoto-lens combinations. The principal
difference between their combinations seems to be that Dall-
meyer uses a " portrait-lens " in connection with the " nega-
tive-lens combination," while Dr. Miethe combines a photo-
graphic lens of the " rapid landscape type " (Steinheirs) with
the negative combination. The construction of both these
telephoto objectives rests upon the same principles, and their
combination is composed of two biconvex lenses interposed
between the camera-lens and the sensitive plate. By changing
the distance between the biconvex lenses more or less enlarged
images will be photographed on the plate. These enlarge-
ments arc made at a sacrifice of the field of view commanded

GENERAL REMARKS ON TELEPHOTOGRAPHY. 405

by the camera-lens alone, and it would require a large number
of plates to cover the entire horizon with telephoto views; still,
in phototopography the general panorama views will be taken
with the simple camera-lens, adding the telephoto attachment
only for special views of distant " heads," or saddles of valleys,
inacessible mountain peaks, etc.

Since the " negative element " of the telephoto combmation
lens produces a picture of the distant view in the image plane
of the camera with a sharp definition and a richness in detail
far surpassing the perceptive power of the eye, and, on the other
hand, the wide angle type of camera-lens (the " positive ele-
ment ") reproduces, with an evenly good definition, views sub-
tending angles far surpassing the field of view of the eye, it will
be plain that the combination of these two elements into one
optical system will give results that cannot be obtained in any
other manner. The telephoto combination works particularly
well for picturing objects that are about equidistant from the
lens (for picturing objects that are in the same frontal plane),
but when objects are photographed that are at different dis-
tances from the instrument the image will be more or less dis-
torted by the effects of spherical aberration. This spherical
aberration may be reduced in a measure by stopping down
the camera-lens; this, of course, decreases the intensity of the
illumination of the image, reduces the rapidity of both the lens
and the plate, contracts the already small field subtended by
the telephoto combination, and consequently necessitates an
increase in the time of exposure, with the incident risk of com-
municating tremors to the camera which would be detrimental
to the definition of details in the telephoto-plate.

Dallmeyer recommends, therefore, for a " really useful tele-
photographic lens system" the combination of the rapid recti-

[F F\
linear type of lens (^ or - ), together with a negative lens of

half its focus. In this proportion the latter may be made of
larger diameter than the lenses of the positive element, thereby

4o6 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

increasing both the included angle and the illumination of the
plate, as when using a negative lens of smaller diameter than
that of the positive element. With observance of this recom-
mended proportion of the foci the negative element (attached
to the positive lens) will not become inconveniently large and
heavy. Negative lenses with foci a little longer than half the
focus of the positive set may safely be used to increase the field
of view (the included angle) at a reduction in the magnification
of the image.

When using a telephoto-lens tl^e instrument should be well
and rigidly supported, as the smallest tremors of the camera
become magnified in their effect upon the image and probably
would spoil the plate for phototopographic purposes. The
focusing should be very carefully made, using the same stop
that is to be used for the subsequent exposure. Generally speak-
i.ig, the best results will be obtained on a calm day after a rain
or when the atmosphere is perfectly free from smoke and dust.
To obtain good results for distant mountain views, of course
a yellow color-screen or ray-filter will have to be provided. It
is also of great importance that the photographic plate be in
perfect contact with the metal frame of the camera, as even a
slight irregularity in this respect would spoil the negative for
iconometric purposes.

Telephotographic cameras combined with phototopographic
methods no doubt will play an important part in modem wars
and maneuvers where smokeless powder will be in general use,
to reveal the positions of hostile troops and enable the observing
officers to obtain a correct idea of the positions of the enemy,
and consequently of his plans (inasmuch as these may be
deduced from the recognized disposition of the forces in the
field). Light transportable ** conning towers" supplied with
'* telescoping *' or extension tubes and prism reflectors will
probably form the most satisfactory support for the telephoto-
lens combination when used for strategic reconnoitering pur-
poses. For use in the army a telescoping-tube system could

GENERAL REMARKS ON TELEPHOTOGRAPHY. 407

be devised to serve in place of the center pole of the dark-
tent.

The following are some of the better-known publications on
long-distance photography and telephoto-lens attachments:

T. R. Dallmeyer. "The Telephotographic Lens," published by J. H. Dall-*

meyer. London, 1892.
T. R. Dallmeyer, F.R.A.S., etc. "Telephotography: an Elementary

Treatise on the Construction and Application of the Telephotographic

Lens." London, 1899.'
M. le capitaine du G^nie Bouttreaux. "Mtooire sur la td^photographie."

Nro. de Sept., 1897, Revue du Gdnie.
M^moire du chef de bataillon Allotte de la Fuye, commandant TEcole du

G^nie de Grenoble. "Sur I'Emploi des appareils photographiques pour

les observations k grande et k petite distances." Autographic k I'Ecole

du GCnie de Grenoble, 1891.
Aug. Vautier-Dufour. "Sur la TdlCphotographie." Bulletin de la

SociCtd Vaudoise des Sciences natxuielles. No. 143. Lausanne, 1902.
NicoDEMO Jadanza. "II Teleobbietivo e la sua Storia." Tonno, Carlo

Clausen, 1899, Estratto dalle Memorie della Academia delle Scienze di

Torino, Serie II, T. XLIX.
Max Loehr, Chef de la maison C. A. Steinheil, Paris. "Du Td^bjectif."

