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Full text of "The optics of photography and photographic lenses"

I-NRLF 




or PHOTOGRAPHY 

AND PHOTOGRAPHIC LENSES 





J.TRAILL TAYLOR. 




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THE OPTICS OF PHOTOGRAPHY 



AND 



PHOTOGRAPHIC LENSES. 



GEOMETRICAL OPTICS. An Ele- 

mentary Treatise upon the Theory and its Practical 
Application to the more exact Measurement of Optical 
Properties. By T. H. BLAKESLEY, M.A., F.Phy.S. 
With 32 Diagrams, zs. (>d. net. 

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of the student. Mr. Blakesley has, moreover, effected a 
notable advance in geometrical optical theory.' Nature. 

LENS WORK FOR AMATEURS. By 

H. ORFORD. With numerous Illustrations. y. 

' The book is a trustworthy guide to the manufacturer 
of lenses, suitable alike for the amateur and the young 
workman . ' Natu re. 

'The author is both a sound practical optician and 
is able to convey his knowledge to others in a clear 
manner.' British Journal of Photography. 

MODERN OPTICAL INSTRUMENTS. 

By the same Author. With 88 Illustrations. 2s. 6d. 

CONTENTS : The Eye as an Optical Instrument 
Properties and Aberrations of Lenses Aberrations of the 
Eye Examination of the Eye The Ophthalmoscope 
Ophthalmoscopes and their Uses The Morton Ophthal- 
moscope Various Forms of Ophthalmoscopes Retino- 
scopy Spectacles and their Selection Various Forms 
oi Spectacles Illustrated and Described Stereoscopic 
Projections Anderton's System Principles of the 
Optical Lantern The Stereoscope The Spectroscope. 

' To those of our readers who wish to inquire into the 
elements of optical instrument construction and the 
principles involved therein, we can cordially recommend 
the little book.' Photographic News. 



WHITTAKER & CO., LONDON, E.C 



THE 



OPTICS OF PHOTOGRAPHY 



AND 



PHOTOGRAPHIC LENSES 



BY 



J. TRAILL TAYLOR 



3fllUj3tration0, 



THIRD EDITION (REVISED). 

WITH AN ADDITIONAL CHAPTER ON ANASTIGMATIC LENSES BY 

P. F. EVERITT. 



WHITTAKER & CO. 

2 WHITE HART STREET, PATERNOSTER SQUARE, LONDON, E.G., 
AND 66 FIFTH AVENUE, NEW YORK. 

1904 



/ / / L// rl 

Ifl/b * 1 1 / 

/ 

QPTOMETflY 




753 



PREFACE 



THIS little treatise is not theoretical but practical, 
and it is not intended for the makers but the users of 
photographic lenses. 

Some of it is already familiar to readers of The 
British Journal of Photography and its Almanac, and 
such portions are reproduced by the kind permission 
of the Proprietor ; while other portions are collated 
from my contributions to the Society of Arts, The 
Photographic Times, the Camera Club, and various 
other London and Provincial Societies. There are, 
however, several chapters written expressly for this 
work, while in every case the other matter has been 
entirely revised or re -written and brought up to 
date. 

If it be said that there are innumerable lenses in 
commerce which are not even mentioned by name in 
this volume, I reply that each maker has his idio- 
syncrasy he may vary the diameters, foci, and curves 
of his productions, and select special trade terms by 



vi PREFACE. 

which to distinguish them, but I have preferred in all 
cases to associate each class of lens with the name of 
its first inventor, and believe that no lens in use at 
the present day has been omitted. 

It is in the hope that the work will prove useful 
to photographers both professionals and amateurs 
that it is issued. 

J. TRAILL TAYLOR. 



PREFACE TO THIRD EDITION. 

ADVANTAGE has been taken of the demand for a 
new edition to thoroughly revise the work and bring 
it up to date by including the recent period of anastig- 
matic construction. This has been done by omitting 
the short chapter on Lenses of Jena Glass, written by 
the late Mr. J. Traill Taylor, and inserting in its place 
one on Anastigmatic Lenses, specially written for this 
book by Mr. P. F. Everitt. 



CONTENTS. 



CHAP. PAGE 

I. WHAT CONSTITUTES PHOTOGRAPHIC OPTICS NATURE 

AND PROPERTIES OF LIGHT I 

II. PHOTOGRAPHIC DEFINITION, REAL AND IDEAL FORMS 

OF SINGLE AND ACHROMATIC LENSES ... 7 

III. THE CAUSE OF AN INVERTED IMAGE .... 12 

IV. SPHERICAL ABERRATION 1 5 

V. THE NATURE AND FUNCTION OF THE DIAPHRAGM OR 

STOP 20 

VI. PROPERTIES OF DEEP MENISCUS LENSES COMPEN- 
SATING SINGLE LENSES 27 

VII. THE OPTICAL CENTRE OF SINGLE LENSES ... 34 

VIII. THE OPTICAL OR FOCAL CENTRE OF A COMBINATION . 38 

IX. SINGLE ACHROMATIC LENSES 42 

X. DISTORTION : ITS NATURE AND CURE .... 49 

XI. NON-DISTORTING LENSES . . . . . .58 

XII. WIDE-ANGLE NON-DISTORTING LENSES ... 65 

XIII. PORTRAIT LENSES 73 

XIV. RAPID LANDSCAPE, GROUP, AND COPYING LENSES . 80 
XV. UNIVERSAL LANDSCAPE LENSES 87 

XVI, FLARE ANP THE FLARE SPQT ,,,,,, 93 



viii CONTENTS. 

CHAP. 

XVII. THE EQUIVALENT FOCUS IOO 

XVIII. CONJUGATE FOCI Io6 

XIX. THE PRINCIPLE OF CONJUGATE FOCI APPLIED TO 

HAND CAMERAS AND FOR ENLARGEMENT . .112 

XX. A MECHANICAL MEANS OF ESTIMATING CONJU- 
GATE FOCI Il6 

XXI. DEPTH OF FOCUS 123 

XXII. DIFFUSION OF FOCUS 128 

XXIII. TESTING LENSES 135 

XXIV. THE SHAPE OF THE APERTURE IN THE DIAPHRAGM 147 

XXV. EQUALISING THE ILLUMINATION OF SUBJECTS-- 
SKIES AND FOREGROUNDS 150 

XXVI. ADJUSTING DISSIMILAR LENSES . . . .154 
XXVII. THE DETERIORATION OF LENSES BY LIGHT . .158 
XXVIII. HOW TO ASCERTAIN THE ANGLE OF VIEW IN- 
CLUDED BY ANY LENS 165 

XXIX. REFINED FOCUSSING BY MEANS OF A TELESCOPE . 1 70 

XXX. ANASTIGMATIC LENSES . . . . - -177 

XXXI. MOUNTS AND CELLS .... 207 

XXXII. LENS GRINDING 214 

XXXIII. OPTICAL CONTACT CEMENTING LENSES . .224 

XXXIV. SELECTION OF LENSES 230 

XXXV. ON THE CURE OF EXISTING DISTORTION . . 237 

XXXVI. LANTERN OPTICS ENLARGING AND PROJECTING . 241 

XXXVII. PHOTO-TELESCOPIC LENSES 2 = 8 

XXXVIII. EXCEPTIONAL RAPIDITY WITH HIGH DEFINITION . 260 

XXXIX. MISCELLANEOUS . , 262 



THE OPTICS OF PHOTOGRAPHY 

AND 

PHOTOGRAPHIC LENSES 



CHAPTER I. 

WHAT CONSTITUTES PHOTOGRAPHIC OPTICS NATURE 
AND PROPERTIES OF LIGHT. 

PREVIOUS to speaking of the lenses employed in 
photography, or the principles which underlie their 
construction, it will be necessary to explain what we 
mean by the term, the Optics of Photography, as con- 
tradistinguished from the optics of any other science, 
such as those which involve the use of the microscope 
or telescope. 

The chief distinction lies in this: that in photographic 
optics, not only must those rays which are transmitted 
directly through the lens, or the axial rays, as they are 
designated, be brought to a focus, but also those which 
pass obliquely, or in a direction other than axial. The 
principal lenses, or object-glass, of a telescope or micro- 



I CHEMICAL AND VISUAL FOCI. 

scope will not give a sharp image if removed in even a 
slight degree from perfect squareness of position in 
relation to the line of light. Hence, the sharpness of 
image produced by even the finest telescope object-glass 
is confined to a very small space in the centre, the rest 
of the image being indistinct, owing to the inability of 
an objective of this class to form a sharp image of an 
object, the light from which is transmitted obliquely. 

In photographic optics, on the other hand, the con- 
struction of the lens must be such as not only to give a 
sharp image of the object to which it is directed, but 
also of those which lie within a certain extent on either 
side of the centre. In proportion as a lens embraces 
objects situated at a considerable distance from the 
point to which it is directed, so does such lens become 
entitled to the designation of being a { wide-angle ' lens. 

But, further, the chief end of any optical instrument, 
such as the telescope or microscope, formed for visual 
examination, has been attained when it is made to 
produce an image that is sharp when examined with the 
eye. But with a photographic lens something more is 
required. The corrections of the lens must recognise 
the absolute necessity of all the chemical rays being 
brought to a focus at the same spot as the visible rays, 
so that not only will the image appear sharp to the eye, 
but it will be equally sharp when, as the result of the 
action of the chemical rays, it is developed upon the 
photographic plate. Such coincidence of the visible 
and chemical focus does not exist either in the telescope 
or microscope, but only in the photographic lens. 



CONCERNING LIGHT. 3 

The optics of photography, therefore, takes cog- 
nisance of rays transmitted obliquely as well as axially, 
and of bringing both the chemical and visual rays to a 
focus on the same plane. 

This paves the way for a consideration of the 
principles upon which the various classes of lenses are 
constructed. 

Concerning Light. As a fitting introduction to the 
subject of lenses, it is necessary that an explanatory 
remark be made on light. Without entering upon this 
abstruse topic, it is enough for our present purpose to 
observe that the undulatory theory of light is now 
generally accepted. This assumes light to be a certain 
result of setting in motion the ether which pervades all 
space, and owing to that motion we see objects upon 
which such ether waves fall. 

But the functions of light are not confined to 
rendering objects visible ; they also include heating 
and chemical action, or actinism. These three properties 
of lighting, heating, and actinism may be very easily 
demonstrated by the following simple experiment : 
Cover up a south window by an opaque screen, allowing 
the sun's rays to be admitted only through a small 
aperture. Now intercept the rays thus admitted by a 
prism, so as to have them spread out upon a sheet of 
white paper, and observe the gorgeous spectacle these 
rays then present. The beam of white sunlight is 
decomposed into its primary constituents, as shown 
in the diagram (Fig. i), in which B represents the 
aperture through which the beam of light is admitted ; 



PROPERTIES OF COLOURED LIGHT. 



and which beam, but for the interposition of the prism I' 
would, without deviating from its straight path, fall at 
W. But the prism bends the ray, and decomposes il 




W 



FIG. I. 



into the primary and secondary colours indicated by the 
initial letter of the spectrum, the violet ray v having 
been bent or refracted in a greater degree than the red 
ray R, from which circumstance the violet and blue rays 
of the spectrum are popularly spoken of as the visible 
rays of greatest refrangibility. 

Temonstration of Properties of Coloured Light. 
If a strip of sensitised paper be pinned up so as to 
receive the spectrum it will soon be found that it 
becomes dark ; but the darkening power of the light is 
confined to the rays at and beyond the violet end. If, 
however, a thermometer be placed at the various colours 
of the spectrum, the mercury will rise in the most pro- 
nounced manner at, and even beyond, the red end ; 
hence the application of the term ' heat rays ' to these. 



REFRACTION INFLUENCED BY DENSITY. $ 

That the yellow is the luminous or light-giving ray is 
sufficiently demonstrated by the sense of sight. Now, 
while the foregoing is correct in the popular significa- 
tion, it is also the case that all the rays induce chemical 
change, and it is possible to prepare a sensitive surface 
upon which the red rays will exercise more prompt 
action than the so-called actinic or violet light. But 
this need not here be considered. 

We shall here sum up the truth or law to be deduced 
from what has been said. Light always travels in a 
straight line as long as the density of the. transparent 
medium through which it is passing remains unchanged. 
Upon entering a denser medium obliquely it suffers 
refraction or bending, the amount of the refraction 
depending altogether upon the density of the medium. 
Pure water refracts more powerfully than air; water 
containing a salt such as nitrate of silver in solution 
exceeds pure water in its refractive power ; crown glass 
exceeds salted water, and is in turn exceeded by flint 
glass, which last must yield the palm to the diamond 
and other gems. Suppose, then, we had four simple 
lenses, all precisely alike, so far as curvature and out- 
ward form were concerned, but one of them was formed 
by water encased by glass shells, the others being made 
of crown glass, flint glass, and diamond, respectively ; 
each would have a different focus from the other, the 
water having the longest and the diamond the shortest. 

Optical glass of greater density is being utilised at 
the present time much more extensively for photo- 
graphic lenses than it was several years ago. One 



6 TOPAZ LENSES. 

practical advantage arising from this may be perceived 
from the principles just enunciated. It is this : that with 
a given diameter and form of lens it is possible to obtain 
a shorter focus, and, consequently, greater intensity of 
illumination than when the objective is formed of lighter 
materia]. 

Pebble Lenses. Transparent pebbles, such as the 
Brazilian topaz and other similar crystalline bodies of 
which spectacle glasses are sometimes formed, have in 
former times been strongly recommended as media for 
the construction of portrait lenses. Sir David Brewster 
advocated this on account of the greater softness obtained 
by a single lens of this nature than by an achromatic 
lens. But we now know that the softness desiderated 
arose from un corrected aberration, and not from the 
material of which the lens was formed. 

Aberration What is it? Seeing that throughout this 
work there will necessarily be much said concerning the 
aberration of lenses, it is well here to give such a general 
definition of the term as will embrace the ramifications 
afterwards to be specially treated under their proper 
headings. 

Aberration merely denotes that deviation of the rays 
of light, when inflected by a lens, whereby they are pre- 
vented from meeting in the same point or geometrical 
focus. It is of a two-fold nature : (a) that arising from 
the figure of the glass, and (b*) that caused by the un- 
equal refrangibility of the rays of light. The former is 
' spherical,' and the latter ' chromatic ' aberration. 



CHAPTER IL 

PHOTOGRAPHIC DEFINITION, REAL AND IDEAL FORMS 
OF SINGLE AND ACHROMATIC LENSES. 

SHARP definition being an essential requisite in a 
photographic lens, we shall here make some observations 
on this quality, and try and assign a place to the well- 
defined photographic image. For reasons which will be 
adduced we are unable to give it a higher than a third 
place. 

Ideal Definition. Definition of the first order is 
ideal, existing only in imagination. It is that kind 
of definition which presupposes perfection in mathe- 
matical principles, in mechanics, and in atmospheric 
conditions. It is tolerant of things as they exist, 
merely because they cannot be helped. Optical tran- 
scendentalism, when indulged in by the photographer 
demands a lens which shall define so perfectly that the 
application of unlimited magnifying power will only 
serve as a means of unlimited penetration into Nature's 
arcana ; a lens having an aperture abnormally great in 
proportion to its focus, with a range of lateral definition 
so extensive as to include a panorama ; and a penetrative 
depth sufficient to embrace everything from within a 
few feet to infinity. This is the ideal or hypothetic 



3 DEFINITION, REAL AND IDEAL 

lens. Optical conservatives say that such a lens cannot 
possibly exist save in the brain of some enthusiast ; 
but recent progress made in Jena, in the production 
of glass having wonderful and valuable optical pro- 
perties, warrant us in being very cautious in assigning 
a limit to the capabilities of any lenses yet in futuro. 
For its productions, however, when they come, we 
reserve the first place in our classification. 

Telescopic Definition. The second order of definition 
is that which we find existing in a well-constructed 
telescope or microscope. The image formed by their 
object-glasses is never examined by the unaided eye, 
but invariably through powerful magnifying glasses, 
technically known as ' eye-pieces,' or l oculars.' This 
demands a perfection of definition altogether unknown 
and unrequired in artistic photography. 

Photographic Definition. Definition of the third order 
is of a lower grade than that just described. Photo- 
graphic definition may be considered as fulfilling every 
requirement of our art-science, when not only is there no 
portion of the picture noticeably deficient in sharpness, 
even at its margin, but also when it bears the test of 
examination by a glass magnifying three or four times. 
There are many otherwise excellent lenses which will 
not permit of this last test being applied to their pro- 
ductions unless when used with a very small diaphragm, 
and it is sometimes desirable that one should have the 
power, both with single and combination objectives, of 
reproducing a scene or subject with less sharpness than 
that which it appears to possess to the eye of the 



CHROMA TIC ABERRA TION. 9 

observer. The appliances for obtaining such effects will 
be considered in a subsequent chapter. 

Kefraction by Lenses. We have seen in Fig. I, Chapter 
I., in what manner a ray of light becomes decomposed 
when it is transmitted through a prism. Now, a lens 
may be considered a series of prisms formed by a 
single piece of glass, its faces being spherical instead of 
an unlimited number of flat surfaces. The property 
possessed by a wedge-shaped piece of glass of bending 
and decomposing a ray of light applies equally to the 
glass, whether it be purely prismatic or lenticular in 
form, and no single l?ns formed of one piece of glass 
can possibly bring the rays transmitted through it to 
one focus ; for, as we have shown, the violet rays, being 
bent so much more strongly than the red and all the 
others, are brought to a focus nearer to the lens than 
these. This defect is entitled 'chromatic aberration,' 
from chroma (colour) and aberro (I wander from). Its 
nature is shown in the diagram, Fig. 2, which represents 
rays a a, incident upon a double-convex lens L. These 




FIG. 2, 



rays are not only bent or refracted but are also de- 
composed, which is what we have to do with at 



to SINGLE AND ACHROMATIC LENSES. 

present. The violet rays, in consequence of their 
greater refrangibility, are brought to a focus at V, the 
red rays finding a focus at R. By the term 'focus' 
is here meant that place where rays cross the axis c 
of the lens. This definition is only strictly accurate 
when applied to direct rays ; a more comprehensive one 
will be given when we come to treat of oblique pencils. 

Chromatic aberration is avoided by the employment 
of an achromatic (without colour) lens. The construction 
of an achromatic lens is based upon the fact that flint 
glass effects a much greater separation of the elementary 
colours of a ray of light than crown glass. A convex 
lens of the latter material would, undoubtedly, cause the 
rays to be decomposed, as shown in Fig. 2, but by being 
placed in juxtaposition with a concave lens formed of 
flint glass, the refracting power of which is exerted in 
a contrary direction while its power for dispersion is 
greater, the inward dispersive tendency of the crown is 
opposed by the outward dispersive proclivity of the flint, 
the result being that the ray is transmitted intact, or 
without colour, to its focus. 

Forms of Single Lenses. In Fig. 3 are shown, in 
outline, various forms of simple lenses, the names given 
having reference to the external configuration of the 
lens, no matter of how many elementary parts of other 
forms it may be composed. 

In this diagram, I and 2 are respectively plano- 
convex and plano-concave lenses ; 3 and 4 are double 
convex and double concave ; 5 is a concavo-convex ; 
and 6 a periscopic or meniscus lens. If 3 had one of 



SI. VOLE AND ACHROMATIC LENSES. 



ii 



its surfaces of greater curvature than the other, it would 
be designated a ' crossed ' lens. 




FIG. 3. 



When lenses are achromatised by uniting a convex 
crown glass with a concave formed of flint glass, Fig. 4 




FIG. 4. 

indicates some of the ways by which such union is 
effected. 

Besides these, in which the achromatism is obtained 
by the union of one crown glass with one flint glass lens, 
the method (first applied to the telescope by John 
Dollond) of uniting two crowns with one flint has been 
advantageously applied to photographic lenses, details 
of which will be subsequently given. 



CHAPTER III. 

THE CAUSE OF AN INVERTED IMAGE. 