Annexe du bulletin de la SociCtC frah^aise de Photographic, 1902.
Max Loehr. "Sur la DCtennination des M&ures du Td^bjectif." Bulle-
tin de la SociCtd franyaise de Photographie, 1902.
Dr. P. Rudolph. "Anleitung zum Gebrauch des Teleobjectifs von der

Firma Carl Zeiss." Jena, 1896.
C. A. Steinheil. "Ueber Femphotographie." Phot. Correspondenz,

1892.
Dr. A. Miethe. "Photographische Optik."
Lechner's Mitthcilungen, Feb. 1892.
Mittheilungen aus dem Gebiete des Seewesens, 1894.
Revue du Cercle Militaire, 1895.
The American Annual of Photography, 1896.
Zeitschrift fuer Vermessungswesen, 1892.
Anthony's Photographic Bulletin, 1892 to 1894, etc.

INDEX.

PAOB

Abbe , Prof. 178

Aberration 176

archomatic , . . 163

chromatic 180

spherical 163

Abruzzi Mountains 24

Academy of Sciences, Madrid 28

Paris 7

Academy, Royal Building, Berlin 14

U. S. Military, West Point 4

Swedish Military 22

Accampamento Reale 89, 103, 106

Accelerating development 357

Accelerator 357, 361

Acetate of lead 383, 384

soda 383

Acetic acid 355

Achromatic aberration 163

lens 179, 181

Acid, acetic , 355

carbonic 353

citric 357,383

hydrochloric. 362, 372

muriatic 353, 385

nitric 385

oxalic 352

pyrogallic 358

sulphuric 355, 361, 367, 385

sulphurous. ... 366, 373

409

4IO INDEX.

PAOB

Ackerblom, Ph 22, 23

Actinic action of light 338

power of light. • . . > 346

Aerial perspective 337

Alaska, southeastern 197

Alaskan boundary 31

Commissioners 31, 32, 135, 197

Phototopographic Reconnaissance 3i» '35

Albumenized paper 378

Alcohol 188, 362, 382

Alidade, Danish plane-table 242

Alidade Holom^trique 260

Alkaline solution 350, 351, 358

Alophe ' 8

Alpenstocks 212, 222

Almeria 28

Altazimuth 195

Alum 357, 367, 382, 383, 385

Aluminum camera-box 186, 189, 199, 201

Amber powder 375

Amidol 366

Amidol developer 366

Ammonia-bath • 378

Ammonium chloride 370

Amrein, Prof 23

Anastigmat lens 164, 222, 226, r 29, 239, 243, 247, 25^, 342

Antiplanatic lens (Steinheil's) 212

Antihalation plates 342

Andreas, F. C 15

Aperture 166, 167, 176, 177, 215

Aplanatic lens (Steinheil's) 183, 222

Apua 25

Arago iv, 6

Archer, Scott iv

Architectural surveys vii

Argentic chloride iii, »v, 372

Arizard 402

Artificial horizon 188, 193

Assistant topographer 187, 193, 195, 196

Axis, optical 202, 206

perspective 65, 66

INDEX. 411

PAGB

Azimutale fotografico 224, 227

Azimuth camera 226

compass ". 203, 223, 227, 266

photographic determination of 224, 225

Backing plates 342

prints 402

Balbreck 255

Barrel-shape distortion 177

Bart Glacier 24

Base, stereophotogrammetric 323, 326

Batuty Arthur 11

Becker, Prof 20, 23

Beautemps-Beaupr6 5, 6, 223

Bennati, L 27

Benzine 375

Benzole 375

Bemhardi, Capt 12

Bcrthaud, Col 11

Bcrtelli, Gi\iseppe 27

Bessel 6i

Bicarbonate of soda 360, 386

Bichloride of mercury 296, 370

Bichromate of potassium 362, 372

sodium 379

Biconcave lens 165, 173, 174

Biconvex lens 165. 168-173, ^79

Bildmesskunst 13, 47

Binocular microscope 318, 321

telescope 309

Blistering 378, 382

Bisulphide of sodiimi 365

Blue haze 337

Bluemke, Dr. A 15

Board, photograph 137, 138, 148

Bock, Major 16, 17

Boettcher, Prof. 368, 372

Bohnenberger 61

Borax 379, 382

412 INDEX.

PAGB

Bomecque, J ii

Bouttr^u, Capt ii

Brasset Frferes 253

Bridges-Lee 33, 262-269

Brit sh Columbia boundary 31

Broken telescope 260

Bromide of potassium iv, 355, 356, 359, 361, 364, 365, 371

silver v

paper 365, 378

Bromo-hydrochinon developer 364

Brown stains 373

Brunner 7, 28

Buc 28

Bunsen 346

Buonome, Giacome 27

Bureau, Swiss Topographic 23

Canadian Topographic 135

Bush, E 128, 183

Calotype iv

Camera-box, aluminum 186, 199

Camera clara v, 6

crosswires 202, 213, 265, 266

diaphragms 186, 205, 259 342

frame graduation 233

levels 184-192, 197, 200, 203, 205, 233

obscura 6, 42

plates, size of (Bridges-Lee) 268

(Ney) 253

(Vallot Frferes) 255

(von Huebl) 253

stations, distances between 3PQ

select'on of 388

surveying 182

(Bridges-Lee's) 262-269

(DeviUe's) 186-196

(DoergensO 184

(Finsterwalder's) 227-230

(Hafiferi's) 242-243

INDEX. 413

PAGB

Camera surveying (Italian) 211, 219-227

(Koppe's) 234-241

(Laussedat's) 244

(Le Bon's) 184

(Lechner's) 184

(Meydenl>aur*s) 185, 186

(Ney's) 231-234

(Pollack's) 241-242

(U. S. Coast and Geodetic Survey's) 196-200

(Vallot's) 253-262

(VogePs) 184

(Werner's) 184

telescope (of Lechner) * 244

(of Paganini) 219, 220, 229

(of Starke and Rammerer) 245, 249

Canadian Department of the Interior 29, 31, 132

Government Printing Office 132

Pacific Railroad 29

phototopographic method ; 132

outfit. . . ; 193

surveys 340

Topographic Surveys Office 135

Carbonate of soda 358, 359, 361, 363-365, 379

ammonia 352

potassium 361, 362, 365

Carbonic acid 353

Carbutt's color-screen 340

orthochromatic films 255

Casella, L. P 262

Caucasus 209

Caustic soda 361, 368

Cazes, L ii

Center, geometrical 165

Centered lenses 164, 165

Central projection 167

Centro linead * 290

Cbamounix base 262

Chemical focus 180

Chevallier, A 8, 15, 28

Chicago Exposition 8, 29

Chloroform 375

414 INDEX.

PAGB

Chloride of ammonium 370

gold 382-384

and sodium 384

platinum 373

, potassium 372, 375

silver iii, iv, 372

sodium 381, 383, 385

Chloroplatinite of potassium 383

Chromatic aberration (secondary) 178-181

Chrome alum 367

Chromium salts 372

Cian del Lei 89, ic6

Citric acid ' 357, 383

Clearing solution 357, 366, 369

Cloud hoods 390

Coast pilot work, Italian 223

Cogne Valley 25

Collinear lens (Voigtlaender's) 183

Collodion iodide iv

Color-screens 187, 198, 205, 339-341

(of Bausch and Lomb) 341

(Carbutt's) 340

(Dallmeyer's) 341

(orange) 187, 340

(yellow) 198, 340, 341

Colson, Capt. R 11, 44

Combined toning- and fixing-bath 379, 383, 384, 386

Commissioners, Alaskan Boundary 3^i 3^i i35> 197

Comparative exposures 343

diagram > 345, 346

table 345

light values 343

Compass, azimuth 203, 223, 227, 266

dial 226, 244

Dixey 203

Schmalkalder 203

Concave-convex lens 165

Concluded points 196

Congress, IX. Geographical, Vienna 26

Conjugate foci 171-174

planes 171-173

INDEX. 415

PAGB

Conjugate points 174

Conservatoire des Arts et Metiers, Paris 8

Constant focal length 183, 185-186, 204

Contouring, iconometric 126, 151, 157

Convex-concave lens 165

Copal , 375

Copper sulphate 371

Cross levek 235

Cross-wires 213, 214, 221, 229, 265, 266

Crown glass 180

Crystallization of fihns 36S

water 350

Curvature 214, 399

Cyanide of potassium 371

silver , 370

Cyanine 339, 340

Cylindrograph, topographic 271-274

D'Abbadie 8

Daguerre iv

Daguerreot)rpe iv

Dalhneyer, T. R 403-407

Dalhneyer's rapid rectilinear lens 183

wide-angle lens 340

Danish plane-table alidade 242

Dark-room 377

Dark -tent v

Daussy 7, 10

Davy iii

Definition, error in 215, 21 7

of bisected points 229

De Geer, Prof. G 22

De Iriarte, C 28

De La Fuye, A *. 11

Denison's orangie tissue 349

Denkmaeler Archiv 14

Density of a negative 336

Department of the Interior, Canadian 29, 31, 132

Derogy, G 402

4l6 INDEX.

PAGB

Developer 353-366

amidol 366

bromo-hydrochinon 364

eikonogen 366

eiko-cum-hydro 365

ferrous oxalate 356

hydrochinon 361, 362

metol 360

(bicarbonate of) soda 360

hydrochinon 363

pyro 358

Developing agents 350

Deville, Capt. E viii, 11, 29, 30, S3y 132, I35> 136, 186, 197, 199, 228,

3<^» 342, 353» 355, 394, 398, 401

Deville's photographic surveying method 132

De Zea, Lieut.-Col. Dom Pedro 28

Diagram of comparative exposures 345, 346

Dial compass 226, 244

Diaphragms of cameras 186, 259, 342

' lenses 166, 167, 176, 177, 201-205, 212, 215, 344

Diffused light 337

Dispersions 178, 179

Distance line 47, 51-55, 190

Distance between camera stations. 399

Distilled water 352

Distortion, barrel-shape 163, 177

optical 163

pin-cushion 163, 1 77

radial 164

tangential 164

in backed prints 402

negatives 401

prints 401

produced by diaphragms 177

Dixey compass 203

DjamS^ht 15

Doergens, Dr. R viii, 12, 13, 15, 17, 184

Doergens' camera 184

Dolezal, Prof. E 19, 22

Dolland 178

Dominion land surveyors (Canadian) 155

INDE3C. 41?