HAVING spoken of the nature of lenses we next advert 
to their properties, particularly to that special character- 
istic upon which depends the formation of an image. 

If a double convex lens formed of one piece of glass, 
such as a hand magnifier of the simplest kind, be held 
up so as to allow the sun's rays to be transmitted on to 
a sheet of paper held at a certain distance behind where 
the rays come to a point, the brightness at the apex of 
the cone is owing to the formation of a minute image of 
the sun there, its intensity either for luminousness or 
burning being dependent upon the dimensions of the 
lens. This applies also to the formation of an image 
of any terrestrial object to which the lens may in like 
manner be directed. In every case in which an image 
is produced in this way it will be seen to be inverted, or 
upside down. Why this is so we shall explain by the 
aid of the following diagram (Fig. 5), in which the 
dart A may be considered as representing anything in 
external nature, such as a church, a house, a landscape, 
or a figure. The rays of light from every point of this 
pass in straight lines everywhere, and hence through 
the small hole B in the opaque sheet, which may be 



PINHOLE APERTURES. 



assumed to be the front of a box ; some of these rays 
from every point pass straight on until interrupted by 




FIG. 5. 

the screen c, on which they fall, forming an inverted 
image of the object in front. 

Pinhole Apertures. The smaller the aperture B is 
the sharper will be the image. It is, therefore, quite 
possible to take a photograph without any lens what- 
ever ; but, owing to the attenuation of the light by 
transmission through a pinhole aperture, a protracted 
exposure is required in order to obtain a picture. By 
greatly enlarging the aperture and inserting a lens, 
however, it will be found that, while the dimensions of 
the image formed by the pinhole aperture are not 
sensibly altered, there is at once a great increase in 
both the brightness and sharpness of such image. It 
may here be remarked that the size of the image is 
determined by the distance at which the receiving screen 
upon which the image is depicted is situated from the 
aperture a fact that will be self-evident on inspecting 
the foregoing diagram, and imagining the situation of 
the screen C to be only half the distance from the pin- 
hole at which it is now represented. 

Size of Image determined by Focus of Lens. From 
what has been said it will be seen that the longer the 



14 ANGLE OF VIEW DETERMINED BY FOCUS. 

focus of a lens by which an image is to be formed the 
larger will be that image. If a lens of ten inches focus 
be employed in the production of a picture of a scene, 
such as a house and its surroundings, and another picture 
of the same scene be taken by a lens of five inches focus, 
when both are examined side by side it will be observed 
that the house produced by the lens of the shorter focus 
will only be one-half the dimensions of that obtained 
by the lens of longer focus ; but, as a set-off against this, 
there will be twice as much of the subject depicted on 
a plate the same number of inches in dimension. From 
this it will be correctly inferred that a wide-angle lens 
that is, a lens intended to include a wide angle or large 
amount of the subject to be photographed must be of 
short focus relatively to other lenses. Another deduction 
from this is that dimension or size of image depends 
exclusively upon the focus of the lens, and is entirely 
unconnected with its diameter. If we have a lens of 
ten inches focus and only one inch diameter, and another 
lens the same focus and four inches in diameter, the 
images formed by them will be precisely alike in di- 
mensions. The influence of the diameter of the lens 
is confined to giving greater or less brightness to the 
image, and we shall consider this more fully when 
treating of the requirements of quick-acting lenses. 



CHAPTER IV. 

SPHERICAL ABERRATION. 

A SINGLE lens of the class of which we have been 
hitherto treating does not give an image possessing 
more than a very low degree of sharpness, even to the 
unaided eye. This arises from spherical aberration, 
which we may define as an inability in a lens having 
a spherical surface to bring to one focus all the rays 
which are transmitted through it. A ray transmitted 
by the margin of a lens (Fig. 6) is more deflected 





FIG. 6. 

or refracted than one which is transmitted nearer the 
centre. Observe in what manner the representative 
rays O and S are refracted by the lens. The former, 
being bent in a greater degree than the latter, comes 
to a focus at o', the focus of S being carried farther to 



i6 



SPHERICAL ABERRATION. 



S' ; and the absolute focus of such a lens will be 
nowhere in particular, but anywhere between o' and 
where the rays which are more nearly central cross 
the axial line. Now, this has no connexion whatever 
with the aberration of colour, but is true of a lens 
even if achromatised. It is possible to correct a single 
achromatic lens so that it shall with its full aperture 
bring direct rays to a focus, which is the case with 
telescope lenses ; but for oblique rays it would be quite 
worthless. Photographic correction of lenses, therefore, 
partakes of the nature of a compromise ; it is content 
with an inferior order of axial definition in order to 
secure an equal degree of oblique sharpness. 

A plano-convex lens, or one of a slightly meniscus 
form, if directed, convex side out, to an object will give 
a fairly well-defined image of what is directly in front : 




FIG, 7. 



but those objects not axially situated will be very 
imperfectly rendered indeed. Now, by reversing the 



POSITIVE SPHERICAL ABERRATION. 



position of the lens that is, placing its flat side out- 
wards quite a different aspect is presented ; for the 
central sharpness now gives place to a certain kind of 
indistinctness of image inferior in this respect to the 
former crispness of delineation ; but this inferior dis- 
tinctness is distributed over a larger area of the plate. 
The reason for this will be seen from an inspection of 
the diagram (Fig. 7), in which a few oblique rays are 
represented before and after transmission. It will be 
perceived that V and w suffer less refraction than Y 
and z, and this being the case there is a great degree 
of confusion at the focus, which, as in the former 
instance adduced with the axial rays (Fig. 6), is really 
* nowhere.' 

Positive and Negative Spherical Aberration. In the fore- 
going instances and illustrations, in which the margin 
of the lens refracts the light to a much greater extent 
than does its centre, the aberration is positive. But it is 



FIG. 8. FIG. 9. 

quite easy to combine two glasses, one a convex and 
the other a concave, with an air-space between them 

C 



iS NEGATIVE SPHERICAL ABERRATION. 

in which this condition will be reversed, or, to put it 
popularly, in which the centre of the compound will be 
possessed of a great magnifying power and the margin 
not necessarily any at all. The two illustrations here 
given (Fig. 8 and Fig. 9), in which the inner surfaces 
are of dissimilar radii of curvature, afford a fair idea 
of the conditions requisite to attain this end. This 
property is known as negative spherical aberration, and 
its use in flattening the field of certain combinations will 
hereafter be pointed out. 

No single lens can be made that shall be entirely 
free from spherical aberration, but by giving a lens a 
certain form it may be very greatly reduced. If a lens 
be a plano-convex, and its flat side be directed towards 
the object, the aberration is 4*5 ; but if the position is 
reversed, and the convex side held toward the object, 
the aberration is reduced to ri/. 

The longitudinal aberration is ascertained by noting 
the difference between the focus given by the margin of 
a lens and that of its middle. While making this trial, 
opaque masks must be employed to prevent the trans- 
mission of light through any but the part being tested. 

In a lecture on lenses at the Society of Arts, Mr. 
Conrad Beck gave ' in a nutshell ' a synopsis of the 
aberrations of the various forms of lenses, both convex 
and concave. Premising that, as a general rule, when 
parallel rays enter from a less refracting medium (air) 
into a denser medium (glass), the more curved the 
surface that is turned towards the parallel rays the 
less is the aberration, while the flatter the curve or 



ABERRA TION OF SINGLE LENSES. 



the more nearly it approaches a flat surface the greater 
the aberration, the amount of such aberration is shown 
in the following figures, the parallel rays being assumed 
to enter each individual lens from the left-hand side. 





S.Aberraticn 

4-2-07 +1-071 



.1-17 



S. Aberration 
-1071 

FIG. 10. 



-2-07 



-4'5 



In the above, the upper or convex series are of 
the same focus as the lower or concave series, so that 
any one of the former will just balance that of the 
negative focus below. The amount of aberration, plus 
or minus, is placed underneath each. By comparing the 
figures attached to any of these lenses, even those of the 
same form, such as the first and last in upper series, it 
will be perceived to what extent aberration is affected 
according to the side which is turned towards the light. 



CHAPTER V. 

THE NA'iURE AND FUNCTION OF THE DIAPHRAGM 
OR STOP. 

How, by whom, or at what time a diaphragm came 
to be designated a ' stop ' we need not here wait to 
inquire. Photography has given rise to so many new 
terms and new applications of pre-existing terms that its 
literature, and especially its vernacular dicta, must not 
be considered as amenable to strict etymological rules. 
A diaphragm, in all other branches of optical science 
than that of photography, differs from a stop, but in 
our young art-science they are held by the vox populi 
to be synonymous ; hence the indiscriminate employ- 
ment of the two terms in what we have further to say in 
these chapters. 

Use of a Diaphragm. A diaphragm fulfils two alto- 
gether dissimilar functions in photography, according to 
whether the lens to which it is attached be a single or a 
compound instrument. In the former it is usually a neces- 
sity ; in the latter only an expedient. It has been shown 
in what manner rays are transmitted by a single lens, and 
that those impinging upon one part of the surface are 
not brought to a focus with such rays as are permitted 
to fall upon another portion. Now, by placing a 
diaphragm at a little distance in front of the lens, it 



DIAPHRAGMS OR STOPS. 



21 



cures all the evils arising from spherical aberration by 
debarring access to those rays which, if transmitted, 
would interfere with ultimate sharpness. 

In Fig. 1 1 we show in what manner the ' curative ' 




FIG. II. 



powers of the diaphragm are exercised when employed 
as a stop to obstructant rays, both central and oblique. 
Observe what havoc would be played as regards focal 
sharpness if the mass of the rays were permitted indis- 
criminate access to the lens. The dotted lines represent 
those by which definition would be entirely marred 
vere they not stopped by the diaphragm, which, sentry- 
like, guards the access to the lens. What has, therefore, 
to be effected in this case by the diaphragm is this 
no rays are allowed to take part in the formation of the 
central portion of the picture but those transmitted 
through the centre of the lens ; and, in like manner, 



22 MISCONCEPTIONS REGARDING DIAPHRAGMS. 

none but rays transmitted through the margin of the 
lens are allowed to form any but the margin of the 
picture. This is the law regulating the margin of a 
diaphragm to a single achromatic lens, and from what 
has been said it will be seen that to a lens of this class 
the stop is a necessity. 

Misconceptions Regarding Diaphragms. Before pro- 
ceeding further we may allude to a very prevalent and 
popular misconception, which finds expression in the 
suggestion that by making the lens of only the diameter 
of the largest diaphragm an equal degree of sharpness 
would be secured. While this is quite true as regards 
the formation of the centre of the picture which would 
be equally well defined if an opaque disc of paper having 
a round hole in its centre were pasted upon the surface 
of the lens, and by which it would be practically reduced 
to the dimensions of the aperture in the paper it is not 
so with the sides of the picture, which, although equally 
well lighted as before, are now badly defined. The 
following experiment is both suggestive and instructive : 
Let a plano-convex or meniscus lens (the front lens of 
a portrait combination answers the purpose well) be 
mounted, flat side out, and without any diaphragm. 
Now try to focus the image, and observe that while no 
part of it is sharp, it is rather more so in the centre than 
towards the sides. Next make a cardboard diaphragm, 
with an aperture about one-fourth the diameter of the 
lens, push it close up against the flat surface, and then 
focus the centre as sharply as possible. This will now 
be well defined, but only over a very limited area. 



DIAPHRAGMS CONFER SHARPNESS. 23 

Without altering the camera or lens pull the diaphragm 
slowly away from the lens, and it will be found that, by 
the simple act of increasing the space between the 
diaphragm and the lens, the area of sharpness extends 
outwards, till a point is reached at which further with- 
drawal of the diaphragm cuts off the light from the 
corners of the plate without further increasing the 
marginal definition. At this stage the requirement has 
been fulfilled that the centre of the picture be formed 
by the centre of the lens, and, in like manner, that 
no rays have taken part in the formation of the 
margins of the picture but those transmitted by the 
margin of the lens. 

When applied to a combination of lenses such as 
that employed in portraiture the function of the dia- 
phragm is different from that just described ; for such 



FIG. 12. 



combination, being corrected in itself for spherical 
aberration, gives a sharp image with its full aperture. 



24 DIAPHRAGMS GIVE PENETRATION. 

But it is characteristic of all portrait lenses and others 
having a large working aperture that they lack the 
power of bringing objects situated at different distances 
to a focus on one plane ; or, as it is commonly said, they 
have no ' depth of focus.' By reducing the aperture a 
portrait lens can be made to possess as much of this 
depth of defining power as may be required. The way 
by which this is secured is shown in the diagram, Fig. 12, 
in which the dotted lines A A represent the rays trans- 
mitted through a lens worked with its full aperture. 
Observe that this focus partakes of the nature of a definite 
point, at which it is imperative that the ground glass of 
the camera be situated in order to obtain sharpness. 
Now this is all very well, and it is the easiest thing in 
the world to place the focussing-screen in that precise 
position. But here lies the difficulty : this spot of precise 
focus is that for rays only which come from a definite 
distance in front of the lens (say 12 feet), while 
those rays emitted by an object either ten or fourteen 
feet away do not focalise at the same point or 
distance behind the lens one set of rays coming to 
a focus nearer and the other further than the twelve- 
feet set. 

To meet the difficulty just stated we must have 
recourse to a diaphragm by which all rays outside of 
SS are excluded, with this result that the point at 
which the rays crossed the axis of the lens has now in 
effect become elongated, and a fairly good focus is 
obtained without the necessity that formerly existed for 
having the ground glass situated in one definite position. 



DIAPHRAGMS INCREASE LA TERAL DEFINITION. 25 

The reduction of the aperture has given such a range to 
the focus that, while the sharpness of the object originally 
focussed upon with full aperture remains unimpaired, 
this quality is now imparted to objects situated both 
nearer to and further from the camera. 

No * Depth of Focus' in Large Portrait Lenses. For 
the reason just given a portrait lens of very large 
dimensions cannot be used with its full aperture in 
taking a head unless the sitter be placed a considerable 
distance from the instrument, because such is the lack 
of depth of definition in a lens of this character that if 
the nose were sharply focussed, the eye, ear, and other 
portions not situated upon the plane 'of the nose would 
be so much out of focus as to destroy the pictorial value 
of the head; while by focussing merely the eye, the 
nose and chin would be equally out. By employing a 
diaphragm, however, all the features may be brought 
into pictorial sharpness. 

We may here foreshadow what we shall have to say 
afterwards in its proper place relative to the use of stops, 
by observing that a portrait or aplanatic lens (an 
aplanatic lens being one which is capable of working 
with full aperture) not only has its focal range, as 
regards depth, increased ad libitum by the employ- 
ment of a diaphragm, but it has its lateral definition 
improved in similar ratio. A lens when worked with 
full aperture is unsuited for photographing anything 
requiring great marginal sharpness, such as copying a 
large sheet of printed matter or photographing a house 
pn a plate otherwise within its capacity. By inserting a 



26 FOCUSSING WITH THE WORKING DIAPHRAGMS. 

diaphragm the range of sharpness will be so far extended 
as to enable the lens to execute work for which, without 
having recourse to this expedient, it would have been 
altogether unsuited. 

Focussing with the Working Stop. Unless a lens be 
quite free from spherical aberration, or, in other words, 
be aplanatic, it is well to focus with the same stop with 
which the picture is to be taken. There is often a great 
temptation to focus with a large diaphragm on account 
of the superior illumination of the image thus obtained, 
and then insert a small one. But with some lenses this 
ensures the very evil it is intended to avoid, for with a 
small stop the best point of focus is farther from the lens 
than when employing a large one. The reason for this 
will be apparent on studying the diagrams, Figs. 6 and 7. 

For composing a picture, when one cannot have too 
much light upon the focussing screen, it may be well to 
employ the largest aperture possible ; but when the 
subject has been arranged, then should the focussing be 
done as above indicated. 



CHAPTER VI. 

PROPERTIES OF DEEP MENISCUS LENSES-- 
COMPENSATING SINGLE LENSES. 

THE simpler the parts and structure of a photo- 
graphic objective the less danger is to be apprehended 
from flare or false light caused by internal reflections. 
This being the case, why, it may be asked, not employ 
the simplest of all lenses a single meniscus ? 

The Deep Meniscus. A deep meniscus lens, whether 
single or achromatic, possesses properties different from 
all others. Those who desire to see the finest exemplifi- 
cation of the so-called * depth of focus ' possible to be 
obtained have only to procure a meniscus of very deep 
shape, expose its concave side to a bright object, and 
observe the image. This experiment may be performed 
by directing it to the flame of a candle situated at a 
distance of a few yards and receiving the image on a 
sheet of paper held in the hand. Having got the 
sharpest image that can be obtained, observe to what a 
great extent the lens may be moved backwards and 
forwards without the identity of the candle flame ceasing 
to be observed. It is true that it is surrounded with an 
aureola of false light, but the form itself is still there. In 
this respect it is quite unlike an image obtained by any 
other lens, such as a plano-convex, curved side out, in 



28 DEEP MENISCUS LENSES. 

which the slightest motion of the lens from its correct 
focal distance converts the image of the flame into 
a circular disc of light. 

The spherical aberration by which the flare or 
mistiness of the image in the foregoing experiment is 
caused can be practically eliminated by the employment 
of a diaphragm ; and here we may observe that photo- 
graphs of great beauty and even sharpness may be, and 
often have been, taken by means of a simple non-achro- 
matic meniscus lens. For a reason which will be apparent 
to those who carefully study the diagram, Fig. 2 (page 9), 
the photographic image will not be sharp unless care has 
been taken that, after focussing upon the ground glass, 
the lens is then pushed in towards the camera to such 
an extent as to cause the focus of the chemical or violet 
rays to take the place of the visual ones, which, as 
regards the ground glass, will now be quite out of focus. 
The difference between these foci is approximately one- 
thirtieth of the focus of a lens formed of crown glass; 
hence, if a ten-inch lens were employed it would, after 
focussing sharply, have to be pushed in over a quarter of 
an inch in order to secure a sharp image on the sensitive 
plate. Now, this would be of no consequence whatever 
if distant objects alone were to be photographed, because, 
the difference between the two foci being a constant 
one, the ground glass could easily be let deeper, or set 
farther forward, in its frame to effect the requisite com- 
pensation. But while the difference is a constant one 
with respect to proportion, it is not so as regards 
quantity; for upon focussing a. near object the lens, as 



COMPENSATING SINGLE LEASES. i$ 

every one knows, must be withdrawn farther from the 
focussing-screen in order to obtain a focus, and the 
quarter-of-an-inch alteration of the screen in the frame 
would prove totally inadequate when, in photographing 
an object on the scale of the original, the lens had to be 
twenty inches from the plate. 

This would obviously demand an adjustment be- 
tween the visual and the working focus of a measure- 
ment greatly exceeding that employed under the 
circumstances described. Among other reasons, the 
trouble necessitated in effecting this adjustment has 
operated to prevent photographers from making use of 
lenses other than those in which the actinic achromatism 
is effected in such a manner as to ensure a strict coinci- 
dence of the visual and chemical rays. But as, notwith- 
standing the drawback mentioned, there are several 
advantages alleged to be found in simple crown glass 
meniscus lenses cheapness being one, and less loss 
of light another it is fitting that we here give the 
means whereby an accurate adjustment can be made so 
as to ensure the requisite sharpness with such lenses 
when used in either a single or combined state. 

Compensating Methods for Simple Lenses. Propor- 
tional compasses and suitable markings upon the sliding 
mount will suggest themselves as one obvious method 
by which to effect the desired adjustment ; but that 
to which we have long confined ourselves invariably 
recommended as superior to all other methods, and 
which owes its inception to that profound mathematical 
optician, Mr. Robert H. Bow, C.E., of Edinburgh is 



30 COMPENSATING SIMPLE LENSES. 

one more practically perfect (as we have often proved it 
to be under many ramifications) that even its talented 
progenitor could easily have imagined it to be. A weak 
and thin convex lens such as may be obtained from 
spectacle lens opticians must be selected, its strength 
being such that, when added to the focal length of the 
operating lens, it will have the power of reducing the 
focus two per cent, or any other proportion found to be 
the proper amount of adjustment for a very distant 
object. As the focal length of this supplementary lens 
will be very great say from forty-five to fifty times that 
of the camera lens very little error will be caused by 
inserting it at the place of the stop instead of in con- 
tact with the working lens. It has, therefore, merely to 
be dropped in a suitable slit in the mount, like a Water- 
house diaphragm, where it remains till the focus is 
obtained, after which it is removed and the photograph 
taken without it. The simplicity and beauty of this 
system must approve itself to every one. 