PAGB

Dom Panunce 6

Douglas-Archibald, E 114

Driffield, V. C 353

Drouin, F 11

Drying negatives 369

Dry plates 335, 336, 339

Dry-plate process ' v

Dubosque 8

Ducretfit, E 244

Duffield, Gen. W. W 32

Dumas, Fl 11

Eccentric center of gravity 229

Eccentric telescope 238

Eclimfetre 260

Eckholm, Nils 22

Eder, Dr. J. W 18, 19, 340, 355

Edwards, B. J 33

Edwards' isochromatic plates 340

EfiFect of color-rays, qualitative 336

quantitative 336

light, actinic 338

optical ^ 338

Eikonogen '. 264, 265

developer 366

Eiko-cum-hydro developer 365

Elements of a photographic perspective 119

Elevations, determination of 80, 88, 95, 97, 119, 125, 132, 145, 147

Emergent ray .* 166, 176

Emery 7

Engineering surveys vii

Eosine 339

Eritrea 88, 200

Error in definition * 215, 217

iconometric angular measures 393*395

stereoscopic base 328

stereotelemeter readings 315

swing of stereoscopic plates 328

Erythrocine 339

41 8 INDEX.

PACK

Exposition, Paris 7

Chicago 8, 29

Exposure, length of 177, 336

correctly timed 336

overtimed 336

undertimed 336

Exposures, comparative 343-346

test 346-348

Eyepiece, Ramsden 220, 221, 239

Fahrlaender, Col 23

Fa verges 7

Fenner 26

Ferrero, Gen •. 24

Ferricyanide of potassium 362, 377

Ferrous oxalate 353

Fihns, Carbutt's 258

crystallized 365

Finsterwalder, Dr. S 15, 17, 20, 27, 157, 227, 228, 245, 400

Finscerwalder's phototheodolite 227-230

Five-point problem 55*59

Fixed focus 183, 185, 186

Fixing-bath 365-368, 377, 3^4

Fixing and toning-baths 379, 382-384, 386

prints 381

Flafe spots 166

Flexible prints 382

Fhnt glass 180, 181

local length of pin-hole camera 44

constant 183, 185, 186

to compute 89, 90, 92, 98, 102, 104, 107, no, 112, 128, 129,

215, 217, 228, 233, 236

lengths 161, 170-175, 183-186, 189, 191

planes 168-175

scales 209, 217, 220, 247

variation 180

Foci, conjugate. 1 71-174

Focus, chemical. . .1 180

optical 180

INDEX. 419

PAGB

Focus, principal 168

visual ." 180

Foerster, Dr 15

Fourcade, T 8

Fraunhofer lines 338, 341

Freiburg 14

French ministry of culture 184

Fribourg, Commandant 8

FriUing 351, 367, 369

Fritsch, G 19

Front pl?ines 37,40,310

Fuming-box 378

Galileo 25, 224

Garibaldi 7

Gassr Dachel 14

Gautier-Prandl prism 313

Gay Lussac iv

Gelatine emulsion v

General staff, Prussian 12

Swedish 22

Swiss 23

G^nie corps, French 258, 271

Geographic latitude, photographic 224, 225

Geological surveys vii

Geometric center of lens f65

Gimbal support for camera 225

Girard, J 11, 16

Glass, crown 180

flint 180, 181

Jena 178

Gleaves, Albert 33

Glycerine 382

Goerz, anastigmat of 183, 187, 199, 201, 227, 229, 239

Gold 385

bank-envelope paper 349

solution 379-389

Golfarelli, Prof. 1 27

Goulier, Col. Th 254, 260

420 INDEX.

PAGB

Gran Sasso 24

Graphic hypsometer , 284

protractor 275-27S

sector 278-283

Grenoble 9

Grimsimski, R 20

Grousillier, H 309

Guenther, Oscar 238

Guillemont 9, 402

Gum guaiacum 188

HafiPerl, F 16, 19, 26, 107-110

Hagstrom, H. L 22

Halation 166, 342

Halo 166

Haloid, silver 335-354

Hamberg, Dr. A 22

Hanot, Alfred i r

Hardener and short -stop 385

Hartl, Lieut 16

Hauck, Dr. G 15, 17, 62, 63, 80, 81, 139, 388

Hecker, Dr. 20

Heights, scale of. 147

Heine, L 20

Heliography iv

Heifer, Prof. J 17

Helmholtz 312

Herring, E 20

Hess, Dr. H 15

Heun, Dr. K 21

High School, Technical, Berlin / 14, 63

Prague 16

Hildebrandson, H. H 21, 22

Hintze 13

Hondaide 9, 402

Hondaille, Capt 11

Hood for lens % 186, 194

Horn, Photographische Mittheilungen 13

Horizon, artificial t88, 193

INDEX. 421

PAGB

Horizon line 185-193, 197, 206

Horizontal angles 230

contours 126, 151, 157

intersections 115, 117, 124, 131, 140, 149

Huebl, Baron von 243, 250

plane-table photogrammeter of. 950-253

Hurler, F 353

Hydrochinon 361-365

developer. '. 361, 362

Hydrographic surveys vii

Hydrometer tests 350

Hyposulphite of sodium 357, 365, 367, 370, 373, 382, 383, 385

tests 368

Iconometers 275

Iconometric angular errors 393-395

contouring 126, 151, 157