The rule for finding the focus of the lens that must 
be inserted as a stop (when focussing) to effect the cor- 
rection of the working lens is this f being the focal 

f x f 

length of the required lens : -f~- -^ or when 

/i ~/n 

/j 50, / n = 49, /- 2450 inches. This rule will be 
found useful in another direction when we come to 
speak of over-corrected lenses ; for the means described 
for curing the annoyances arising from the use of non- 
achromatised lenses apply equally to those in which the 
achromatism for colour is cawed further than is re- 



DEEP MENISCUS LENSES. 3t 

quired for photographic working as to those in which it 
is not carried sufficiently far. 

Deep Meniscus Lenses require Small Diaphragms. A 
deep meniscus, whether achromatised or not, requires a 
small stop placed comparatively close to the lens. This 
permits of the transmission of a very oblique ray, the 
incidence of the ray being more normal than in the case 
of a flatter lens. For this reason all wide-angle lenses 
must partake of the external form of the deep meniscus, 
and the diaphragms must be placed near to the lens. 

When single meniscus lenses are mounted in doublet 
form that is, one lens in front of and the other behind 
the diaphragm there is a help towards correction 
accomplished naturally in the case of oblique rays, the 
nature of which we may explain as follows : Let a 
symmetrical or, by preference, a non-symmetrical doublet, 
of which the back element is shortest in focus, be im- 
agined, its two elements being deeply curved crown- 
glass menisci. When an oblique ray impinges upon the 
anterior lens in such a manner as to enable it to be 
transmitted through a stop placed between both lenses, 
it undergoes decomposition, and its violet constituent, 
being more strongly refracted than the yellow, falls 
upon the surface of the posterior lens nearer its margin 
than does the yellow ray, which, as we have said, is less 
refrangible than the other. But the nearer to the centre 
of a lens that a ray falls for transmission, the less is 
it refracted ; or, on the contrary, the margin of a lens 
possesses the refractive power in a greater degree. The 
yellow and violet rays which, therefore, were separated 



$i ELASTICITY OF FOCtfS. 

by the action of the front lens are, to some extent, made 
to reunite by the back lens, seeing that the violet falls 
under the influence of a portion of this back potent to 
cause it to reunite with the yellow, which, being less 
refrangible in itself, is also transmitted by a portion of 
the lens possessing less power for refracting. 

Accommodating Elasticity of Focus. The deep me- 
niscus lends itself wonderfully to combinations intended 
to have an easy, accommodating elasticity of focus. A 
single achromatic, deep meniscus, which is properly 
corrected for actinic achromatism, may have wedded to 
it as a back combination a lens formed of a single 
crown-glass meniscus, which shall not only correct the 
distortion of figure necessarily caused by the former 
when used alone, but shall do so without much inter- 
ference with its actinic correction. In other words, the 
achromatised front when used alone has its chemical 
and visual foci coincident ; yet when a single, non- 
achromatic, crown-glass meniscus is added to this, 
although there is a diminishing of the focus to about 
one-half, the chemical and visual foci are still practically 
coincident as before. 

A practical outcome of this fact is that, when a 
photographer has a lens of the achromatised, wide- 
angle, non-distorting class, which may not be of pre- 
cisely the focus he desires, he may temporarily lay aside 
its posterior element and substitute for it a simple lens 
of another focus, by which he can arrive at almost any 
focal result required. Having determined upon the 
focus desiderated he must start with this fact as a basis 



SEFARA TION IN DETERMINING FOCUS. 33 

that no two lenses of only half that focus will enable 
him to obtain what he desires. An important factor in 
the calculation is the distance that must intervene be- 
tween the two lenses forming a combination. Knowing 
the foci of the particular lenses about to be employed in 
the formation, temporary or otherwise, of a new objec- 
tive, the combined focus of the pair may be ascertained 
by multiplying together the individual foci and dividing 
by the foci added together, subtracting from the divisor 
the distances apart at which the lenses are to be 
mounted. 

It will be obvious that when a combination is very 
near the focus desired, that focus may be lengthened or 
shortened till the required power is obtained by slightly 
separating or bringing the lenses nearer together. The 
nearer they are together, the shorter the equivalent 
focus. 

This question will be found treated with greater 
fulness in the chapter on ' The Adjustment of Dissimilar 
Lenses/ 



CHAPTER VII. 

THE OPTICAL CENTRE OF SINGLE LENSES. 

PREVIOUS to the consideration of either the solar, 
the equivalent, or conjugate foci of lenses, it is necessary 
that we speak of the 'optical centre,' this being the 
point from which focal measurements must be made. 
Our remarks will, at first, have reference only to the 
optical centre of a lens, by which we mean just what is 
expressed by this name and not of an objective or com- 
bination of lenses, which is quite another matter ; for, 
if one choose to be too nice with definitions, it is not 
difficult to show that a combination has not an optical 
centre at all, or, to put it more intelligibly, that any 
given combination may have its optical centre at several 
places, according to the circumstances under which it 
is being employed. 

The situation of the optical centre for focal centre, as 
it has by some been designated) of a lens is determined 
by its form. In some forms it is within, and in others 
outside, of the lens. In a double-convex it is in the 
middle, or equi-distant from both surfaces ; in a crossed 
lens it is situated at a point between the middle and the 
more convex of the two surfaces ; a plano-convex has its 



CENTRE OF LENSES. 35 

optical centre on the curved surface; while in a meniscus 
it is outside altogether, its distance from the lens being 
determined by the degrees of curvature of the surfaces. 

To find the Centre of a Single Lens. The method for 
finding the optical centre of a lens is this : Draw two 
parallel radial lines, one from the centre of each cur- 
vature, and both being oblique to the axis; then connect 
the points at which they touch the curved surface by a 
line which, in the case of a meniscus, must be prolonged 
till it meets the axis. The point at which this junction 
line touches the axis is the optical centre. We shall 
now illustrate this law by applying it to the case of 
three of the four lenses just named. 

Centre of Double Convex Lens. In Fig. 13 we have a 
double convex lens, the radii of curvatures of both 
surfaces being a and a, the lines from which to the 




further surfaces in this and the two following figures 
are parallel to each other. From their points of impact 
on their respective surfaces, as s /, a connecting line is 
drawn, and at the point o, where this line touches the 
axis, is situated the optical centre, 



36 CENTRE OF LENSES. 

Centre of Crossed Lens. By the flattening of one of 
the curves of the lens it becomes, as in Fig. 14, a crossed 
lens, having its optical centre at 0, which in this case is 
not centrally situated. 




FIG, 14. 

Centre of a Piano-Convex Lens. This centre being 
situated on the convex surface of the lens, it is not 
necessary here to give an illustration 

Centre of Meniscus. It is in the case of the deep 
meniscus now so much in use for many purposes, both 
singly and combined, where the greatest discrepancy 
exists between one's ordinary or crude conjectures as to 
the situation of the optical centre and its true position. 
In Fig. 15 it is demonstrated not only to be outside of 
the lens, but a long way outside. We have heard the 
question put to one who was reputed to be fairly con- 
versant with optical matters : ' Where must I measure 
the focus of my lens from ? ' the lens spoken of being 
a wide-angle, deep meniscus, having a stop in front. 
The response was : ' You will be sufficiently ac- 
curate by measuring from the centre of the curved 
surface of the lens.' Now, this reply is not correct 



PROPERTIES OF OPTICAL CENTRE. tf 

in the case of a lens of this form, although it would 

be so if one surface were plane instead of being 
concave. 




FIG. 15. 

Properties of Optical Centre. One of the properties 
of the optical centre of a lens is this that any ray 
refracted by the lens which passes through this centre 
emerges in a direction parallel to that of its incidence. 
In most of the class-books on optics, the rule for finding 
the optical centre is expressed thus : ' Multiply the 
thickness of the lens by the radius of one surface and 
divide the product by the sum of the radii, and the 
quotient is the distance of the centre from the vertex 
of that surface.' The position of the centre is the same 
in every lens of the same dimensions, whatever may be 
the material of which it consists. 

What has hitherto been said applies to single lenses, 
to which alone the term 'optical centre' is strictly 
applicable ; and, although we have confined the illus- 
trations to those of the positive or convex class, the 
rules equally apply to concave lenses. 



CHAPTER VI1L 

THE OPTICAL OR FOCAL CENTRE OF A COMBINATION. 

IN a combination of lenses, whether symmetrical or 
non-symmetrical, there is no fixed point which can be 
termed the * optical centre.' The mistake, however, is 
frequently made of assigning it a position where the 
stop is placed. But the best position for the stop has 
not necessarily any relation to that of the centre, which 
can only have its position determined upon knowing 
the precise circumstances under which the combination 
is to be used, for it has strict relation to the conjugate 
foci. If these weie so definitely fixed as to be invariable, 
then the position of the centre could be definitely allo- 
cated, but not otherwise ; for every alteration of focus 
would be attended with a displacement of the central 
point. What is commonly termed the ' optical centre ' 
in a combination is in reality the centre of conjugate 
foci, and this is determined by the conjugates, which, 
as already said, are changed with nearly every change 
of picture taken. 

How to find the Focal Centre. The place of the centre 
in a combination of any nature cr form may be easily 
found, and for any special purpose it may be marked 
upon the brasswork of the me. anting. The method 



TO FIND THE FOCAL CENTRE. 39 

now to be described is one which involves no special 
apparatus. Suppose the lens to be a large portrait 
combination, and it is desired to know its centre when 
employed in portraiture. Let us assume the anterior 
conjugate (the sitter) to be at an average distance of 
(say) eighteen feet from the camera, then let the lens 
be brought into a darkened room and placed upon a 
board on the table. On this board must be laid a 
small square block of wood about two inches in height, 
and the upper surface of which is brought to a wedge 
shape. Now rest the lens across the face of the wedge, 
and let it be directed to a lighted candle placed in front 
at a distance equal to that at which the sitter is expected 
to be placed, and having erected, a few inches behind the 
lens, a white sheet of cardboard on which to receive the 
image of the candle, hold the lens (the weight of which 
rests upon the wedge-shaped block) level by the fore- 
finger of the left hand, and with the right hand rotate 
the lens gently on the extemporised rotary axis formed 
by the wedge below and the finger above. Now 
observe if the image of the candle flame stand perfectly 
motionless, or whether, as will most likely be the case, 
it moves across the card with every rotation of the lens. 
In this latter case, move the lens a little farther back- 
wards or forwards on the supporting block and try 
again. Do this until that position is found at which 
the image of the candle remains motionless while the 
lens is being rotated from side to side, and then put a 
small mark on the tube, which ever afterwards will in- 
dicate, with the degree of accuracy practically required, 



40 THE MECHANICAL CENTRE. 

the optical centre of the combination, whenever em- 
ployed under circumstances in which the position of the 
conjugates assimilates to those under which the trial 
was made. 

But to prove that the centre in question is really 
only that of these conjugates : after having made the 
mark on the tube, let the candle be brought to within 
six feet of the lens, and by another course of experiments 
let its centre be again found, and it will be seen that 
it now differs materially in position from that of the 
previous trial. The new centre is quite right for, and 
under, the altered conditions, but wrong as regards all 
others. 

We are aware of some gentlemen who are so 
dexterous in examining a combination for its optical 
centre (we are now using the term under a kind of 
protest) that they will take it up and, poising it between 
finger and thumb, examine the stability of the image 
on the wall opposite a window while rotating the lens, 
and in this way will in less than half a minute have 
acquired more knowledge concerning it than another 
would in some days. 

The Mechanical Centre not the Focal Centre. To de- 
monstrate that the focal centre is not situated in the 
mechanical centre, let us take the case of a combination 
of the cemented doublet class so commonly used, and 
let us further assume that it is a symmetrical compound, 
that is, that its front and back lenses are identical in 
figure and focus. Now while the mechanical centre of 
such a combination is midway between the lenses, the 



MECHANICAL VERSUS FOCAL CENTRES. 41 

focal centre may be anywhere according to circumstances. 
This will be readily understood from the following con- 
siderations : To adduce an extreme instance, let each 
lens of the combination be twenty laches in focus. 
These, if placed so close together as to be merged into 
one, and that one infinitesimally thin, would have a 
focus of ten inches, and its focal centre would measure 
from that of the lens. But in proportion as they are 
separated so does the focal centre move forward in 
advance of the mechanical centre ; until at last when 
our hypothetic mount is nineteen inches long and the 
back lens is within an inch of the ground glass, the front 
lens being over nineteen inches away, where, under 
such conditions, would be the optical or focal centre for 
a distant object ? It would be in the vicinity of the 
front lens and many inches in front of the mechanical 
centre. 

The focal centre, or point from which the focus 
must be measured, varies therefore according to the 
distance of the object in front or its anterior or major 
conjugate. 



CHAPTER IX. 

SINGLE ACHROMATIC LENSES. 

Historical Memoranda. When photography was youn^, 
various devices to work with a large aperture, and at the 
same time to secure sharp definition, were had recourse 
to. It had been early found that simple lenses would 
not answer because of their actinic plane of representa- 
tion being situated nearer to the lens than that of the 
visual focus ; accordingly the single lens of the camera 
obscura was supplanted by the achromatic lens of the 
telescope, the surface of maximum convexity being 
placed to the outside. Owing to the circumscribed area 
of definition, the lens was afterwards reversed as regards 
position, and a diaphragm placed in front. The value 
of Wollaston's meniscus lens was in time duly recognised 
as a means of securing an extended field ; and we find 
in a manual by Daguerre, published in 1839, a single 
meniscus achromatic, which is practically that manu- 
factured at the present time, subject in some cases to 
modifications of internal curvature, in others to none. 

Diaphragms Necessary with Landscape Lenses. Single 
achromatic landscape lenses are usually either of plano- 
convex or meniscus form, and this latter is the more 



OF LANDSCAPE LENSES. 43 

pronounced according to the width of the angle of view 
it is intended to include. The deeper is its meniscus 
shape the smaller must be the stop, and the nearer must 
that stop be to the lens. If a single lens be intended to 
include only a very narrow angle, then may it be a 
crossed one, that is, both sides may be convex, the rela- 
tion of the radii of the surfaces not being arbitrary, 
although approximately as one to six, the flatter side 
being outside or next the stop. We examined an old 
lens of this form, constructed by Goddard, and found, as 
might have been deduced a priori, that it bore a dia- 
phragm unusually large, and placed at a considerable dis- 
tance in front, but that its covering power was not great. 

It is absolutely necessary that to ensure the best 
definition, a landscape lens must have a diaphragm, for 
in this respect it differs from combinations which may 
be made aplanatic. The reason for this is shown in 
Fig. n, page 21. But this lens lends itself admirably 
to those who desire definition of a low order, to secure 
which, all that is necessary is to use it either without any 
stop at all, or with one much wider than the fixed dia- 
phragm which the optician places in the mount. 

Forms of Landscape Lenses. In Fig. 16 we 
give the earliest form of landscape lens (that 
referred to in Daguerre's manual), and, as stated, 
it is much employed at the present time. It 
consists of a bi-convex crown cemented to a bi- 
concave flint. It is also modified by being 
externally a plano-convex, the flint glass lens 
in this case being plano-concave. FIG. 16. 





44 FORMS OF LANDSCAPE LENSES. 

GruWs Aplanatic. The first departure from the 
above form was made in 1857, by Thomas 
Grubb, who reversed the relative positions of the 
flint and crown, as shown by Fig. 17, in which 
the lens, meniscus externally, is formed of a 
concavo-convex flint cemented to a meniscus 
crown. This lens was found to bear a larger 
working aperture than the one previously men- 
tioned and to have less spherical aberration, 
FIG. 17. hence his selection for it of the name 'Aplanatic/ 
Dallmeyer's Wide -Angle Landscape Lens. In 1865, 
J. H. Dallmeyer introduced a modification of the Grubb 
aplanatic, shown in Fig. 18. In this he divided the 
power of the crown glass into two, one placed in 
front and the other behind the flint glass. In 
this way, by sandwiching the flint concave, which 
was soft, between the two hard crown - glass 
menisci, the twofold purpose was attained of 
securing the softer glass from abrasion and of 
effecting better correction, for he was not con- 
fined to making both the crown elements of 
glass of similar refractive power. A subsequent 
FIG. 1 8. modification of this lens has been made by T. 
R. Dallmeyer, who, while adhering to the same arrange- 
ment and configuration has, by the adoption of other 
kinds of glass than that employed by his father, adapted 
it for working with a larger aperture than was formerly 
employed. 

An American Landscape Lens. A lens achromatised 
in the same way as the ' Globe ' has been employed as 





LANDSCAPE LENS MOUNTING. 45 

a single landscape combination. As shown in the 
figure (Fig. 19) it consists of a concavo-convex flint 
and a meniscus crown, its components being placed 
so that the concave surface of the flint is 
outside. From the fact that this class of 
objective is very seldom to be met with, 
the inference may be deduced that it is 
inferior in general utility to the others pre- 
viously described. 

When, for the purpose of more effective 
correction, the inner or contact curves of a 
lens are of short radius, the thickness may FIG ' I9< 
be reduced by grinding the margin of the concave 
surface flat, as in Fig. 20. By comparing this 
with the previous lens, it will be seen to 
what extent a gain is effected. With contact 
surfaces of the same radius carried out to 
the extreme edge, the lens would be abnor- 
mally thick. The flattened portion is of course 
protected by opaque varnish and a metal 
annulus. The light being transmitted from a 
diaphragm rather close to the concave surface, 
no loss is sustained by the smaller dimensions of 
the less dense positive element. The drawing 
is one of a wide-angle combination formed of 
light and heavy flint rlass, instead of crown and FIG . 20 . 
flint. 

Landscape Lens Mounting. By whichever method a 
single landscape lens is corrected, it must be mounted 
with the stop next to its flatter side, as indicated in 



46 GODDARUS DOUBLE PER ISC OP 1C LENS. 

Fig. 21, which represents one of tie best forms of 
wide-angle lenses. 




FIG. 21. 

Non-distorting Landscape Lenses It was long held to 
be impossible for a lens of the single genus, having a 
diaphragm in front, to give a rectilinear picture ; but, in 
1859, tne l ate James T. Goddard, under the title of the 
double periscopic lens, made several which, however, 
never came into general use. This lens, externally, was 
a double convex, and all the parts were cemented 
together in such a manner as to afford no clue as to its 
internal figuration. One of these which soon afterwards 
fell into our hands was dissected by placing it in warm 
v.ater and melting the balsam. It was then found that 
the interior portion of the lens was a double convex 
crown lens cemented to a double concave flint, the two 
neutralising each other in respect to magnifying power, 



DALLMEYER'S RECTILINEAR LENS. 47 

and that the back element was a meniscus of rather 
deep curvature and formed of crown glass. This lens 
was mounted with a stop in front of it. It will be seen 
from this description that there was a considerable air 
space between the back lens and the cemented portion. 

Dallmeyer's Rectilinear Landscape Lens. In 1887, T. R. 
Dallmeyer introduced a lens which, as implied by its 
name, gave freedom from distortion. In it he makes 
a species of compromise between the purely landscape 
lens and the rectilinear combination, and he effects this 
by displacing one of the crown elements of his triple 
landscape objective and transferring it in a reversed 
position to the opposite side of its confreres. It is known 
to those who have studied this landscape lens, that when 
one of its crown elements is removed, a flint negative 
and crown positive still remain, in which there is little 
or no power either for magnifying or diminishing. It 
is, however, a powerful corrector of the aberration of the 
third element. In Dallmeyer's new lens, he makes as it 
were a separate element of these two glasses and turns 
the convex surface towards the diaphragm, while im- 
mediately behind, he places the crown meniscus, by 
which the focus is determined. As the concave surfaces 
of both are next each other, there is thus an air-space 
left between them. Although this lens possesses some 
general features in common with Goddard, it cannot, in 
any sense, be considered to be a copy of his, because, 
in the first place, the only Goddard lens, the existence 
of which we can learn, has never been out of our own 
possession since it was made, and secondly, that Dall- 



48 ADVANTAGES OF SINGLE LENSES. 

meyer's lens has a totally different internal construction 
which, we may add, gives it great advantages in covering 
power with better definition. 

Advantages of Single Lenses. Single lenses possess an 
advantage over double ones in respect to the pluck and 
vigour which they yield, a landscape being the subject. 
Being formerly constructed with only small diaphragms, 
they were necessarily slow in action, but this drawback 
has now been surmounted, some of the better class 
working with an aperture of even f-S. With an aperture 
of this width it need scarcely be said portraits can be 
easily obtained ; indeed, for portraiture, there is a special 
charm in this class of lens, on account of the delicate 
softness which can be obtained when working with an 
abnormally great angular aperture. Of course, to cover 
an extended field sharply, a small stop must be employed, 
and as we have shown in anothei chapter the stop in 
the lens must be so placed as not to give a flare-spot. 



CHAPTER X. 

DISTORTION : ITS NATURE AND CURE. 

THERE are several kinds of distortion capable of 
being produced in photography. These include that of 
' violent perspective/ caused by placing the camera too 
close to the object to be taken, whether portrait, land- 
scape, or building. 

Point of Sight too Near, This kind of distortion is 
seen in portraits in which the feet or hands are pro- 
jected forward and represented on a scale of magnitude 
greatly surpassing that of the figure itself. The remedy 
for this consists in removing the camera to such a 
distance from the object as to reduce all the parts 
practically to the same uniform scale of representation. 
The employment of lenses of too short a focus has much 
to answer for in the production of this distortion of per- 
spective. In these cases the perspective itself is not 
necessarily false it is only violent ; but it conveys an 
erroneous idea. In landscapes it causes insignificant 
ponds in the foreground to become considerable lakes, 
and tiny rivulets to assume the magnitude of rivers. 

Distortion of Convergence. There is also a very common 
form of distortion exemplified in the contraction of the 

E 



50 DISTORTION. 

scale of representation towards one margin of the 
picture. It is usually seen in photographs of buildings, 
and gives them an appearance as if they were leaning 
towards an imaginary central line for support. This 
may be termed the ' distortion of convergence.' It 
arises from no fault in the lens, but from the want of 
care or of knowledge in the photographer, who, desirous 
of including the whole of a building in his plate, has 
tilted his camera slightly upwards without utilising its 
swing-back to bring the plate into a perfectly vertical 
state ; for one of the rigid conditions which govern the 
taking of a building properly is this that, no matter 
how much the lens or camera may be pointed upwards, 
the plate itself must be perfectly vertical. 

Curvature of Straight Lines, There are other kinds of 
distortion, but none that is justly chargeable to the lens 
save that very important one known as ' curvilinear 
distortion,' the chief characteristic of which is the curva- 
ture imparted in the photograph to lines that are quite 
straight in the original. This defect is produced solely 
by the lens, and no skill in the photographer can obviate 
it so long as a lens of that description is employed. 

Every ordinary objective having its stop between the 
lens itself and the subject to be reproduced will give 
distortion. No matter how perfect a landscape lens 
may bo how superb its definition or penetrative its 
range ; though it may reproduce the finest line of the 
finest engraving with all the crispness of the original 
and delineate the very structure of the stones of which 
an edifice is formed, yet it will not be either a copying 



btSTORTION. 51 

or an architectural lens. These demand not only all the 
qualities mentioned but also something more, namely, 
absolute rectilinearity in projection. The appearance 
presented in a photograph taken with a landscape lens 
in which a building is made to cover nearly the entire 
plate suggests the form of a huge, wide barrel, owing to 
all the straight lines curving inwards towards the centre. 
In such a picture only two lines are quite straight 
those which pass vertically and horizontally through the 
centre of the photograph. No matter how much the 
lens distorts, these centre lines are always straight ; but, 
in proportion as we proceed towards the margin, we 
find them becoming more and more curved. As this 
defect is not much noticed near the centre, it follows 
that one may take a view of a house or church without 
any apparent distortion so long as its position is kept 
near the centre of the plate ; but for copying a map, 
chart, or any kind of engraving in which accuracy is a 
sine qua non, it is altogether unsuitable. 

Cause of Distortion. Having indicated the nature, 
we shall now consider the cause of distortion. Bearing 
in mind what has been said in a previous chapter con- 
cerning the possibility of taking a photograph without 
any lens whatever, merely by transmitting the rays from 
the object through a pinhole aperture in front of the 
camera, we remark that any copy of a picture or repre- 
sentation of a natural object made by such pinhole will 
be quite rectilinear, for with such an arrangement the 
light passes in straight lines without refraction. Let 
us consider in what manner these rays are influenced 



52 BARREL DISTORTION. 

by a lens so as to disturb rectilinearity of projection. 
It has been shown that the margin of a lens refracts 
in a greater degree than its centre ; that, in short, one 
of a set of parallel incident rays is transmitted through 
the centre of a lens without undergoing any refraction 
at all, and that in proportion as the point of trans- 
mission is near the margin or towards the centre so 
does the ray thus transmitted become refracted in a 
greater or lesser degree. All rays which come from 
a perfectly square map or building are quite right 
while passing through the diaphragm and up to their 
passage through the lens, when they are brought under 
the influence of its dimensions, with the result already 
described. The square original becomes barrel-shaped 
in the photograph, as shown in Fig. 22, in which the 




FIG. 22. 



curvature is, in order to show the principle, more pro- 
nounced than it would be with an ordinary photographic 
lens, because as the margin of a picture is taken with 
the margin of the lens, that margin, owing to its superior 
refractive power 'condenses' the rays into a smaller 
space or bends them towards its axis, thus causing a 
given portion of the original to occupy a smaller space 
near the margin than it would do at the centre of the 
photograph. 



PINCUSHION DISTORTION. 53 

This is the invariable result of employing a lens, 
such as a single landscape objective, in which the stop 
is in front. What, it might be inquired, would be the 
result if the objective were turned round so as to allow 
the light to pass through the lens before it reached the 
diaphragm ? Simply this : that the nature of the dis- 
tortion would be changed. There would be an expansion 
of the scale at the margin instead of a reduction as in 
the former case, and the resulting picture would have its 
marginal lines bent outwards like a pincushion, as shown 
in Fig. 23, from which has arisen the term ' pincushion ' 




FIG. 23. 

distortion, now recognised as the antithesis to the 
' barrel ' distortion already described. 

The nature and cause of distortion having been 
explained with all the fulness required in a popular 
disquisition like the present, we now come to speak 
of the various methods adopted for effecting its cure. 
During a long period it was the earnest aspiration of 
both opticians and photographers to obtain a lens which 
would give freedom from distortion, and here in this 
connexion we would record one of the most remarkable 
things in optical history. While opticians were straining 
to devise a lens which should give freedom from dis- 
tortion, it was already in their hands, although seemingly 



54 



DISTORTION. 



they knew it not. So long ago as 1844, Geo. S. Cundell 
had published in the October number of the Philosophical 
Magazine of that year a symmetrical combination of 
lenses of form similar to the rapid rectilinear class of 
the present period, being meniscus lenses, although un- 
corrected, with a diaphragm midway between the lenses. 
How opticians failed to recognise in this combination 
the panacea for the evils of distortion is truly surprising. 
Cundell's lens, which was apparently entirely lost sight of 
by opticians, having been achromatised, is now the lens 
of the day. And here, as a sequel to the immediately 
foregoing illustrations of the two kinds of distortion 
produced by single lenses, we show in what manner the 
combination referred to cures distortion (Fig. 24). It 




FIG. 24. 

eventually began to dawn upon those people who gave 
thought to the matter, that if a diaphragm placed in 
front of a lens gave barrel-shaped distortion, and a 
diaphragm behind the lens gave distortion of the opposite 
or pincushion character, a diaphragm placed midway 
between two lenses would give no distortion at all. 
And so it was. 

But at a date three years anterior to the publication 
of the Cundell lens, the late Andrew Ross had con- 
structed for Henry Collen a portrait-objective, composed 



ORTHOSCOPIC LENS. 55 

of two plano-convex achromatic lenses with a stop 
midway between, and during the subsequent years of 
the life of this optician it does not appear to have 
occurred to him (as it did twenty-three years afterwards 
to his son, Thomas Ross), that this form satisfied the 
conditions of freedom from distortion, and that it was 
only necessary to make its components of a meniscus, 
instead of a plano-convex form, to flatten its originally 
round field. 

Contemporaneous with Andrew Ross was Thomas 
Davidson, an Edinburgh optician, who produced some 
symmetrical achromatic lenses which were quite free 
from distortion, which will be described in another 
chapter. 

The Condition for ensuring N on -distortion. The con- 
dition that must be fulfilled by any combination of 
lenses in order that there shall be no distortion, is this 
each ray that enters the combination must emerge from 
it in a direction parallel to that of its entry. If the 
immergent ray makes a certain angle with the axis of 
the lens, the emergent one must make a similar angle. 

Advent of the Orthoscopic Lens. The cry for non- 
distorting lenses was at its loudest, and all those just 
described had been forgotten or ignored when, in the 
beginning of 1857, Voigtlander introduced his orthoscopic 
lens, which was constructed on a formula supplied by 
Petzval. The orthoscopic lens became the ( rage.' 

Several claims were put forward on behalf of this 
lens, and Thomas Sutton, editor of Photographic Notes, 
who was a facile writer on mathematical optics, descanted 



56 ORTHOSCOP1C LENS. 

in the most rapturous terms upon its numerous virtues 
its entire freedom from distortion, its flatness of field, 
equality of illumination, perfection of focus, and freedom 
from spherical aberration. The orthoscopic lens was 
to prove the panacea for every ill. It was everywhere 
spoken of; and, having become the fashion, there were 
some weak-minded photographers who scarcely dared 
venture to assert that any specially fine picture they 
had taken had perchance been obtained by the aid of 
the old-fashioned landscape lens. But fashions change 
in lenses as in other things, and subsequently it was 
found that the once-idolised orthoscopic lens did not 
possess freedom from distortion, that its field was not 
flat ; that in equality of illumination and perfection of 
focus it was not a whit better than the old landscape 
lens. And so the orthoscopic lens was deposed from 
its position of reigning favourite and well-nigh lost sight 
of. What is to be regretted is that the foolish claim 
implied in its name was ever put forth, because any 
careful observer could upon close examination have dis- 
covered that it did distort, although from the position 
of the diaphragm the distortion was of an opposite 
character to that previously experienced. 

In a subsequent description of lenses we shall not 
be disposed to treat the orthoscopic objective as defunct, 
because, when the absurd claim made for it upon its 
introduction has been set aside, it possesses special 
features and virtues of a marked order which may 
eventually secure for this instrument a recognition and 
patronage, doubtless, greatly exceeding that first accorded 



ORTHOSCOPIC LENS. 57 

to it. In saying this we are fortified by the expression 
of opinion of one of the ablest mathematical and practical 
opticians of the present time, to the effect that in the 
orthoscopic lens, when subjected to certain modifications 
of structure, may yet, possibly, be found one of the 
'lenses of the future/ 

A word in passing concerning the name 'orthoscopic' 
or 'orthographic' signifying respectively correct seeing 
or correct delineating. It is much more applicable to 
the doublet lenses of the present time, which are really 
rectilinear (a term having an analogous meaning), than 
to that form about which we have been writing. We are 
rather pleased than otherwise to find that an American 
optician has lately re- adapted the name to a lens of the 
rectilinear class which he makes. 



CHAPTER XI. 

NON-DISTORTING LENSES. 

THE nature of distortion having been fully treated 
in a previous chapter, we now enter upon a consideration 
of the various lenses which have been constructed with 
a special view to freedom from this error. 

The Ortkoscopic Lens. We have already alluded to 
this as having been the first objective presented to the 
public with a direct claim to correctness in linear pro- 
jection, although such claim was subsequently abandoned 
The following is a description of it : It consists of a 
plano-convex or nearly flat achromatic meniscus, similar 
to the front lens of the Petzval portrait combination, 
and used in the same position. At a very short distance 
behind this is placed an achromatic lens, somewhat 
smaller in diameter and concave as a whole ; that is 
to say, it diminishes instead of magnifies. Although 
Voigtlander, the first maker of this objective, con- 
structed it with a smaller back than front these being 
in the ratio of 2\ inches to ij inches yet did some 
other makers form both front and back of equal 
diameters. The first element of this back lens is 
formed of a bi-concave crown glass, the radii of the 



ORTHOSCOPIC LENS. 



59 



surfaces being unequal ; the second element is a flint 
glass meniscus, and this back lens both materially 
lengthens the focus of the front one and flattens the 
field, at the same time correcting the oblique pencils. 




FIG. 25. 

This it does in right of the fact that an oblique pencil 
falling upon a concave lens is powerfully affected by it, 
being considerably lengthened in focus. Indeed, with 
a combination of this nature, it is easy to have a back 
lens of such a kind so adjusted to the front as to cause 
the oblique pencils to be so much longer than the 
central ones, that the field shall be not merely flat 
but bellied in the opposite direction from that in 
which photographers are accustomed to see it. 

Much was said concerning the equality of illumi- 
nation possessed by the orthoscopic lens at the time 
it was introduced, and much was written, even by talented 



60 ORTHOSCOPIC LENS. 

opticians (e.g. Andrew Ross) to prove its superiority in 
this respect over the single achromatic landscape lens ; 
but although the author possesses some of the best 
specimens of this kind of lens that have been made, 
he has quite failed to discover their superiority in this 
respect over ordinary lenses. 

The Causes of Unequal Illumination. These are, first, 
the fact that a pencil transmitted obliquely through a 
circular aperture (the diaphragm) is smaller than one 
transmitted directly or centrally through the same 
aperture ; and, secondly, that the pencil thus transmitted 
obliquely is not merely smaller in diameter, but it has 
farther to travel and more work to accomplish. This 
is the case with every lens by which an oblique pencil 
is transmitted through a circular aperture. 

Position of the Diaphragm in the Orthoscopic Lens. 
The orthoscopic lens was somewhat extensively con- 
structed by opticians after its introduction, and was 
sold under a variety of names. It is worthy of being 
noted that while Voigtlander placed the diaphragm 
behind the back lens, Ross inserted it between the 
front and back, while Goddard placed it outside of 
the front lens. It is difficult to surmise why he did 
so, unless on the supposition that realising the optical or 
focal centre was quite outside of the front lens, he sought 
to minimise distortion by having the stop as near to 
that centre as possible. 

A Unique Property in the Orthoscopic Lens. A special 
virtue possessed by the orthoscopic lens, and by no other, 
consists in the ability of obtaining with it larger sized 



bOUBLE PERI SCOP 1C. 6l 

images in the negative with a given extension of camera 
than can be obtained by any other lens extant. The 
size of the image depends upon the focus of the lens 
by which it has been taken. The focus of a lens is 
measured from and determined by the position of its 
focal centre ; and while this in a single landscape lens 
is rather nearer to the ground glass than the lens itself, 
it is in the orthoscopic combination, as just stated, out- 
side of the lens entirely, so that, with a given length 
of camera, a much larger image of an object can be 
obtained by the orthoscopic lens than by any other. 
This is a property of great value. 

Goddard's Double Periscopic. The name of James T. 
Goddard occupies an honourable position among those 
opticians who have directed their efforts to the intro- 
duction of lenses different from those which previously 
existed, in order to eliminate with more or less success 
their inherent faults. 

Among lenses introduced by Goddard was a recti- 
linear landscape objective which he designated his 
'double periscopic' lens. This was in January, 1859. 
Externally this lens was of double convex form ; but 
there was an air-space inside, and it was constructed as 
follows : The front surface was that of a biconvex 
crown, cemented to a biconcave flint, these two forming 
a meniscus combination without any positive or mag- 
nifying power. Cemented by its margin to this was 
a meniscus of crown glass, the residuum of the over- 
correction for colour of the front portion effecting the 
correction of this meniscus. Used with a diaphragm 



62 GODDARDS TRIPLE LENS. 

in front, this objective was free from distortion. Its 
marginal definition, however, is inferior to another since 
constructed, on the same general principle, by T. R. 
Dallmeyer, and as an independent invention, he not then 
being aware of Goddard's lens. See pages 46 and 47. 

Goddard's Triple Lens. About the same time as the 
' periscopic ' was introduced, Goddard constructed a 
triple objective, the front of which was an ordinary 
shallow achromatic meniscus, the centre lens being a 
biconcave and the back a deep meniscus. The centre 
lens was smaller than the others, but neither it nor the 
back lens was achromatised. The front achromatic and 
the back meniscus were of similar f~ci, the power of the 
intermediate concave being such as to neutralise the 
magnifying power of either of them. 

Goddard's Combination Landscape Lens. This lens, 
introduced at the same period as the two preceding, has 
an achromatised, front of meniscus form. The anterior 
of the back combination is a 
biconcave of crown glass, the 
posterior being a meniscus also 
of crown. The curvatures of 
these two are such as to prove 
that they possess no magnifying n 




power. The distance apart of FIGi 26 

the front and back combinations 

is not an arbitrary one, but may be altered to suit 
the circumstances of each case. When separated some- 
what, the marginal definition is much improved and 
the field flatter than when they are brought close 



SUTTOWS SYMMETRICAL TRIPLET. 63 

together. The focus of the combination is, therefore, that 
of the achromatic meniscus ; and Goddard's idea was 
to supply a variety of these mounted in separate cells, 
so that the photographer having a mount containing 
one correcting back lens for he (Goddard) preferred 
giving the achromatic lens the anterior position in the 
mount could make use of several achromatic lenses of 
any required focus. 

The advantages claimed for the combination just 
described were freedom from distortion, flatness of field, 
and the ability for adapting a number of front lenses, 
each varying in focus from another, to the combination. 
On referring to some notes made when inspecting 
Goddard's work-book after his death, we find that he 
frequently departed from the form shown in the above 
diagram, occasionally, inter alia, adopting the plano- 
concave instead of the double-concave form for the 
crown, and sometimes separating the two crown glass 
lenses to a considerable extent. 

Button's Symmetrical Triplet. In 1860 Thomas Sutton 
introduced a lens under this name. It was composed of 
two achromatised plano-convex lenses of similar foci, 
mounted at either end of a tube, with a simple bi- 
concave lens in the middle. The power of this latter 
was such as to neutralise either of the outer two, but 
only few of them were made. 

Dallmeyer's Triple Achromatic. A special form of 
triple lens which secured a great degree of favour 
among photographers is shown in Fig. 27, which re- 
presents the triple achromatic combination of J. H. 



6 4 



DALLMEYEKS TRIPLE ACHROMATIC. 



Dallmeycr. The flatter surfaces of the front and back 
lenses are slightly concave, differing to this extent from 
a triple lens subsequently introduced by Thomas Ross, 




FIG. 27. 

in which these surfaces were quite flat. The triple 
just shown, together with the others mentioned in this 
chapter, is quite free from distortion. 

By increasing the diameter of the middle lens, Dall- 
meyer subsequently constructed a triple objective having 
an angular aperture sufficiently great to enable it to be 
employed for groups and portraiture. 



CHAPTER XII. 

WIDE-ANGLE NON-DISTORTING LENSES. 