plotting 47, 49, 126, 151, 157, 215, 307

rangp 396-39^

Iconometry 47> 49

Identification of pictured points 138, 139

Image plane 170, 202

plate. '. 215

stereoscopic 319

Imfeld 19

Incident ray 166-176

Inclined photographs 76, 78-80

Index-mark of stereocomparator. 319, 321, 324, 326

Index, refractive 159, 161, 178, 179

Institute, Military Geographic, Italy 112

Vienna 15

Photogranunetric, Berlin 14

Polytechnic, Milan 24

Intensification 369, 370

Intensity of light 205, 346

Interocular distance 309, 317

Intersections, horizontal 115, 117, 124, 131, 140, 149

vertical 134, 142, 144, 3^9

Iodide of potassium 370

422 INDEX.

PAGE

Iodine, tincture of. 369

Iron developer 356

perchloride 373

sulphate 355-357

Isochromatlc dry plates 339

Italian Military Geographic Institute 112

phototopographic method 88

J-

Janet 9, 402

Javary, Capt 7, 10

Jena 178, 309

glass 258

Jesse, 21

Jewell, L. E , 362

Johnson 13

Jordan, Dr. W 14, 16, 83-86, 394

Jouart, A 8, 10

Jungfrau 23, 238

Kennett v

Kernel planes 64

points 63-<58, 304

Kempunkte 63, 64

Kerschensteiner, Dr 15

Kinberg, Major H 22

King, Dr. W. F viii, 31, 135

KIotz,O.J 19,33

Kobsa, R 18

Konistka, Dr. Karl 15

Koppe, Dr. C 15, 17, 234, 238

Krauss 258

Krifka, Col. Otto 27

I^combe 8, 402

Lambert, F. C 43, 44

INDEX. 423

PAGB

Lambert, J. H 5, 35

Lampblack ' 188

Laska, W 20

Latent image 334, 336, 343

Latitude, photographic determination of 224, 225

Laugier 7, 10

Laussedat, Col. A v, viii, 4, 6, 12, 15, 27, 28, S3y 34, 12;, i3o» 253, 298, 308

Leather plate-holder 228

Le Blanc, Capt 6

Le Bon, Dr. G 10, 86-88, 184

Le Bon*s camera 184

Lechner, R 250

Lechner's camera 184, 244

Le Conte, Prof 35

Le Comu, J n

Le Gros, Commandant v, 156

Lejeune, L 244

Length, focal, computation of 89-92, 98, 102, 104, 107, no, 112, 128, 129

Length of exposure 117

stereophotogrammetric base 326

Lens, anastigmatic, Zeiss' 183, 187, 199, 201

aplanatic, Steinheil's 183

antiplanatic, Steinheil's *. 212

biconcave 165-174

biconvex , 165-173

concave-convex 165

convex-concave 165

coUinear, Voigtlaender's 183

combination 166

diaphragms 166-177, 201-212, 215, 344

double anastigmat of Goerz 183

doublet 215

hood 194

negative. 165, 173, 174, 405-406

pantoscopic, of E. Busch 183

periscopic 165

positive 165-170

principal axis of 165

rapid rectilinear, Dallmeyer's 183

stops , 166-177, 201-212, 215, 344

vertices of a i5r

424 INDEX,

PAGB

Lenses .'. 159, 161-163

centered * 164

Liesegang, Prof. R. E 371

Light intensity 205

Light-rays, sources of 337

Light intensity, table 346

Light values, comparative 343

Lime 352

Line, distance 47, 51, 55

front 38

horizon 51-54

principal 38, 51-54

vanishing 41

Literature, Austrian 16

English ^^

French 10

German 16

Italian 27

Spanish 28

Swedish 22

Swiss 23

Loehr, Max u

Long-distance photography .' 402

Lowisoij, Major 22

Liunifere's orthochromatic plates 255

Lundal, A 22

Lunette d'Etat Major 402

Lynn Canal 32

MacArthur, J. J 29, 31, 3^

Maddox, Dr v

Magnesia 325

Magnetic azimuth camera 226

Magnification of the stereotelemeter 313

Mallmann 340

Mandl, Julius 21

Manzi, Michele 24

Marselli Capt. Carlo 27

Martens 8

INDEX. 425

PACB

Matthieu 8, 402

Maurer, M 16

Measuring distorted negatives 238

Mendenhall, Dr. T. C 32

Meteorological observations vii

Method, phototopographic 82

Finsterwalder's 157

French 121, 130

German 83-86, 128

HaflFerl's 107, no

Hauck's 63, 80, 81, 139

Italian 88-115

Jordan^s 83-86

Laussedat's 121, 130

Le Bon's 86-88

Legros' 156

Meydenbanr's 128

Paganini's 88-151

of squares ; 72, 294

Methods, topographic 2

Metol developer 359, 360, 363, 373

hydrochinon developer 363

soda developer 360

Meydenbaur, Dr. A 12, 14-16, 185, 186, 392

camera 185, 186

Miethe, Dr. A 19, 403, 404

Mikiewicz, Lieut. L 16, 17

Military Academy, U. S., West Point 4

Geographic Institute of Italy 112

Austria 15

Ministry of Culture, French 184

Moessard, Conmiandant P 10, 163, 271-274

Monet, E 11

Mont Blanc 9, 253, 258, 261