Defining the Term. It is difficult to draw a sharp 
line of demarcation between narrow angle, medium 
angle, wide angle, and panorama, seeing that they im- 
perceptibly merge into each other. We may, however, 
hazard the opinion that an included angle of subject 
up to 25 fittingly comes under the first of these terms; 
one up to 45 being medium ; whilst a lens that includes 
more than a view of which the base equals the focus, 
may be relegated to those of wide angle. But many 
wide-angle lenses include an angle of 90 on the base 
line, and hence the application of the distinguishing 
terms can at best be only approximative. 

Button's Panoramic Lens. This lens, which doubtless 
covered a wider angle than any previously introduced, 
must at present be spoken of in the past tense, none of 
them being now made. It was composed of two thick 
concentric shells of flint glass, all the surfaces being 
measured from a common centre. It was in effect a 
sphere of glass, the space in the middle being filled 
with water. It was achromatic, and the spherical 

F 




66 THE GLOBE LENS. 

aberration was sufficiently corrected to admit of its 
taking pictorially sharp photographs. An ingenious 
' butterfly ' diaphragm was de- 
vised, by which the extreme 
side of the image was illumi- 
nated with the same intensity 
as the centre. But at the time 
when it was introduced, it was 
imperative that the image had 
to be received on a curved or 
cylindrical plate, the printing- 
frame and other fittings being 

also curved, and this led to its manufacture being 
discontinued. 

The objection formerly existing need not now pre- 
vail, for sensitive celluloid or other flexible plates can 
be placed in flexible or roller slides, and, by suitable 
curved guides at the back of the camera, can be tem- 
porarily bent in the cylindrical form and afterwards 
flattened out. The author has ascertained from actual 
experiment the practicability of the suggestion here 
made, having taken by one of these lenses and on a 
celluloid film a panoramic view embracing an angle 
of 125 in the fractional part of a second an angle 
exceeding by 5 that which the lens was originally 
computed to cover. 

The Globe Lens. This is an American production, 
so named because its external surfaces, like those of 
Button's, formed portions of a sphere relative to each 
other. It is composed of a symmetrical pair of deep 



MORRISON'S WIDE-ANGLE LENS. 




FIG. 29. 



to exorcise the 



meniscus lenses, achromatised by the union of a con- 
cavo-convex flint cemented to a meniscus crown, the 

latter being placed outside, as shown in the cut. The 

diaphragm is in the middle of 

the objective. Some of the 

Globe lenses gave a flare-spot 

or ghost in the centre of the 

picture, and it does not seem to 

have occurred to C. C. Harrison 

of New York, the maker, to 

have set aside the * globe ' idea in 

their construction, and mounted 

them a little closer together. 

This slight modification we found 

ghost entirely. 

The ' Globe ' was subjected to modifications by other 

makers of the period and country, but the same general 

feature pervaded them all. 

Morrison's Wide-angle Lens. Richard Morrison, on 
the death of Harrison, in whose em- 
ployment he had long been, conceived 
an idea that some advantage, especially 
in construction, would accrue by slightly 
over-correcting one of the lenses of the 
'globe/ and supplying the place of the 
other with a simple crown-glass me- 
niscus This idea was not quite original, 
for, so long ago at 1857, it was placed 

upon record that F. H. Wenham had had a lens (a 

narrow-angle one, however) constructed for him in which 




FIG. 30. 



(l 



68 DOUBLETS. 

the front was a plano-convex lens of crown glass, the 
back lens being an over -corrected achromatic, also of 
plano-convex form. We carefully examined a lens 
received direct from Mr. Morrison, and found that 
although the front lens was not really over-corrected 
for colour, yet that the addition of the crown-glass back 
did not appreciably affect its working to visual focus. 
Lenses of deep meniscus form possess a wonderful de- 
gree of elasticity as regards focus. 

Steinheil's Periskop. Dr. A. Steinheil, about a 
quarter of a century ago, introduced 
a symmetrical doublet constructed ex- 
pressly for including a wide angle. 
It was of simple form, being composed, 
as shown in the figure, of two simple 
or uncorrected lenses formed of crown 
glass. It embraced a very wide angle 
of view, but having an exceedingly 

FIG. 31. 

small diaphragm it worked slowly in 
those days of wet collodion, and, besides, the visual 
and chemical foci were not coincident. 

Zentmayei's Lens. Josef Zentmayer, of Philadelphia, 
improved upon the Steinheil periskop by making it un- 
symmetrical, the back lens being of shorter focus than 
the front, and the diaphragm being placed nearer the 
back, in the ratio of the foci of each component. 

The Doublets of Grubb and Ross. We have to link 
these optical productions together, because both were 
introduced at the same period. It became more in- 
timately associated with the name of Ross, as Thomas 



GENESIS OF THE ROSS DOUBLET. 



69 




Ross manufactured it in three different degrees of in- 
cluded angle, while the professional engagements of 
Thomas Grubb, as chief engineer to the Bank of Ire- 
land, prevented him at that time from bestowing much 
attention upon it. A good idea of its nature will be 
ascertained from the figure, in which 
A is the front lens and B the back 
lens. By rendering the components 
of a more pronounced meniscus form 
and bringing them closer together, 
T. Ross made the objective include 
a still wider angle. Owing to the 
proximity of the diaphragm to the lenses, this com- 
bination is singularly free from flare. 

Genesis of the Doublet. Before dismissing this lens, 
we present drawings of both the original and the last 
form assumed by it. Fig. 33 shows the objective made 
in 1841 for Henry Collen. 
In it both lenses were 
plano-convex; they were 
separated by a consider- 
able space, had a rather 
small diaphragm in the 
centre, and gave a round 
field, so much so that when taking portraits by it (for it 
was constructed specially for portraiture) it was neces- 
sary to have the sensitive negative paper pressed in shape 
between two glasses bent in spherical form. The latest 
form of that doublet is shown in Fig. 34, which differs from 
that first shown by having its elements set closer together. 



FIG. 33. 




70 DA VIDSOWS COMBINA TTON. 

The doublet was in all cases made of flint and 
crown glass, and hence required a rather small dia- 
phragm ; but we have lately seen 
some lenses formed of two kinds 
of flint glass and figured like 
that shown on the previous page 
(see Fig. 32), which work with an 
aperture as great as that of our 
most modern lenses. 

Davidson's Combination. In 1841, 
Thomas Davidson, a well-known 
Edinburgh optician, constructed 
symmetrical lenses, concerning 
which it is worthy of notice that 
FIG ' 34 * they were externally similar to the 

most approved rapid doublets of the present day. Each 
lens was formed of a plano-convex crown, cemented at its 
surface to a plano-concave flint. We had a lens of this flat 
class made by a son of Davidson more than a quarter 
of a century since, and so well did it work that it is 
doubtful if, with all our modern appliances, much better 
pictures can be taken now than were produced by this 
lens invented fifty years ago. Why, it may be inquired, 
was it allowed to fall into a state of desuetude ? We 
reply : Davidson introduced it as a portrait lens, for 
which purpose it could not compete with the Petzval 
portrait combination introduced about the same time 
by Voigtlander. The processes practised in those days 
were slow, and the most rapid portrait lens was that 
which secured preference. 



WIDE-ANGLE APLANA T. 




FIG. 35- 



Dallmeyer's Wide-angle Rectil near. In this objective 
the lenses are both of the form in which the denser 

, material of the achromatic lens is 
placed to the outside. Although 
Dallmeyer made them for the 
most part as shown in the figure, 
that is, with a front lens of larger 
diameter and longer focus than 
that of the back lens, yet are 
they also made symmetrical, 
especially those of short focus. 
They are formed of flint and 
crown glass. 

Steinheil's Wide-angle Aplanat. 
Steinheil having recognised 
the advantages accruing from the exclusive employ- 
ment of flint glass of different refractive and dis- 
persive ratios, as employed in his rapid aplanat, 
afterwards constructed one on the same general system 
but of small diameter so as to be quite portable. The 
lenses were thicker than those usually made of similar 
diameter, and were set so closely 
together as in some instances to 
barely allow the diaphragm to 

- be inserted between them. These 
lenses include a very wide angle, 
and are quite free from the flare 
spot. They are manufactured 
by various makers, in many cases 

under the trade designation of 

FIG. 36. 




STEINHEWS ANTIPLANET. 



the Portable Symmetrical, which was first given them 
by Ross & Co., although other makers adopt different 
names. They work for the most part with an aperture 
equalling a sixteenth of their focus. A distinguishing 
characteristic of the Ross portable symmetrical is the 
identity of diameter of mounts and flanges of all the 
usual foci, and the great perfection of detail given by it 
over the large angle included. 

Steinheil's Antiplanet. This lens partakes of the nature 
of the orthoscopic objective to this extent, that the front 
lens is a positive and the back a negative combination, 
although the latter is so to only a 
very slight extent. It will be seen 
from the cut (Fig. 37) that the back 
lens possesses an unusual degree of 
thickness, this being necessary to cor- 
rect the aberrations of the anterior 
combination. Steinheil makes the 
antiplanet in two forms, one having a 
smaller angular aperture than the other. The former, 
which we have figured here, is intended for groups, &c. 
The latter is a portrait lens, which works at/-4, and it 
differs from the group lens by the introduction of an air 
space between the elements of the back combination. 




FIG. 37. 



CHAPTER XIII. 

PORTRAIT LENSES. 

BY a portrait lens, or combination of lenses, is meant 
jne having an aperture so large in comparison with its 
focus as to admit a volume of light of sufficient intensity 
as to enable portraits to be taken in the subdued light 
of a studio in the briefest possible period of time. 

Being aplanatic, a portrait lens is capable of defining 
sharply without any diaphragm, although, as we shall 
eventually show, a diaphragm is indispensable for se- 
curing its full advantages. It may be urged that any 
lens by which a portrait is capable of being produced 
may be entitled to the designation of a ' portrait lens,' 
but in technical language the term is only applicable 
to those of a certain description, between which and 
the original landscape lens there are now so many 
grades as to render somewhat difficult the drawing of 
a hard and fast line. 

History of the Portrait Lens. The portrait combi- 
nation is a triumph of optical skill, and in its original 
and general form is an emanation from the mathe- 
matician, Professor Petzval, of Vienna. The history 
of its inception may be told in a few words : In 



74 PETZVADS PORTRAIT LENS. 

1840 Professor von Ettingshausen, having returned 
from a visit to Paris, where the daguerreotype process 
was engaging the attention of the scientific world, 
remarked to Petzval that Daguerre, with whom he 
had been in direct intercourse, made use of a lens 
having a small diaphragm, by which a great loss of 
light ensued, and inquired if he (Petzval) could not 
devise a better form of lens. Acting upon this hint 
Petzval instituted researches, and the year following 
(1841) gave to Voigtlander at that time an optician 
enjoying a high reputation the formulae for two ob- 
jectives, both of them working without a diaphragm. 
One had a large aperture and short focus, and gave 
great concentration of light over a large area ; the 
other had a longer focus, and was capable of covering 
a large field. The former was the now well-known and 
universally-used portrait lens, the other being the ortho- 
scopic, which was allowed to lie perdu for several years 
afterwards. A becoming distinction not having at that 
time been recognised between actinic and visual achro- 
matism, the lenses of early tirr.cs had what has been 
succinctly designated a ' chemical focus ' a fault which 
is now eliminated from the productions of every lens 
manufacturer of eminence. Thus much by way of 
remark on the early history of the portrait combination. 
What is Angular Aperture? The leading distinction 
between the portrait and other lenses is implied in 
the term ' angular aperture.' This it is which de- 
termines rapidity. Angular aperture has no relation 
to actual size or diameter of lens, except so far 



ANGULAR APERTURE. 75 

as such relates to focal length ; hence a lens only 
one inch in diameter may be a much quicker-acting 
instrument than one of three inches, because of its 
aperture being larger in proportion to its focus. In 
making choice of a lens for rapidity of action care must, 
therefore, be taken to select one of short focus in pro- 
portion to its actual diameter. The acting angular 
aperture of a lens varies with every different stop that 
is used ; and it is frequently necessary to reduce this 
aperture considerably not for the sake of weakening 
the light and thus protracting the exposure, but in ordei 
to confer a greater degree of penetrative power, foi 
'depth of focus' varies in inverse ratio to angular 
aperture. When a comparison of lenses is made in 
order to determine which is the better, both should be 
as near as possible of similar diameter and focus ; because 
two lenses may be of the same diameter say three inches 
but one of them having a focus of six inches and the 
other of twelve inches, the difference between the two 
as regards rapidity will be this that the one of twelve 
inches will necessitate an exposure four times longer 
than that required by the other in order to obtain 
equally exposed negatives. Again : two lenses may 
have the same focus, one of them having a diameter 
of three inches, while that of the other is only one inch 
and a half. The former possesses four times the in- 
tensity of the latter, and will work in a fourth of its 
time. A just comparison cannot be made between 
two lenses of the same focus but dissimilar dimensions 
unless both are stopped to the same extent 



76 PORTRAIT LENSES. 

The portrait objective consists, as shown in the ad- 
joining diagram (Fig. 38), of two achromatic lenses of 
dissimilar form mounted at some distance apart. The 
anterior lens is a plano-convex, or, more usually, a 
meniscus of such a slight external concave curvature 
as to seem to a cursory observer to be plane. Its 
component parts are a crown glass double - convex, 




FIG. 38. 

attached by transparent cement to a piano - convex 
flint lens. The posterior lens is a double-convex com- 
posed of a bi-convex crown and a concavo-convex 
of flint glass. The inner curves of these are not con- 
centric, as in the anterior lens. They are usually 
mounted so as not to touch each other, and when 
tested as a whole will be found lacking in the power of 
bringing rays to a sharp, or even moderately sharp, focus. 
Properties of Back Combination. The back combi- 
nation of a portrait lens fulfils a twofold function : 



&ACK COMBINATIONS. '17 

it shortens the focus, and thus aids in conferring in- 
tensity of illumination ; it also distributes over a 
flatter field the image formed by the anterior lens, 
which, without the correcting influence of the back 
lens, would be sharp only over a very limited area. 
This is the principal function of the back lens, and 
it performs it because of its excess of negative sphe- 
rical aberration a property that will be observed 
readily if the posterior combination be employed as 
a magnifier in the examination of any printed matter, 
when it will be found that the focus of the centre 
is shorter to a considerable extent than that of the 
margin. Seeing that this property of negative aberra- 
tion is modified by the distance apart of the elementary 
components of the posterior lens, it is frequently possible 
to convert a bad lens into a good one by a slight ad- 
justment of this portion of the objective. Many portrait 
combinations have the back lenses placed loosely in the 
cell, with a flat ring of brass between to keep them 
apart. An objective of this class, four and a half inches 
in diameter, intended for 15 x 12 negatives, which per- 
formed very badly in consequence of its roundness of 
field, the centre of the picture only being sharp, had 
the separating ring of the back components entirely 
removed, and with marked advantage. This posterior 
combination was now found to have its negative aber- 
ration greatly increased, for the separating ring was 
half an inch in width. Now, as the anterior lens of 
the objective was of much shorter focus than the back 
.one, it was considered necessary, in consequence of the 



7 8 DALLMEYEKS BACK LENS. 

now increased negative aberration of the back, to bring 
the front and back lenses much closer together. Ac- 
cordingly, after a few trials the tube was shortened to 
the extent of an inch and three-quarters, with the 
gratifying result of the objective working in an exceed- 
ingly satisfactory manner, and taking a sharp portrait 
on a 15 x 12 plate the full size it was intended to 
cover. This incident is mentioned because a bad lens 
was converted into a good one without a necessity 
being experienced for regrinding any of the surfaces. 
Dallmeyer's Back Lens. A form of back lens, dif- 
fering from that of Petzval, was introduced several 
years ago (1866) by J. H. Dallmeyer, in which both the 
forms and the relative positions of the components 




FIG. 39. 

are reversed. Its nature will be ascertained from the 
diagram (Fig. 39), in which the back lens is seen 
to consist of a shallow meniscus formed of a con-* 



WATERHOUSE DIAPHRAGMS. ^9 

cavo-convex of flint (the convex side being nearest 
the ground glass) and a meniscus of crown. The two 
lens are so constructed that when placed as closely 
in contact as possible the objective will give sharp 
definition, but when separated in even a very slight 
degree spherical aberration is introduced to any desired 
extent, thus lowering the definition. This form of back 
combination is now adopted by some of the leading 
Continental opticians, who burnish the two lenses in 
one cell, thus discarding the advantage conferred by 
separation of the constituents. 

Waterhouse Diaphragms, All portrait objectives of 
any pretensions to the highest quality are now fitted 
with diaphragms. At first these were inserted in the 
hood of the lens, and kept in their place by a ring 
the width of the hood. It then occurred to Mr. Lake 
Price to slit the tube so as to drop in one of a series 
of loose diaphragms between the lenses ; but the in- 
vention is now associated with the name of Dr. Water- 
house, who further simplified the system. 

Discoloured Glass. We have spoken of angular 
aperture as the great requisite towards rapidity ; but 
there is another which, while less essential, is of great 
importance. We refer to quality of glass. Both crown 
and flint optical glass are sometimes apt to be a little 
'off' the colour even when made, and it is a well- 
known fact that discoloration occurs in some lenses by 
merely exposing them 1o a strong light. This will 
be more specially alluded to in a subsequent chapter. 



CHAPTER XIV. 

RAPID LANDSCAPE, GROUP, AND COPYING LENSES. 

Nature of a Rapid Lens, What constitutes a rapid lens 
is not very easy to define. That a portrait combination, 
having a large angular aperture, is really the most rapid 
worker of all no one can for a moment entertain any 
doubt ; and yet it is not ' rapid ' in the sense in which 
we have now to speak of the instrument, but must be 
suffered to remain outstanding, and yield the phraseo- 
logical distinction to others much slower. 

The term, first introduced by Mr. Dallmeyer to 
distinguish one of his rectilinears, may be considered as 
now applying to combinations constructed for the 
purpose of including a wider angle than the portrait 
lens on the one hand, and a smaller angle, on the 
other, than can so easily be obtained by the wide- 
angle, non-distorting lenses which were described in the 
preceding chapter. Any combination which will in- 
clude a moderate angle of view, such as two-thirds of 
its focus, with an aperture from f-6 to/-io, and be free 
from distortion, is entitled to be considered a ' rapid ' or 
aplanatic lens. 

The first of this class of which we possess any record 



RAPID RECTILINEARS. 81 

was issued in July, 1866, by the late Dr. Steinheil, at the 
suggestion of the late Dr. Monckhoven, who supplied 
the required data which should be kept in view in the 
construction of such a lens as was at that time con- 
sidered a desideratum. The instrument which resulted 
from a conference between the two savants possessed 
an aperture equalling one-seventh of the focus. It was 
formed of two different kinds of flint glass. But in a 
patent obtained by Mr. J. H. Dallmeyer about the time 
of the issuing of the Steinheil aplanatic lens as the 
new claimant for public favour was designated the 
principle upon which this lens is constructed was em- 
braced ; for in the specification of the patent which has 
primary reference to the wide-angle rectilinear described 
and figured in our last chapter, together with the back 
combination of his portrait objective, and which patent 
was obtained in the course of the year above mentioned, 
he says : ' A lens may be constructed according to my 
invention of flint glass only, necessarily of two dif- 
ferent kinds as regards density for the production of 
achromaticity, instead of, as is usual, crown and flint 
glass.' 

There is ample evidence that these two ivorkers were 
employed in independent investigations, although in the 
matter of publication Steinheil had the priority. 

Modifications. Although the general principle of 
construction is similar in all of the * rapid ' type of 
lenses, with one exception, yet several modifications 
as regards curvature and densities of glass have been 
made by the respective manufacturers of this rapid 



82 



ADVANTAGES OF DENSE GLASS. 




doublet. The accompanying diagram (Fig. 40) is 
sufficiently accurate to describe nearly all ' rapid ' lenses 
(with the one exception alluded to) by whomsoever 
they are constructed. Each achro- 
matic lens in the combination is 
a meniscus formed of dense glass, 
the denser element forming the side 
that is convex. The elements in 
each are a concavo-convex and a 
meniscus cemented together, and 
two of these form the objective, the 
apertures of which, according to 
FIG - 40. the maker, may be considered as 

varying from /-4 to /- 10. The former of these, however, 
implies that glass has been made use of having a degree 
of density scarcely safe to be employed for photographic 
lenses on account of its tendency to become discoloured. 
Advantage of Dense Glass. Why, it may be asked, 
employ glass of such great density ? Or what advantage 
does heavy, dense glass possess over the lighter sort 
known to be unalterable by either light or time? 
We reply : the denser the material of which a lens is 
constructed the greater is its refractive power, and, 
consequently, the flatter is the curvature required to 
produce a lens of any definite focus. We here repeat 
what we have already stated, that if three single lenses 
are required of similar short foci, all being the same 
diameter, and the first be composed of diamond (if 
that were practicable), the second of dense flint glass, 
and the third of light crown glass, then, while the first 



SYMMETRY. 83 

would be comparatively flat, the last would be very thick, 
owing to its short radius of curvature, while the second 
would be between the two. Now, seeing that the radius 
of curvature of a dense glass is so much greater, for its 
diameter and focus, than one of light material, the 
spherical aberration is diminished in a corresponding 
degree. It is impossible to produce with ordinary flint 
and crown glass a combination of the form shown in the 
foregoing diagram which shall work with an aperture as 
great as those formed of dense glass. Hence the advan- 
tage of the latter kind of glass. 

Symmetry. Symmetry in a rapid doublet (by which 
name we shall designate this class of lens, by whom- 
soever manufactured) is not at all a requisite condition 
towards obtaining either a large angular aperture, 
covering power, or rectilinearity of projection. Some 
years ago a statement was made by the author to the 
effect that for all purposes, except that of copying an 
object the size of the original, the lenses of a rapid 
doublet, examined from the non-distorting point of view, 
should not be symmetrical. This drew forth, first, the 
strong animadversions of the deceased Thomas Sutton, 
whose mathematical ability no one doubts ; and, secondly, 
an adverse private expression of opinion from the then 
mathematical adviser of a large optical firm who now 
in practice ignore strict symmetry. Such is the irony 
of fate ! A vast number of the rapid doublets now 
being manufactured have their front lenses of longer 
focus than their backs. This dissimilarity is sometimes 
carried so far as to cause a sensible difference in focus 



84 MORRISONS RAPID DOUBLET. 

of the combination when the full aperture and a small 
stop are respectively employed. The reason underlying 
this dissimilarity of elements in an objective have relation 
to the law of conjugate foci. But photographic optics is 
so much a series of compromises that it is unwise to 
dogmatise upon what should be the way to carry into 
effect a certain idea, as it is impossible to indicate any 
one mode as being the best. The form of rapid doublet 
shown in Fig. 40 (ante) is that which has been adopted by 
all European manufacturers, and it is a necessity of their 
construction that glass of greater than ordinary density 
be employed in their formation. It may be an abnor- 
mally dense crown glass united with flint glass of a 
corresponding ratio of density to secure the requisite 
actinic correction ; or it may be a light flint glass com- 
bined with heavy flint, the result being the same. 

Morrison's Rapid Doublet. - - The rapid doublet of 
Richard Morrison, an American manufacturing photo- 
graphic optician, formerly spoken of and lately deceased, 
appears to have been projected on lines totally different 
from those of European opticians ; for, not only is it 
formed of the ordinary optical flint and crown, but the 
very principles involved in its manner of correction differ 
from them. In Fig. 41 we present a diagram of this 
.\merican rapid doublet, the curves of which are none 
of them deep in any part, differing in this respect from 
the internal or contact surfaces of the European class, 
the radius of which is always very short. From what 
\ve have seen of this American objective when tried 
in comparison with those of the European form there 



RAPW DOUBLET. 85 

does not appear to be much difference between them. 
There are numerous particular instances in both classes 
in which one has proved much superior to the other ; 




FIG. 41. 

but in the best specimens of each the difference between 
the photographic results is not readily apparent. A 
priori, the European form should possess such an ad- 
vantage over the American as is to be obtained from 
the reflecting surfaces being only half the number ; for 
the interior surfaces of the Morrison lenses being dis- 
similar as regards curvature, it is, of course, impossible 
that they can be cemented. This in practice, however, 
is not a matter of the importance that might at first be 
imagined from the ' loss of light ' point of view, because 
a very slightly increased diameter of lens will amply 
compensate this. 

Where the real ^>oint of danger is apt to lie, if care 
be not taken in properly adjusting their various parts 
is in the increased number of images formed along the 
posterior axis by these various reflecting surfaces. The 
Morrison rapid doublet, if gifted with speech, might 
r r 



86 MORRISONS RAPID DOUBLET. 

hurl a tu quoque against its European rivals ; for it is 
the case that by many of the rapid doublets a cjntral 
flare spot will be produced if the conditions are such 
as to favour its production. 

What we have said regarding this objective com- 
paring favourable with the European rectilinears, must 
be held as applying to narrow angles of view only ; for, 
as might be deduced from a perception of its shallow 
curves, the Morrison doublet cannot, from the very 
nature of its construction, transmit an oblique pencil 
in the perfection capable of being attained by the 
cemented combinations of European form just de- 
scribed ; hence for including other than a narrow 
angle of view it must yield the palm to them. 



CHAPTER XV. 

UNIVERSAL LANDSCAPE LENSES. 

What Constitutes a Universal Lens. By ' universal,' in 
the above heading, is here meant adaptability or adjust- 
ability of focus. The photographer has his camera 
pitched at the one point from which alone the composi- 
tion of the subject is perfect, but when focussed upon 
the ground glass it is found that either too much or too 
little of the scene has been got in. Then why not carry 
a battery of lenses, so that when one fails in delineating 
upon the ground glass just so much as is wanted and no 
more, it may be deposed in favour of another which will 
better fulfil the requirements of artistic composition ? 
While such an expedient is to the individual possessing 
ample means the most satisfactory that could be adopted, 
it is open to the serious objection of great expense and 
much bulk especially the former. Having one mount 
it is, of course, easy to adapt to it a variety of lenses 
set in cells, each lens either set far back or made to 
project in its cell according to its focus ; for it is scarcely 
necessary to remark that the longer the focus of the lens 
the greater must be its distance, cceteris paribus, from 
the stop. 

Convenience of the Universal System. This system is 
much to be commended, as it enables the photographer 



88 CA SKE T LENSES. 

to reduce his impedimenta to a considerable extent 
without having to sacrifice efficiency or convenience in 
any degree. During a series of discussions on land- 
scape lenses which took place at the Photographic Club, 
the author, speaking on this subject, showed a mount of 
convenient dimensions to which he had, by suitable 
adapters, fitted lenses by Grubb, Ross, Dallmeyer, 
Darlot, and others. These packed into a pocket-case by 
themselves ; and by making a selection he could have 
every focus, either singly or in combination, for which 
his camera was adapted. These were not mere make- 
shifts, but each was adjusted according to strict rule. 
Many years since M. Darlot, a Continental manufacturer, 
devised and executed a cabinet of lenses for a similar 
purpose. Casket lenses are now being made by several 
manufacturers. 

Universal Lens on New System. Perhaps the most 
useful lens of all, should it ever reach the stage of being 
manufactured, will be that which was referred to by the 
author as having been devised by him, but as yet in a 
too unfinished state for detailed publication, namely, one 
in which, by the rotation of a collar or the movement of 
a button in a slot in the mount, the focus of the lens- 
complete in itself is susceptible of being altered to a 
considerable extent. That such really can be done 
there is no room for doubt, as we have made use of such 
a combination, constructed somewhat roughly, but suffi- 
ciently well to show the action. The alteration of the 
focus is caused by the movement to and fro of certain 
lenses, more especially of a concave achromatic, so con- 



FOCUS ADJUSTER. 89 

structed as not to interfere with chromatic correction no 
matter how effected. A principle analogous to this has 
for some time been applied to a low-power microscopic 
objective by Carl Zeiss, Wray, and others. 

A Focus Adjuster. A convenient form of focus 
adjuster, which we devised and had constructed several 
years ago, consists in a sliding piece of brass, made 




FIG. 42. 

hollow in order to secure lightness, of the form shown in 
Fig. 42. It contains four apertures, into each of which 
is fitted a thin achromatised lens of negative power. 
This piece slides through the lens mount, by means of 
an aperture, shown in Fig. 43. 
There are a series of notches on 
"^Wfcr the slide so as to ensure the lens 

|J ] V$j{ 

PJJ -HI connected there v/ith being kept 

quite central. The combination 
to which this system is accached 
is a doublet composed of two 
\\ slightly meniscus lenses which, 

r ~" O^i w ^ en use< ^ a l ne > do not give a 

flat field. By inserting the slide 
the influence of either of the four 
concave lenses contained in it is to flatten the field and 



96 FOCUS ADJUSTER. 

lengthen the focus the marginal pencils being well 
corrected with a moderately large aperture. With 
i\ T o. I lens the equivalent focus is seven inches, the 
other concaves increasing the focus respectively in the 
following proportions : 

No. i pinches. 

2 9 

3 12 

4 ! 5 

When not in use this slide packs away in a neat little 
pocket-case, six inches long by one and a half inches 
wide, and half an inch deep. This forms a compact and 
useful appendage to a lens. If one of the lenses of the 
combination be removed an entire change of focus is 
produced ; but in this case it is lengthened so much as 
to be useless when employed with a small camera. A 
series of three auxiliary lenses mounted in similar 
fashion was prepared and long used by us in connexion 
with the Petzval orthoscopic system, the performance 
being so good as to have elicited from a clever manu- 
facturing optician an expression of surprise at what he 
termed the great adaptability and elasticity of this 
system. 

Every one knows that there is a horn or shell pocket 
magnifier which can be obtained for a few shillings, and 
which consists of three lenses of different powers set in 
horn and hinged on a common pivot, so as to rotate in 
or out as required. These lenses being of different foci 
form a tiny battery of seven degrees of magnifying 



ELEMENTS OF COMBINATIONS. 91 

power, according as they are employed singly or in com- 
bination with one another ; and something analogous in 
principle to this in photographic lenses is what we 
contend for as a tool that would prove highly useful to 
landscape photographers. There is much optical talent 
lying dormant among photographers. We trust that 
what has been here said will prove the means by which 
some of this inert power may be aroused. 

The Elements of Combinations may be used as Single 
Landscape Lenses. In connexion with this subject we 
may remind photographers who employ combinations 
of lenses, such as those of the rectilinear or symmetrical 
class, that each lens may be used singly as well as in 
combination. The focus will then be about twice that of 
the complete objective. But this is not always the case, as 
many lenses of this class are dissimilar, the front being 
of longer focus than the back. This is all the better as 
regards diversity, as it affords three changes. But when 
employing only one of the elements of this objective as 
a single landscape lens the best effect is not obtained if 
the lens be screwed into the tube in the usual way. It 
is then rather too close to the stop. But by having an 
adapting ring into which it can be screwed, so as to 
allow of a greater distance between it and the diaphragm, 
its full value will then be ascertained. The central 
definition will be good under all circumstances ; but 
when the stop is close to the lens the marginal definition 
is bad, but will improve in proportion as the space 
between the stop and lens is increased, until it reaches 
the maximum extent of improvement. One such 



92 ELEMENTS OF COMBINATIONS. 

adapting ring (which we have had made to adapt to the 
single lens of a combination) of one inch and five-eighths 
in diameter, possesses a width of three-quarters of an 
inch. When the objective employed in its completed 
state is a double combination everything is right ; but 
when the front lens is removed then the stop is found to 
be three-quarters of an inch too near to the remaining 
lens to produce the flattest field when using it alone. 

Incidentally and apropos of what has just been said in 
relation to increasing the flatness of field by placing the 
stop at the proper distance in front of the lens, we may 
here remark that sometimes, even when making use of a 
single achromatic lens, a flare spot is found on the centre 
of the plate. This has been denied by some; but the 
fact remains that, under certain circumstances, some 
single achromatic lenses do offend in the manner indi- 
cated. This subject is more fully treated in the chapter 
on flare and the flare spot, in which the remedy is 
described. 

If the combination which is to be separated for the 
purpose of employing only one of the lenses be a wide- 
angle one, then the back lens may be removed and the 
front one left in situ, convex surface to the view. This 
is an entire reversal of the circumstances under which a 
landscape lens is usually employed ; but in the case of 
the lens just indicated it will prove best, especially if the 
angle to be included is not great. 



CHAPTER XVI. 

FLARE AND THE FLARE SPOT. 

FLARE may be described in general terms as 
an abnormal transmission of light through the lens 
whereby the brilliance of the image is impaired. It 
is sometimes caused by reflection from the mount of 
the lens, and more usually by reflections from the lens 
itself. 

Flare from Imperfect Mounting. In some objectives 
the lens is retained in its cell by a counter screw formed 
of a short piece of tube having a thread on its outside, 
its inside being blackened, occasionally by staining the 
metal, and not unfrequently by means of a coating of 
dead black varnish. The former of these is altogether 
bad. To realise this it merely suffices to point the 
camera towards a brightly lighted scene, and, having 
laid aside the ground glass and thrown a large focussing 
cloth over the head, direct the eyes towards the mount 
of the lens to observe what an amount of light is re- 
flected from the various parts of the setting. Then let 
it be remembered that all such reflected light thus 
observed will fall upon the sensitive plate and degrade 
the brilliance of the image. 



9* FLARE FROM THE MOUNT. 

When the counter screws are finished with dead 
black varnish, there is but little light reflected at first ; 
but after a while, when the surface of the lens has been 
frequently wiped with a soft cloth to free it from dust, 
the action of the cloth upon the black varnish of the cell 
ultimately converts the dead surface into a shining one 
which is a powerful reflector of light. 

In some of the lowest priced objectives the lens is 
dropped into a recess at the end of the mount and is 
retained in its place by a ring screwed in. This is a 
more fertile source of flare resulting from mounting 
than any other. We have known an offensive flare 
produced in a landscape lens by a high-class maker by 
the hollowing of the cell around and outside of the 
lens, which after its dead black varnish got brightened 
by cleaning the convex surface of the glass with a wash- 
leather, reflected as would a parabolic reflector the light 
radiated from the surface of the sensitive plate. 

The Eemedy for Flare from Mounting. By coating 
the brass work with dead black varnish, a receipt for 
which will be found in another chapter, flare of the 
nature described will be greatly diminished if not en- 
tirely cured. The edges of all lenses should also be 
blackened previous to their being set in their cells ; this 
is done by applying the black varnish by means of a 
camel's hair pencil. 

The Optical Flare Spot. No lens, not even one of the 
simplest class, has ever been made that does not give 
two images of any luminous body in front. One of 
these is, of course, the primary image formed at the 



CAUSE OF THE FLARE SPOT. $5 

principal focus ; but there is another which is to be found 
in the axis, and usually very close to the posterior 
surface of the lens. 

Take any lens, a common reading-glass for instance, 
and interpose it between the flame of a lamp or gas. 
Now look at the lens with both eyes, and a small, 
bright, and inverted image of the flame will be seen at a 
distance of an inch, more or less, from the lens. It is 
very easy to locate its precise position and to receive the 
image upon a small bit of tissue paper or ground glass ; 
while, if desired, the primary image of the flame may be 
simultaneously received at the principal focus farther 
back. Now, an achromatic lens gives a small secondary 
image just the same as does the reading-glass, and 
arises from the same cause. What we wish the reader 
to bear in mind at present is the fact that the relation 
of the small image to the gas flame is that of conjugate 
foci, demonstrated by causing the lens to approach close 
to or recede from the flame, when the image changes its 
position accordingly. 

Cause of the Secondary Image. Most of the rays are 
transmitted through the lens to the principal focus, but 
a few are arrested by the back surface, and are reflected 
to the anterior surface only to be re-reflected back again 
and transmitted. The result of the refraction and re- 
flections they undergo is to bring them to a focus quite 
close to the lens. Of those rays which do not undergo 
the reflection from the front surface, but which come 
to a focus on the opposite side, we shall presently 
speak. 



g6 FLARE IN COMPOUND LENSES. 

The Flare Spot in Landscape Lenses. When adiaphragm 
is placed before a lens, the aperture therein has the 
same relationship to it as had the gas flame in the 
former case ; that is to say, the small bright area of the 
stop will be reproduced as a circular spot of diminished 
brightness behind the lens. As this has a conjugate 
relation to the lens, it is possible by bringing the 
diaphragm moderately close to the objective to form 
an image of the aperture at the primary focus, or upon 
the sensitive plate. But as a very slight alteration in 
the position of the anterior conjugate (the diaphragm) 
makes a great difference in the posterior one, it merely 
suffices to make such slight alteration in order to effect 
a cure. In some cases such a trivial alteration as an 
eighth to a quarter of an inch suffices to convert a bad 
lens into a good one, as it may bring the ghostly imigc 
forward from the plate to a position near to the lens 
whence it is distributed over the the entire surface in a 
state so attenuated as to be harmless. 

Flare in Compound Leases. In proportion to the 
number of reflecting surfaces in a combination so does 
the number of false images increase. Let a Petzval 
portrait combination be taken into a darkened room 
and directed to a lamp, and it will be found that along 
its axis no fewer than fourteen images of the flame wiil 
be seen, four of them erect, and ten inverted. Of all 
combinations this one seems the worst to work with a 
diaphragm and escape the presence of a more or lc:>s 
bright flare spot. To avoid this evil it is much the 
better way, when using it out of doors with a bright sky 



FLARE IN RECTILINEARS 97 

in front, not to employ any stop at all, but to use it with 
full aperture. 

Flare in Rapid Rectilinears. In cemented doublets of 
the ' rapid ' type,' now in such general use, it will be 
found, when directing it to a lamp or gas flame, as in 
the previous experiment, that the number of reflected 
images is reduced to five or occasionally to six. Of 
these, one depends to a greater extent than the others 
upon the degree of the separation of the front and back 
components, and there is a special distance at which 
they may be separated where the concave surface of 
the front lens will be seen to be one blaze of light. To 
show that this arises from reflections from the back lens, 
it is only necessary to increase or decrease the amount of 
separation ever so slightly to cause it to disappear. The 
flare spot in this class of lens is most pronounced when 
the distance at which the lenses are separated is such 
as to give this reflection, the relation between the 
diaphragm and the back lens also being such as to have 
the image of the former thrown on the sensitive plate. 

To Ascertain whether a Lens gives FUre. A good 
way by which to discover the presence of this flare 
propensity in any lens is to screw it into a camera 
and focus a view of an ordinary gas flame on the 
screen, the room being otherwise darkened. This image 
will be sharp, bright, and inverted. Now move the 
camera slightly so as to cause the inverted image 
to be a little to one side of the centre of the focussing- 
screen, and in nine cases out of ten there will be seen a 
ghostly image at the opposite side of the centre. This 

H 



98 CURING FLARE. 

secondary image is non-inverted, and upon rotating 
the camera it moves in the opposite direction to the 
primary image. The nature of this secondary image 
and the cause of its formation may be examined in 
the following way : move the camera so that the 
ghostly image shall be near the margin, and then, 
placing the eye in the line of that image and the lens, 
withdraw the ground glass, when the posterior surface 
of the lens will be found to be quite luminous. 

That the false image is, in this case, caused by a 
reflection from the back surface of the anterior lens 
is demonstrable by unscrewing the cell containing it 
until it is almost ready to drop out of its tube, and 
then, keeping an eye upon both the primary and the 
secondary images on the ground glass, move or slightly 
wriggle the front cell, which, by its looseness in the 
mount may now be easily done, when it will be seen 
that, while the primary or legitimate image of the flame 
remains motionless, the flare image, caused by the re- 
flection from the surface of the front lens, dances about 
all over the plate. But observe, further, that there is a 
certain distance between the front and back lenses at 
which this secondary image is sharp and bright ; and 
in proportion as either the front or the back lens cell 
is screwed out or in, so does the image become more 
attenuated and expanded till at last it ceases to be seen 
altogether, while all this time the real image is not; seen 
to suffer in any way. 