Mont Cenis 24

Mueller, Wilhehn 250

Muriatic acid 353, 385

426 INDEX.

PAGR

Nadar 8

Nathorst, Prof. A. G 22

Navarro, L 28

Negative 45

lens. 173, 174, 405, 406

Negatives, to dry 369

measure distorted 238

intensify 369, 370

reduce 373

varnish 375

stained 370, 373

distortion in 401

Neuhauss, Dr 228

Ney's phototheodolite 231, 234

Niepce, J. N iv, 6

Nitrate of ammonia 374

lead 383

silver iv, 371

sodium 374

Nitric acid 385

Nodal planes 165, 170, 174, 175

points 165-168

Nonactinic paper 349

Nonhalation plates 34a

Oasis Dachel 14

Obemetter 340

Objective scale 201, 209, 217-220, 247

Observatory at Upsala, meteorological 21

Observing angles from negatives 236-241

Ocular lens of telescopic camera .*. 219

Oetz Valley 15

Olson, K. P 22

Optical axis 202-206

center .v 165, 166

distortion 163

focus 180

INDEX. 427

PAGE

Optical lens 161. 162

sensitizers 339

Oreo Valley 25

Orienting picture traces. . . 47-49i Si-54, 62, 78, 92-94, 99, 124, 130, 137, 226

Orthochromatic plates 339

Orthogonal projection 210

Ott, Max 227

Ottawa 31, 132. 195

Overexposure 348, 356, 363, 364

Oxalate of iron 353

potash 355

Oxalic acid 352

Pacific Ocean 196

Packing-case, light-tight 235

Binini, L. P viii, 3, 22, 25, 26, 88, 260-209, 211-227, '44

ini's camera 200-209, 212

Panoramic cameras 270-274

Pantograph arms 306

Parallax 311, 318, 321, 326, 330

Paris, map of 7

Pat^, E 8, 10

Peep-sights 260

Perchloride of iron 373

Period of reversal 336

Permanganate of potash 368

Perspective axis 65, 67

elements 35, 37,^19

photographic 36-42

Perspectographs 275, 298-302, 303-307

Perspectometer, Deville's 294-297

Phosphate of soda 352, 375

Photogenic drawings iv

Photogrammeters 182

Photog ammetric plane table of von Huebl 250-252

Photogrammetry and phototopography 13, 387

Photograph board, Deville's 137, 138, 148, 290-294

protractor 154

Photographic azimuth compass 223-227

428 INDEX..

PAGE

Photographic detennination of magnetic azimuth 224. 225

outfit. Canadian 193

perspectives 36-42

plates, inclined. . 76 78-80

printing 376

Photometrography 13

Phototach^mMre 253

Phototheodolite of Bridges-Lee 262-269

Capt. E. Deville 186-188

Dr. S. Finsterwalder 227-230

Dr. C. Koppe 234-241

Col. A. Laussedat 244

R. Lechner 243

L. P. Paganini 211, 219

O. Ney. . 231-234

V. Pollack 241- 242

Pollack and Hafferl 242-243

Starke and Kanmierer 245-250

the U. S. Coast and Geodetic Survey 196-200

Vallot Frferes 253-262

Phototopographic plane table of Chevallier 270

Picture traces 47-49, Si-54, 62, 78, 92-94, 99, 124, 130, 137, 327

Pietsch, Dr 14-16

Pie-y-AUu^, Don Juan 28

Pin-cushion distortion , 177

Pin-hole -camera constants 44

exposures 43

cameras 42

diameter 42

photography 42

Pizzighelli, Major 16, 17

Planchette photographique 8, 28, 270

Plane, conjugate 171-173

datum 37

focal 168-175

front 37, 40, 310

ground 37

horizon 37

image 170, 202

kernel 63-68

nodal 165, 170, 174, 175

INDEX. 429

PAGB

Plane, principal 37* 3^

table photogrammeter of von Huebl 250-253

visual 39

Plate carrier 186

holder 186, 193, 194

Plates, overexposed 348, 356, 363, 364

size of, Bridges-Lee's 268

Finsterwalder's 227

French 245

Huebrs 253

Italian 201, 202, 222, 225, 226

Meydenbaur's 185

Ney's 233

U. S. Coast and Geodetic Survey's 199

. Vallot's 255

stereoscopic 31

test 346-348

underexposed 349, 363

verticality of 100-102

Platino-bromide paper 136, 378, 401

Platinum 379, 383

Plotting contours 126, 151, 157

iconometric 49, 115, 117, 124, 131, 140^ 149, 215

picture-traces 47-5i» 54, 62, 78, 92-94, 99, 124, 130, 137, 327

shore lines 126, 151, 157

stations 55

Pneumatic shutter 226. 234

Point, distance 39

kernel 63-^

nodal 3^3^f 165-168

principal 38, 47, 51-55, 190, 202

reference 93, 134, 139, 140, 152

vanishing 39> 4i

Pointer, station 60

Points, concluded 196

conjugate 174

iconometric plotting of 115, 117, 124, 131, 140, 149

identification of 138, 139

trigonometric 218, 229

Polar iconometric method 387, 391

precision of 391

43© INDEX.