The Cure. This tendency of the ghostly image to 
pass out of focus with such extreme rapidity upon 



FLARE. 99 

separating the lenses by a few turns of the screw, or 
even by making them come nearer to each other, pro- 
vides the means by which this annoying evil may be 
cured. A rapid doublet may be excellent for groups, 
copying, and every other purpose, and yet may break 
down when employed with a small stop in landscape 
work. This class of flare-spot is seldom, if ever, seen 
unless a small stop be used. 

It does not follow, because there may sometimes be 
a mistiness or haze on the whole or a portion of a nega- 
tive, that this indicates a defect in the lens. We have 
known it to be so attributed when in reality it was 
caused by the admission of light into the camera through 
a chink almost imperceptible to the eye. A tiny crack in 
the front of the camera, a pinhole in the bellows body, 
the absence or bad fitting of a screw in the flange 
these and other causes may produce deleterious effects 
which may be wrongly attributed to the lens. 



CHAPTER XVII. 

THE EQUIVALENT FOCUS. 

PREMISING that the solar focus of a lens is that point 
at which objects situated at a great distance are brought 
to a sharp focus, we now consider the nature of the 
' equivalent ' focus of a combination a term which arises 
from a comparison with a single lens that would produce 
the same-sized image, one being equivalent to the other. 

What is the Equivalent Focus ? The equivalent focus 
of a lens may be said to be the focus measured from the 
optical centre of the combination when such centre has 
been determined for a distant object. The term 'back 
focus,' in popular use, is altogether misleading, or, rather, 
it conveys no idea at all in cases in which accuracy is 
required. We give an instance, and in this case an 
extreme one : An objective may be formed having a 
back focus of only one inch, yet the real or equivalent 
focus of which shall be eight inches ; in other words, the 
size of the image produced by the combination shall 
equal that produced by the use of a single lens of eight 
inches focus. 

Out of several portrait combinations to be met with 
every day, and by makers of high reputation, a large 



EQ UIVALENT FOCUS. 101 

number may be selected almost identical as regards back 
focus, but not two alike as regards real or equivalent 
focus. We were present in the establishment of a dealer 
in lenses of home and foreign production when two 
portrait lenses were selected from a. large stock and 
accurately paired, as was imagined, for the purpose of 
being employed in the taking of instantaneous stereo- 
scopic views. Thorough care and honesty were bestowed 
upon the selection, the mounts were identical in every 
respect, and both were then brought under the influence 
of a single rack-and-pinion. So far all was right, and 
the images on the ground glass were sharp. Soon after- 
wards they were returned as not being a pair, in the 
sense of their producing images of different dimensions. 
This was an illustration of the misleading nature of back- 
focus measurement. It being of importance that the 
photographer should know the real focus of his lenses, 
we shall now give some methods by which this can be 
ascertained. 

Rough Method of ascertaining the Equivalent Focus. 
We commence by giving one which is, at frequent inter- 
vals, being discovered by some whose reading of photo- 
graphic literature is limited, and paraded, especially in 
non-photographic serials, with all the trumpet-blowing 
of a great discovery. It is, unfortunately, not an accurate 
method, being so only in an approximate degree. For 
'rough and ready' purposes, where exactness is not 
essential, it may prove useful. Focus upon any subject 
such as a map or engraving and let the arrangement 
be such that the image on the ground glass is precisely 



102 GRUBB'S METHOD. 

of the same dimensions as the original. Now, measure 
the distance between the ground glass and the subject, 
and divide by four, which gives the figures required. 
But, as we have said, this method is not accurate in the 
case of a combination of lenses. 

G-ruWs Method. Fortunately, there are several other 
methods by which the equivalent focus may be ascer- 
tained with unfailing accuracy, and in describing a few 
of them we commence with that which we almost in- 
variably employ in preference to all others, being that in 
which the late Mr. Thomas Grubb has made the camera 
itself to do duty as a theodolite. In front of a window 
place a table covered with a sheet of smooth paper, 
which must be fastened to the table top. Now make a 
pencil mark at each side of the ground glass of the 
camera, a slight distance from the margin. This mark 
may consist of a line about an inch or more in length. 
Next direct the camera to any well-defined object at a 
distance say, the top of a chimney, a flag-staff, the 
corner of a building, or any other suitable object and 
rotate the camera so as to bring this object directly upon 
one of the pencilled lines on the focussing-screen. This 
having been done, with a pencil draw a line on the paper 
cover of the table, making use of the right-hand side 
base of the camera as a straight-edge for this purpose. 
Now, without disturbing the table, move the camera 
round until the object of which we have already spoken 
is brought directly upon the pencil mark at the opposite 
margin of the focussing-screen, and again draw a pencil 
line on the sheet of paper, using the right-hand side of 



P1NHOLE METHOD. 103 

the camera for this purpose as before. (We may here 
state, par parenthhe, that the two lines thus drawn show 
the angle of view included within the space, hence this 
forms a simple method of determining the angular field 
given by any lens.) To resume : if necessary, extend 
the lines thus projected on the table and connect them 
by a line, as in the cross of the letter A, which is equal 
to the distance apart of the two pencil marks on the 
ground glass. The distance of the intersection of the 
first two lines and the third line is the equivalent focus 
of the lens. 

A modification of the system described consists in 
determining the central point of the focussing-screen by 
drawing diagonals from the corners. Then select two 
distant objects, so arranged as that their images shall be 
equidistant from the central point. Measure with a pair 
of compasses the distance between the two objects on 
the ground glass, and, rotating the camera so that one 
of them shall ' cut ' the centre mark, draw a line on the 
sheet of paper as before directed ; then turn the camera 
until the second object shall in like manner correspond 
with the central mark, a second line being drawn on the 
table. Now connect these two angle lines by a third 
equal to the space between the compasses, and the dis- 
tance between the junction point of the angle lines and 
the cross line is the focus. 

The Pinhole Method. Another method by which the 
equivalent focus of a combination may be ascertained is 
to observe very carefully the size of the image of any 
distant object given upon the ground glass, then remove 



104 SINGLE LENS METHOD. 

the lenses from the mount and insert most conveniently 
in the cell for the stops a thin plate of metal in which 
is a very small hole, such as a pinhole. Now move the 
lens mount in or out until the image thus obtained coin- 
cides in dimensions with that given by the lens ; then 
measure the distance between the pinhole and the ground 
glass. This will be practically equal to the equivalent 
focus of the lens. Owing to diffraction, or the tendency 
of rays of light to bend when passing an opaque edge, 
it will be impossible to secure a very sharp image by 
this pinhole system. On this we may observe that 
although in geometric optics light is assumed to travel 
in straight lines in physical optics this is not the case, 
for on passing by the edge of an opaque body it is bent 
round the corner to some small extent. 

Single Lens Method. Instead of the pinhole system a 
better way is to obtain a cheap biconvex spectacle glass, 
which can be obtained in nearly any large town at a cost 
of one or two shillings per dozen. Select one that gives 
with a small stop an image the same size as the com- 
bination. Measure the distance between the centre of 
the glass and the ground glass, although, owing to the 
thinness of the lens, the measurement may practically 
be made from the outer surface. Greater accuracy is, 
of course, secured by adding to the measurement thus 
obtained the semi-thickness of the spectacle lens. 

Rule-of -Three Method. But it is not at all necessary 
that a large number of spectacle glasses be obtained for 
determining the equivalent focus of a combination, seeing 
that it may be effected by the use of one alone of any 



RULE-OF-THREE METHOD. 105 

known focus. Having taken the precise dimensions of 
any subject and which we may designate the ' test 
object' on the ground glass with the photographic 
combination whose focus is as yet unknown, do the same 
with the spectacle glass of known focus, and compare 
the two results. The relation of the sizes of the two 
images to each other is the same as that of the foci of 
the lenses by which they were produced. It is a simple 
rule-of-three problem. 

Several other methods for ascertaining the equivalent 
or solar focus have been suggested, but those here given 
will serve every purpose, and may be practised very 
simply. Hence to avoid complications we confine our- 
selves to tnern. 



CHAPTER XVIII. 

CONJUGATE FOCI. 

IF a lens which has been carefully focussed upon a 
distant object be then directed towards one compara- 
tively near at hand, the nearer object will be found to be 
out of focus, necessitating the withdrawal of the ground 
glass from the lens before the image will assume its 
maximum sharpness. This establishes the fact that 
there exists a relation between the object that is focussed, 
as regards its distance from the camera, and the focus of 
the lens. This relation is termed * conjugate foci' In 
what we have now to say we will speak of the distance 
between the lens and the object as the anterior or major 
conjugate, and that existing between the lens and the 
ground glass of the camera as the posterior or minor 
conjugate focus. 

Conjugate Focus Illustrated. Parallel rays a a that 
is, rays from a great distance falling upon a lens come 
to a focus at f; but those from b, which may serve to 
represent any object ten or twenty yards distant, have 
their focus at c (Fig. 41). f is the solar focus, b and 
c are conjugate foci, and the former of these is the 
anterior, and the latter the posterior conjugate. To 



LAWS GOVERNING CONJUGATE FOCI. 107 

facilitate reference, the lines indicating the conjugate 
foci are solid, while those relating to the solar focus are 




>c 



FIG. 44. 

dotted. The points b and c are interchangeable ; an 
object placed at either is sharp at the other. 

Laws governing Conjugate Foci. The laws which govern 
the conjugate foci are to be found not, perhaps, so 
clearly expressed as the practical photographer would 
require in several old optical treatises. The following, 
which amount to nearly the same thing, although ex- 
pressed differently, will be quite sufficient for introduction 
in this chapter : 

Claudet's Rule for estimating Conjugate Foci If the 
principal or solar focus of a lens be regarded as the unit 
of measure, an object situated in front of the lens at a 
distance from a certain point, equivalent to a multiple 
of the said unit, will have its conjugate posterior focus 
at a distance from another certain point equal to a 
corresponding fraction of the same unit This relation 
of the conjugates to each other, although probably first 
published in aa old work (Dr. Smith's Optics), was first 
brought before the world in relation to photography by 
the late M. A. Claudet at the Aberdeen meeting of the 



icS BREWSTER'S RULE. 

British Association (1859). The following popular illus- 
tration, which was given at the time of the first publication 
of the proposition in The British Journal of Photography, 
serves to make it more readily understood : Suppose 
we have a lens of twelve inches solar focus an object 
situated at a distance of six feet from a certain point in 
front of the lens that is, at six times the unit of measure 
will have its conjugate posterior focus at a distance of 
one-sixth part of the same unit that is, at two inches 
distance from a corresponding point behind the lens. 

The ' point ' here spoken of before or behind the lens 
is the solar focus measured from the optical centre of 
the combination, or, as we described it in the previous 
chapter, the centre of conjugate foci. 

Brewster's Riile. Previous to the publication of this, 
one of the methods usually adopted to calculate the 
conjugate foci was that of Sir David Brewster, which, 
however, was of little or no use when applied to other 
than a simple lens : Multiply twice the product of the 
radii of the two surfaces of the lens by the distance of 
the radiant point from the centre of the lens for a divi- 
dend. Multiply the sum of the two radii by the same 
distance, and from this product subtract twice the pro- 
duct of the radii for a divisor. Divide the above dividend 
by the divisor, and the quotient will be the focal distance 
required. 

From what was said in the previous chapter, it will 
be understood that the range of posterior conjugate focus 
extends only from the solar focus, which is the nearest 
point to the lens at which a focus of any kind can be 



GRUBffS METHOD. ^09 

obtained, and that focus which results from having the 
object so near to the lens as to give an image of the 
same dimensions as the object, and which, as we have 
shown, is twice the solar focus. 

Grubb's Method and Table. Soon after the publication 
of M. Claudet's method, as just described, the late Mr. 
Thomas Grubb directed his attention to the proposition 
with a view to its still further simplification and per- 
fecting for photographers' use. We here present two 
tables in juxtaposition No. T containing four ratios con- 
structed in accordance with M. Claudet's method; No. 2 
being based upon the shortcoming of the other, in which 
there is nothing to indicate any ratio required except 
that of I to i, and in which (viz., in No. 2) Mr. Grubb 
adopts in preference the more simple and natural ratios 
ot the actual distances from the lens. 

No. i. No. 2. 

1 /and i / 2 /and 2 / 

2 /and i/ ... 3 /and |/ 
3 /and i/ 4 /and |/ 
4/andi/ 5 /and |/ 

In table No. 2 the proportions required are at once 
apparent. The numbers denote the actual distances 
required to be used for a focus of one foot, and the ratio 
is still of so simple a progressive nature that a table 
of any required extent may be constructed almost as 
quickly as the figures can be written. 



no GRUB&S METHOD. 

Having given Mr. Grubb's table (No. 2), we here 
present in a condensed form his argument based upon 
it, and the simple arithmetical rule deducible therefrom, 
by which to determine the conjugates : 

Let it be borne in mind, first, that / represents the 
focus of the lens, and that this focus is assumed to be 
= i foot, or unity ; and, secondly, that we do not alter 
the poiver of a lens by using it, whether for bringing 
parallel rays to a focus or for forming conjugate foci. 
What we do in the latter case is simply to use a portion 
of its power on one side, leaving the balance of its 
power to be exerted on the other side the simplest case 
of this being that where we use the lens for forming 
equal conjugate foci, and where, the lens being one foot 
in principal focus, a power equivalent to a focus of two 
feet is used at one side, leaving an equal power to be 
exerted at the other side. Now it requires very little 
mathematical knowledge to perceive that we can only 
perform the operation of adding and subtracting such 
powers by treating them as fractions that is, by using 
their reciprocals ; and thus, as we express the adding of 
two halfpennies, namely, 

\ + | = i = i penny, 

we in like manner must, in adding the two before- 
mentioned of two feet each in focus, adapt the formula 
(p and /" l being put for the respective powers) : 

\ + = y (and / and p 1 being each = 2 feet). 

% 4- J = Y or focus = i. 

From this simple equation (calling the whole power of 
the lens I, or unity) we gather that the sum of the 



GRUB&S METHOb. Hi 

reciprocals of the powers, which are at the same time 
the required distances from the lens, must equal unity ; 
that is, any two fractions whose sum is unity will, in 
their reciprocals, give relative distances of the object 
and image for a lens whose principal focus is I foot, 
yard, &c. 

The rule deducible from the foregoing for finding 
the required distance for any proportional size of object 
and image, and for any given focus of lens, is : Add 
the required proportions together for the denominator 
of two fractions whose numerators are the separate 
numbers. Invert these fractions, and multiply the focus 
of the lens by each of these for the respective distance. 



CHAPTER XTX. 

THE PRINCIPLE OF CONJUGATE FOCI APPLIED TO HAND 
CAMERAS AND FOR ENLARGEMENT. 

Hand Cameras. One practical application of the 
principle of conjugate focus is the construction of scales 
of distance for hand-cameras, rendering the focussing of 
each object unnecessary. Every photographer is now 
aware that, if he focus a distant object very sharply, and 
then make a mark on the adjusting portion of his 
camera, no re-focussing will ever afterwards be required 
when taking a distant object, all that is necessary being 
to slide out the camera until the previously made 
adjustment marks coincide. In like manner adjust- 
ment marks may be made for objects situated at 
shorter distances. The value of this will be specially 
appreciated under a twofold class of circumstances, 
namely, when by accident the focussing-glass gets 
broken ; but more especially when the object to be 
photographed is in motion, precluding the possibility of 



ENLARGING AND REDUCING. 113 

staying in order to have it focussed, To focus ships in 
motion, especially from the deck of another ship also in 
motion, is altogether out of the question when the whole 
powers of the photographer are taxed in observing 
the fitting moment at which to touch the exposing 
trigger. In such a case the proper procedure is to 
estimate as nearly as possible the distance at which the 
ship is from the lens (the acquisition of such guessing 
power being by no means difficult), and then adjust 
the sliding portion of the camera or lens to the corre- 
sponding mark. 

Enlarging and Reducing. It is, however, in the pro- 
duction of enlargements and enlarging requirements, 
together with those employed in copying of every 
description, that the use of a knowledge of the laws 
of conjugate foci will be exceptionally useful. A photo- 
grapher is supposed to be desirous of knowing what 
dimensions, as regards length, he should adopt in con- 
structing a camera in which he will be able to copy a 
picture or object several times larger or smaller than the 
original, and to know how far from the lens must be 
the object on the one hand and the ground glass on 
the other. He is further supposed to have two or 
three lenses of different foci, but of the precise equi- 
valent focus of each of which he has made himself well 
aware by one of the methods described in our last 
chapter. 

Now let that focus whether five, six, eight, or 
nine inches be represented by / This is the only 



ii4 TABLE OF VTEW ANGLES. 

known element in the inquiry at the present stage. 
What is now required are the conjugates at which 
to place the negative to be enlarged (represented by 
n) and the focussing -glass respectively, so that a 
sharp image shall be produced, no matter what may 
be the degree of enlarging. Expressing one focus 
of the lens by u and the other by v we have the 
following : 

(1) u (n-\- i)/ and 

(2) V= 



which, when converted into simple language, means 

(1) Add one to the times of enlargement (or reduc- 
tion) desired, and multiply the sum by the equivalent 
focus of the lens. The product is the length sought 
for. 

(2) To find the other conjugate focus : Divide the 
equivalent focal length of the lens by the times of 
enlargement (or reduction) required, and add it to the 
equivalent focal length. The sum is the length sought 
for. 

The above embraces the whole subject of enlarge- 
ment and reduction, even though the degree of en- 
larging be such as extends to the production of a life-size 
picture from a small miniature. 

Table of View Angles. The following useful table, 
calculated by Dr. C. E. Woodman, of New York, was 
published in the Photographic Times during its editor- 
ship by the author. 



WOODMAN* S TABLE OF VIEW ANGLES. 115 



DIVIDE THE BASE* OF THE PLATE BY THE EQUIVALENT Focus OF 
THE LENS. 



If the quo- 
tient is 


The 

angle is 


If the quo- 
tient is 


The 
angle is 


If the quo- 
tient is 


The 
angle is 




Deg?-ees. 




Degrees. 




Degrees. 


282 


16 


748 


41 


i '3 


66 


'3 


17 


768 


42 


1-32 


67 


317 


18 


788 


43 




68 


]335 


19 


808 


44 


1-375 


69 




20 


828 


45 


i'4 


70 


'37 J 


21 


849 


46 


1-427 




389 


22 


87 


47 


i'45 


72 


407 


23 


89 


48 


1-48 


73 


425 


2 4 


911 


49 


i '5 


74 


'443 


25 


'933 


50 


i -53 


75 


462 


26 


954 


51 


1-55 


76 


48 


27 


'975 


5 2 


i '59 


77 


;s 


28 


i* 


53 


1-62 


78 




29 


1*02 


54 


1-649 


79 


536 . 


30 


041 


55 


1-678 


80 


555 


31 


'063 


56 


17 


81 


573 


32 


086 


57 


1-730 


82 


592 


33 


108 


58 


1769 


83 


611 


34 


"I *2 


59 


8 


84 


631 


35 ; 


155 


60 


833 




65 


36 


173 


61 


865 


86 


67 


37 


2 


62 


898 


87 


689 


38 


225 


63 


93* 


88 


7c8 


39 


25 


64 


965 


89 


728 


40 


274 


65 


2' 


90 



Example. Given a lens of 13 inches equivalent focus ; required the 
angle included by it on plates respectively 3^ x 4^, 4^ x 6], 6 x 8^, 8 x 10, 
lox 12, and II x 14. 

(i) Dividing 4-25 by 13, we have as. quotient -327 midway between 
the decimals '317 and '335 of our table; therefore the required angle is 
18" 30'. Similarly 

Degrees. 

13 = '5 : corresponding to 28. 

13 = '654; 36. 

o 77 ; 42i 

'3 = "923: , 49^. 

13 = i 'OS; 57- 



',2) 
(3) 
(4) 
(5} I2 
(6) 14 



6-5 
S-5 



* Th's is rot strictly accurate, but if the dhftonal of the plate be substituted for the base, the angle 
found will be correct, if the lens be placed opposite the centre of the plate. 