PAr.K

Pollack, V i6, i8, 27, 243

Porro, Prof 24, 223, 232, 30^

Porta, G. della 6

Portland Canal 32

Positive lens 170-173

Positives 45

Potassium bromide 355, 356, 359, 361, 364, 365, 371

chloride .' . . . 37^

cyanide 371

Pothenot 60

Prague. 15

Precision of polar-iconometric method 391

Pribilof Islands. . . .' 3a

Principal axis of a lens 165

foci of a lens 16S

line of a perspective 38, 51-54, 185-193, 197, 206

planes 168

point 38, 47, 51-55, 190, 202

ray - 38

Printing by development 377

frame 376

out papers 376, 377

photographic 376

Prints, distortion in 401

distorted through backing 402^

flexible 382

"splotchy" , 385

Problem, five-point 55^59

three-point 59-62

Protractor, photograph 154

three-arm 60

Prussian Topographic Bureau 319

Prussiate of potash 352

Pujo, Th 8

Pulfrich, Dr. C 20, 21, 308, 314

Projection, central 36, 167

orthogonal 36, 40, 48, 21a

outward 36

Punta Bivula 113

delP Erbetet 114

delP Invergnan 114

INDEX.

431

PAGE

Punta del Lei 89, 106

di Breuil 114

di Nomenon 95» 96, 100-106, 114

di Toss 114

Gran Paradiso 113

Percia 95» 99> ioo» "4

Rouletta 95> 9^) 99^ looj ii4

Ruja 91, 103, 105, 106

Pyro developer 358

lyogallic acid ("Pyro") 358

Pyro-ammonia 373

-metol 373

-soda 373

9-
Quidde 13

Radial method 387, 391

precision of 391

Ramsden eyepiece 26, 220, 221, 239

Randhagen, F 236

Rangie-finder, stereoscopic 309

Range of stereoscopic vision 312

Rapidity of lens 176, 177

Rapid rectilinear lens, Dallmeyer's 183

Ray, emergent 166-176

incident 166-176

Ray-filters 187, 198, 205, 339-341

Rax, principal 38, 51-54, 185-193, 197, 206

Reducer 373

Reed, Lieut. Henry A 4

Reference points 93, 134, 139, 140, 152, 195

Refraction ^ 160, 178, 179, 214, 338, 399

Refractive index ' 159, 161, 167, 170, 179, 180, 188

Regnault 294

Reichel 232

Relief efiFect 310

Retina of the eye 180, 219

432 INDEX.

PAOB

Restminer 356, 361

Rhymes t 95, 113, 114

Rhodamine 339» 34^

Rholf, G 14

Ringertz, Major N. C 22

Ritter, Herman 17

Rocky Mountain Park 29, 30

Rockwood-Schallenberger panoramic camera 271

Roscoe 346

Rosen, Prof 22

Rosenmund, M 23

Ross, Thos 28

Rousson, H. 11

Rubber 375

Ruby light 349

Sal ammoniac 375

Salmairaghi 24

Salt (chloride of sodium) iv, 381, 383, 385

Saltpeter 375

Santa Cruz Island 5

Scale, objective 209, 217, 220, 247

of heights 147

stereotelemeter 317

transverse, aerial 310

vanishing 74, 75

Schallenberger 271

Scheele iii

Schell, Prof 16

Schepp, A 25

Schmalkalder compass 203

Schott, Dr. A 178

Schiffner, Prof. F 16, 19, 20, 56, 303

Schreiner ; • 13

Schroeder, Dr 15, 17, 18

Schroeder & Co 298

Schumann 340

Schwassmann, A 20

Scolik 340

INDEX. 433

PAGB

Scott's exposure table 345

Screens, color 187, 198, 205

test 163, 164

Secondary chromatic aberration 178

Sector, graphic 278-283

ordinary 146

Seeing, stereoscopically 314

visual 35

Selecting camera stations ^8S

Seliger, P 319

Sensitizer, optical 339

Sensitized albumen paper 378

Sensitometer number 205, 336

Settone Grafico 278-283

Shipping-box for plates 195

Shore line, to plot 68-71

Short-stop and hardener 385

Silver bromide v

chloride of iii, iv, 372

cyanide of 370

haloids, reduction of 335, 354

nitrate iv, 371

printing-paper 378

Simon, S 23

Size of photographic dry plates, Bridges-Lee's 268

Finsterwalder's 227

French 245

HuebPs 253

Italian 201, 202, 222, 225, 226

Meydenbaur's 185

Ney's 233

U. S. Coast and Geodetic Survey's '199

Vallot's 255

Snellius 60

Society for Natural Research, Munich 309

Sodium carbonate 358, 359, 361, 363-365

Sodium phosphate 352

Southeast Alaska 197, 340

Spectrum, solar iii, 337-339, 34i

Speed of photographic plates ^^

^irit-levels for camera 184-192, 197, 203, 205

434 INDEX.

PAGB

Spitaler, Dr. A 403

Spitzbergen 22

Splotchy prints 385

Sprung, A 20

Squadro Grafico 284-290

Squares, method of 72

Stained negatives 370, 373

Stanley 33

Starch 368

Starke, G 19

Starke and Kanunerer 254, 249

Station, foot of ^S

plotting 55

pointer 60

Stegemann's camera 183

Steiner, Prof. F 16, 18, 54, 55

Steinem, Baron von 209

Steinheil, Dr 183, 212, 222, 403, 404

Steinheil's antiplanatic lens 212

aplanatic lens 183, 222

Stereocomparator 308

Stereophotognunmetry 308, 318

Stereoplanigraphs 308

Stereoscopic power 309

surveying 308

telemeter 308, 318

Stolze, Dr 15, 17

Strachy, R ^3

Strassburg 13

Sugar 383

of lead (acetate of lead) S^S^S'^A

Sulphate of copper 371

iron 355-357

sodium 375

Sulphite of sodium 358-367, 382, 385

Sulphocyanate of potassium 374

Sulphocyanide of ammonium 382, ^S^

Sulphuric acid 355, 361, 367

Superintendent U. S. Coast and Geodetic Survey, Report of 34

Surveying-cameras, requirements 183

Surveyor-General Dominion lands of Canada 13a

INDEX. 435

PAOB

Siurveying, phototopograi^c, in Alaska 29

Austria 15

Canada 29

France 5-9

Germany 12

Italy 24

Spain 27

Sweden 21

Switzerland 23

Survey's Office, Canadian Topographic 135

Sutton 28

Swiss Topographic Bureau 236

Table salt (chloride of sodium) iv, 381, 383, 385

Tachfemhre 253

Talbot, Fox iv

Talbotype iv

Tannin solution 353

Tasmania 5

Technical High School, Berlin 14, 63, 302

Braunschweig 234

Teeth, camera. 214

Telemeter, stereoscopic 313, 318

errors in reading the stereoscopic 315

Teleobjective 259, 402, 403-405

Telephotography 402

Telescope, "broken" 260

Telescopic camera, Italian 220

Lechner's 244

ocular lens 219

of Starke and Kammerer 245, 249

diaphragm 219, 229

Temperature for toning-bath 380, 385

Test exposures ^ 346-348

plates for stereoscopic vision 314

photographic 334, 346-348

screens, photogmphic 163, 164

Testing water 352

Tests for "hypo" 368

436 INDEX.

Tetrachlor-tetiaethyl-rhodamine-chlorhydrate 340

Theodolite of Col. Goulier 254, 260

Three-arm protractor 60

Three-point problem S9j 62

Tiflis 209

Tincture of iodine 369

Tissandier, G 12

Toning process 379-381

Toning- and fixing-baths, combined 379, 383, 384, 386

separate (or plain) 379, 382, ^83

Topographic Bureau of Switzerland 236

cylindrograph, Moessard's 271-274

Surveys Branch of Canada 135

Topography, definition of i

Tourniquet 165

Transit 187, 188, 195, 198-

theodolite 231

Transverse scale 310

Trial plates 334, 346-34S

Tribrach (triangular instrument base) 187, 198, 263

Trikolograph, Dr. Hauck's 302-306

Trilinear systems 63

Tripods for cameras 185, 187, 193, 197, 204, 211, 222

stereotelemeters 313

Underdevelopment 349

Underexposed plates 349^ 363

Uniform diaphragm system 344

U. S. Alaskan Boundary Conmiissioner 197

U. S. Coast and Geodetic Survey 25, 31, 32, 34, 185, 196-198, 243,

340, 342
Upsala Meteorological Observatory 21

Valenta, Dr. E j^

Vallot,H ^253

Vallot,J 9,253

Valsavaranche Valley 25, 91, 113, .114

INDEX. 437

PAOB

Valsoana Valley 25

Van Diemensland (Tasmania) 5

Vanishing points 39~4i

scale 74-75

Variation, focal 180

Varnish for negatives 375

Vautier-Dufour, A 12

Verein, Oesterreichischer Ingenieur iind Architecten 27

Vemach glacier 15

Vemer, C. W 35

Vertical angles 230

intersections 134, 142, 144, 389

Verticality of plates 100-102

Vertices of a lens 165

Vienna i6

Vision, range of stereoscopic • 312

Visual focus 180

Vogel, Dr. H. C 401

Vogel, Dr. H. W i3i i5, 1 7, 184

VogeFs camera 184

Voigtlaender's coUinear lens 185

von Alster, Gen 12

von Guttenberg, A. K. 20

von Huebl, Baron 16, 19, 20

von Moltke, Coimt 12

von Steinem, Baron 209

von Stemeck, Col 26

von Ziegler, Ch Z2

Volkmer. 19

Wallon, E 12

Wang, F 19

War, Franco-Prussian 12, 13

Water tests 352

Wedgewood iii

Wei^t of phototheodolite, Finsterwalder's 227

Laussedat's 245

Paganini's 200

Vallot*s 261

438 INDEX.

PAOB

Weight of phototopographic magazine camera, Meydenbaur's 185

photogrammetric plane table, von HuebPs 253

stereotelemeters 313

Wenz, E la

Werner 243

Werner's camera 184

Westman, J 2a

Wet-plate process iv, v

Wheatstone stereoscope 308

Whipple, G. M 33

Wide angle lens 340

Wiganowsky 8

Woodbury antipyr. 374

Wollaston 6, 298

YeUow color-screens. 198, 340, 341

stains '. 370, 373

Zeiss' anastigmat lens. 164, 183, 187, 199, aoi, 222, 226, 227, 242, 243, 247, 258

Zeiss, Carl ; 178^ 309

Zeitschiift fuer Instrumentenkimde. 230

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