CHAPTER XX. 

A MECHANICAL MEANS OF ESTIMATING CONJUGATE 
FOCI. 

IN the previous chapter the means for ascertaining 
conjugate foci involve a certain amount of calculation, 
although not much. 

Sir Howard Grubb's System. But for the numerous 
class of photographers who dislike mathematical cal- 
culations, a method has been devised by Sir Howard 
Grubb, F.R.S., a method so simple and withal so accurate 
as to have elicited the highest encomiums from those 
competent to form an opinion. We give it in Sir 
Howard's own language. 

Draw on a board, wall, or floor, a square A B C D, 
each side of which is equal to the focus of the lens ; 
produce two adjacent sides of the square C B and c D. 
At A insert a pin or nail. Now place a rule or straight 
edge and rocking it on the pin or nail there inserted, 
observe where it cuts the prolonged sides of the square, 
as at M and N or M' and N'. 

No matter what position you place the rule in 
(always provided it rests against the pin at A and 
cuts the prolonged sides of square), the distances C M 



HOWARD GRUBB'S SYSTEM. 



117 



and C N will represent a pair of conjugates for that 
particular lens. If it be required to enlarge or diminish 
by four, six, or any definite number of times, it is only 
necessary to rock the rale on the pin till one of the 
distances C M is four or six times more or less than the 




FIG. 45- 

other C N. In other words, a lens of any focus equal to 
C B will form an image of an object placed at a distance 
of C N at the points C M, c. 

Similarly, if the focus of the lens be not known, but 
that the distance is known at which an image is formed 
behind lens of any object at a known distance in front 
of same, and that it is desired to know the focus of that 
lens : measure off the distance of the object from lens 



ii8 CONJUGATE FOCt. 

on a horizontal line as at C N and the distance of imag 
from lens on a vertical line as at C M, lay straight-edge 
across them and observe where this cuts the diagonal 
line as at A, then draw A B parallel to horizontal line, 
and C B or A B is the solar focus of lens. 

The above, which may prove useful to those engaged 
in enlarging operations, depends upon the fact that in 
the figure given : 

I I i C M + c N 

C~M + C~N = cTe or c = cTi "+C~N 

Now as this addition and subtraction of reciprocals 
enters very largely into many optical calculations, it 
will be seen that the above is only one of many cases 
in which this graphical method may be utilised. 

Immediately after Sir Howard sent us the account 
of this system for publication we lost no time in having 
it constructed, which we did by fixing on a thin slab of 
wood two ordinary rules graduated to feet and inches, 
one placed vertically as at C M, the other horizontally as 
C N. The diagonal line C A was a slot in which travelled 
a pin or stud with a pinching screw behind, by which 
it was capable of being adjusted to suit the focus of any 
lens, the distance between the stud and vertical or 
horizontal rules equalling the focus of the lens. 

A piece of apparatus of this kind, which every one 
can make for himself at a very trifling expenditure of 
money or labour, is a thing which we can strongly 
recommend to all who have to do copying or enlarging, 
as the major and minor conjugates of the lens the 
positions respectively of the negative and the sensitive 



THE CAMERA CI.UB FOCIMETEK. 



119 



surface can be ascertained at a moment for any given 
degree of enlargement or reduction. 

The Camera Club Focimeter. When devising a foci- 
meler for the use of the Camera Club, Mr. Lyonel 
Clark, C.E., selected as a basis that of Sir Howard 
Grubb, just described, to which he made some additions, 
so as to render it applicable for any establishment where 
enlarging on a large and varied scale is carried on. For 
the following drawing and description we are indebted 
to Mr. Clark. 




FIG. 46. 

This apparatus is constructed for lenses of any focal 
length, but for amateurs who only use a lens of one 
focal length it can be made in a more simple form. 



120 CONJUGATE FOCt. 

To construct the simpler form of apparatus you lay 
off a right angle, BAG, and divide its two sides, A B, 
A C, into feet and inches ; the length of A B, which 
represents the major conjugate, is of course determined 
by the extension of the enlarging camera or ease). Not 
to have too bulky an apparatus, the sides will best be 
divided to some scale, say one-quarter or one-eighth. 

You next divide the right angle into two equal parts 
by the diagonal A D ; to obtain the correct position of 
the pin P, on which the straight-edge rocks, you have to 
erect a perpendicular on either side at the division on 
the scale corresponding to the focal length of the lens 
to be used. 

In the cut the slide is set for a 10" lens, and there- 
fore the perpendicular, O P, is erected at the 10" mark 
on A C, and the spot, P, where O P cuts the diagonal, 
A D, is where the pin has to be placed. Against this 
pin any ordinary straight-edge is rocked. It is, of course, 
best to let a small piece of brass into the straight-edge, 
through which the pin is inserted; this prevents shifting. 
The straight-edge is furnished with an index, or pointer, 
P P'. This is best placed, for the sake of symmetry, 
not at right angles to the scale, but 22 J less. 

For a large establishment, where lenses of different 
foci arc used, the straight-edge, with its pivot, pointer, 
and quadrant, are carried on a moving piece and can 
slide up and down the diagonal A D, which is now 
divided off in a continuous scale of foci. These, of 
course, are obtained in the same manner as the single 
focus was obtained. 



CONJUGATE FOCI. 121 

The manner of graduation is done by calculating out 
a series of diameters of enlargements for one known 
lens. We need only deal with one conjugate, preferably 
the major, A C. The equation for this length is (n+ i)f y 
where n = number of times of enlargement, and / the 
focal length of the lens. Now we can take the focal 
length of our lens as anything, we will make it unity in 
inches, and the equation becomes n + i ; that is, the 
length of the major conjugate is the number of diameters 
of enlargement plus one (expressed in inches). To 
enlarge one diameter that is, to obtain an image of 
equal size it is I + 1, that is, 2 ; for 2 diam. 3 ; for 3 
diam. 4 ; and so on. As one inch is so small a thing 
to deal with, it is best to take 10" as the focus of the 
lens. This only alters the decimal point ; 2 diam. is 
still 30 inches, and has the advantage that each added 
inch represents a tenth of a diameter. So practically, 
setting our index (thefaur de lys) at 10", we swing the 
straight-edge until it cuts the 20" mark on A C, and 
there mark the spot at which the pointer, P P', stands 
as I diam. ; moving the straight-edge to 21" we mark off 
from the pointer ri diam., at 22"= 1*2 diam., at 23"^= 
1*3 diam., and so on for each succeeding inch. 

The scale of diameters of enlargement thus laid out 
is true for whatever lens we like to adjust our slide to. 
Whatever focus we arc using, if we set the fleur de lys 
to it, and then swing our pointer, P P', to the number of 
diameters we wish to enlarge, we shall read off where 
the straight-edge cuts, A B and A C, the length of the 
two conjugates. 



122 CONJUGATE FOCI. 

In the cut thefaur de lys is set for a 10" lens, and 
the pointer indicates 1*65 diameters. We read off on 
the major conjugate, A B that is, the distance from the 
lens centre to the enlargement 2'*2 J", and on the minor, 
that is, the distance from the lens to the negative, i''4$". 

Let us check this by calculation. 

The major conjugate = (n+i) f 

(1-65 + 1) 10" 



major conjugate 
minor conjugate = - 

n 
26-5 . . 

m inches - 



1 6"' i 



The accuracy of result must, of course, depend on 
the care in the manufacture of the instrument 

It is, perhaps, hardly necessary to point out that 
in the case of reduction the figures remain the same, but 
the major axis is now the distance of the negative from 
the lens, and the minor axis the distance of the lens 
from the reduction. 



CHAPTER XXI. 

DEPTH OF FOCUS. 

A Paradox. In discussing this subject, we begin some- 
what paradoxically by stating that there is no such thing 
as depth of focus. Optically speaking, the focus of a 
lens is a point ; and in cases where, from aberrations, 
the rays from any object do not converge to a point, 
of such a lens it may then be said that it possesses no 
true focus at all. 

But, it may be asked, How does it happen that if 
an object at a reasonable distance say, a quarter of a 
mile be sharply focussed, all objects beyond that will 
also be sharp ? To meet this we say that if a lens of 
long focus capable of yielding a sharp image be em- 
ployed, this will not be found to be the case. If the 
focus of an object at the distance of a mile be carefully 
found in a telescope of, say, eight inches aperture and 
proportionate focus, another object situated a quarter of 
a mile away from the former will be quite out of focus. 

Brewster, in his Treatise on New Philosophical 
Instruments, shows that he was quite aware of this 
property in lenses, for he gives instructions how, by 
means of a graduated eye-tube, a telescope may be 



124 NATURE OF DEFINITION. 

constructed which shall, within certain limits, show the 
distance at which any object is from the observer. 
Were there such a property as depth of focus, it is 
evident that such a telescope could not be constructed. 

The Nature of the Definition required in Photography. 
But the image produced by means of a photographic 
lens is of a different quality so far as concerns sharpness 
from that formed by either a telescopic or microscopic 
object-glass, for the conditions required to be fulfilled 
by the former differ from the others. The sharpest 
possible definition of objects situated in various planes 
of distance this definition not being confined to a 
limited spot in the axis of the object-glass as in the 
telescope, but spread over a field of considerable width 
is required in the photographic lens. 

A lens fulfilling the requirements of the photographer 
should not have a mathematical focus or a definite focal 
point, but should possess such a degree of aberration as 
to yield, with a moderate aperture, good pictorial sharp- 
ness of objects in various planes. We possess a whole- 
plate portrait lens, four inches in diameter, in which 
there was so little depth of definition that in taking a 
portrait when the tip of the nose was sharp, the eyes 
and mouth were quite out of focus. Of course we could* 
by the insertion of a small diaphragm, bring both into 
equal apparent sharpness, but this entailed a prolonged 
exposure. But by destroying the optical perfection of 
focus which characterised this lens, we have now obtained 
such a balance of advantages, that with a wide aperture 
we have still pictorially good definition of the various 



CONTRACTING THE APERTURE. 



125 



planes of the face and body, and a more photographically 
useful, although, optically, a less perfect instrument, is 
the result of the alteration. 

Depth of focus, or, more correctly, of definition, is 
increased by the employment of a smaller aperture. 
By one of the diagrams in the second chapter we have 
shown the effect of a stop in producing sharpness by 
shutting out rays which would confuse. In the following 
figure we show the influence of the stop in extending 
the range of focus. With full aperture as indicated by 




FIG. 47. 

the two outside converging lines, the focus is at a definite 
point, the slightest removal of the sensitive plate from 
which would impair the definition. But suppose a 
diaphragm is inserted which admits only the acute 
angle of rays at the centre, then will it be seen to what 
extent the focal plane may be varied from the true focal 
point, without very seriously impairing the definition. 

Objects served by Contracting the Aperture. In a land- 
scape lens, or, for that matter, in many other lenses, 
the contraction of the aperture by a stop serves the 
threefold purpose of enhancing definition by diminution 
of spherical aberration; depth of focus by causing the 



126 FIXED FOCUS LENSES. 

converging pencil of rays to fall on the plate at a more 
acute angle ; and flatness of field by extending the 
oblique pencils. When a stop is employed with a 
landscape lens, the focus received on the plate is not 
a mathematical intersection of lines forming a point, 
but is composed, so to speak, of a cylinder which can be 
cut at varying distances from the lens, within certain 
limits, without greatly impairing the definition. 

Fixed Focus Lenses for Landscape Work. Previous to 
the advent of the detective or hand camera, since which 
this has been better understood, the question has been 
frequently raised as to the expediency of having a rigid 
camera with a fixed lens for landscape work. The 
principle of depth of focus, or penetration, enables this 
to be successfully accomplished, for when the lens is 
focussed on distant objects, it is found that everything 
desired to be included in a view will be well defined. 
The shorter the focus of the lens, the greater is the depth 
of definition, so that in the case of two lenses one long 
and the other short in focus which are focussed on 
distant objects, the latter will include a greater range 
of sharply-defined objects in the foreground than the 
former. We have seen it laid down as an approximative 
rule by some writer on optics ^Thomas Sutton, if we 
remember aright), that if the diameter of the stop be a 
fortieth part of the focus of the lens, the depth of focus 
will range between infinity and a distance equal to four 
times as many feet as there are inches in the focus of 
the lens. Assuming this to be correct, let us suppose 
that an operator in the field has two cameras, one with 



TABLE OF FOCI. 



127 



a lens of four and the other of fifteen inches focus ; in 
taking views with these from the same spot, the nearest 
objects which in the case of the larger instrument can 
be introduced will be at a distance of sixty feet, while 
with the smaller camera objects situated sixteen feet 
distant will be included with equal sharpness. 

A few years ago a Committee of the Society of 
New York Amateurs was appointed to compile a table 
showing the range of focus for detective camera lenses. 
The following is the result. It shows the number of 
feet beyond which everything is in focus when the 
equivalent focus indicated is used. The disc of con- 
fusion is less than one hundredth of an inch. 



Equivalent 
focus lengths. 


Stop. 
5~ 


Stop. 

10 


15 


20 


25 


3 


i 


f 

50 


60 


2 inches 




ioi 


ll 


i 


2 

3 


1 

2 3 


'i 

2 


i 


4 


I 


3 






i5 2 


8 


5 


4 


3 


3 


2" 


J 4 


i^ 


4 






27 


14 


9 


7 


si 


5 


3? 


3 


2 2 


5 






46 


21 


14 


ii 


9 


7* 


6 


ti 


4 


6 






60 


3 


20 


15^ 


12} 


ioi 


8 


4 


sl 


7 






82 


42 


27 


21 


17 




ipj 


9 




8 






107 


54 


36 


27 


22 


19 


14 


ii 


TO 


c, 






137 


68 


45 ! ^4. ! 28 


2 3 


18 


14 


12 


IO 






167 


84 


5 


42 34 


30 


21 


18 


15 


1 1 






202 | 101 


67 


5< 


41 


37 


26 


21 


18 


12 






211 


T2I 


8c 


61 


49 


4* 


3 1 


25 


21 


1 } 

14 






283 
328 


142 
I6 4 


109 


71 
83 


I 


48 
56 


37 
42 


34 


25 
29 








376 


I8 9 


125 


95 


76 


64 


48 


39 


33 



CHAPTER XXII. 

DIFFUSION OF FOCUS. 

Meaning of 'Diffusion.' The term ' diffusion of focus* 
is another name for spherical aberration. Some imagine 
that portrait lenses possessing this property have an 
advantage, not shared by others, of equalising the de- 
finition of varying planes ; this, however, is an error, for 
there is no equalising of such different planes. But 
there is this advantage : that, whereas with a spherically 
corrected lens, when employed with a large aperture, 
one plane of the face the eye, for example is rendered 
microscopically sharp, the other planes such as the ears 
and nose are indistinctly delineated from being out of 
focus, in a ' diffusion ' lens these various planes appear to 
possess a greater equality of definition, owing to the 
destruction of that excessive sharpness of one plane by 
which the others, by comparison, were degraded. 

Advantage of Diffusion. We are not now speaking of 
that depth of focus (which, we have shown, cannot exist 
from the strictly optical point of view), or depth of 
definition which arises from reducing the working aper- 
ture of a lens, but of that quality of non-optical definition 
arising from spherical aberration in the objective. Now, 



APLANATJSM. 129 

while we like a lens that shall ' cut sharp as a razor/ we 
also like the power, when occasion demands, of making 
a picture that shall not be quite so sharp. 

This is a very natural want felt by every photographer 
who does not consider the acme of perfection to lie in de- 
finition. Mr. Fox Talbot found the need of such a power 
even when using paper negatives, and recommended the 
separation of the negative from the sensitive paper by the 
interposition of a sheet of thin paper or gelatine as a 
means of obtaining this requirement. Others have sug- 
gested putting the sensitive plate a little out of focus ; 
but an objection to this is found in the fact that if the 
face of the sitter be out of focus some other portion will 
be sharp, and Charybdis is no better than Scylla. If a 
lens have a moderately large aperture, and is not only 
properly achromatised but aplanatic, it is impossible to 
escape this extra-sharp definition of one plane. Every 
possessor of a large telescope is well aware that if it be 
focussed sharply upon an object situated at a distance 
of a mile an object only half a mile away is altogether 
out of focus ; and so it is with photographic lenses 
within a more limited range. In order to remove this 
property some means must be utilised by which the lens 
can be rendered non-aplanatic. 

The term ' aplanatic/ we here pause to say, was first 
employed by a Scotch savant, Dr. Blair, who in 1791 
made use of it to signify certain points of superiority in 
lenses which he had constructed. Its application since 
that time has been narrowed down to signify freedom from 
Spherical, in contradistinction to chromatic, aberration. 

K 



130 DIFFUSION OF FOCUS. 

The first Diffusion of Focus Lens, The first account 
upon record of any lens in which the aplanatism could 
be modified at will, so as to secure either sharpness or 
'diffusion/ was given in April, 1864, in the course of a 
paper read before the Photographic Society of Scotland 
by the author. When exhibiting a lens which he, as an 
amateur, had constructed for his own use, he directed 
special attention to the fact that by a slight re-arrange- 
ment of the lenses, operated by a projecting button 
working in a slot in the mount, the fine, crisp definition 
given by the lens in its original state was eliminated, 
and that in the altered condition it gave a picture 
generally sharp all over the plate, but particularly sharp 
nowhere. ' The lens,' he said, * suddenly becomes pos- 
sessed of a new property, which is the much-disputed 
one of depth of focus, or, more strictly, depth of definition, 
covering a large flat field without any stop whatever.' 
This, it is believed, is the first exhibition of any lens for 
which such a property was claimed, and special attention 
was at the time directed to the advisability of securing 
a lowered degree of sharpness in this mode rather than 
by the common method of putting the subject a little 
out of focus. It is fortunate that lenses both by home 
and foreign makers are now easily procurable in which 
by a separation of the back lenses the focus may be 
blunted in any desired degree. 

Dallmeyer's Diffusion of Focus Objective. We have 
already, when describing portrait lenses of large an- 
gular aperture, referred (at page 78) to the back lens 
introduced by J. H. Dallmeyer, with the special object 



OPENING THE FIXED DIAPHRAGM. 131 

of introducing any desired amount of spherical aberra- 
tion by the separation of its components. The posterior 
of these is set in the main cell in such a manner as to 
be separated from its fellow by turning a graduated ring. 

Optical Perfection not necessarily Desirable. When, at 
a series of discussions on lenses, at the London Photo- 
graphic Club, the author took occasion to attribute a 
certain degree of blame to the manufacturers of lenses 
especially those of the * rapid ' and ' portable ' class of 
compounds for curtailing their usefulness by limiting 
the aperture in the fixed stop to that point at which 
optical crispness terminated, the representative of a large 
manufacturing firm who was present good-humouredly 
hurled a jocular anathema at the individual in question, 
whose first act, he said, upon obtaining one of their 
lenses was invariably to put it in the turning lathe and 
open out the fixed stop to the diameter of the lenses. 
This is precisely the course we are now about briefly to 
advocate, and its reasonableness will stand or fall by the 
soundness of the reasons adduced. 

Advantage of Opening the fixed Diaphragm. When a 
lens of the description specified gives, with its fixed 
diaphragm, black definition by which we mean the 
rendering of a piece of printed matter in an unmistakably 
sharp, black manner without greyness or fuzziness it 
may be considered as being optically perfect ; but as 
every lens will do this when it is stopped down to a 
sufficient degree, the question for consideration is What 
price do we pay for this, or what do we suffer in the 
way of cutting off the illumination ? The larger the 



132 OPENING THE FIXED DIAPHRAGM. 

aperture of the lens that does this the better is such 
lens ; and in making a selection of a 'rapid' objective 
this is one of the points to which we always pay special 
attention, for some will not define ' black ' unless the 
fixed stop be very small. Let us suppose that we have 
got an objective the diameter of the lenses of which is 
two inches, the fixed stop between the two being one 
and a quarter inch. If with such a working aperture it 
gave black definition, we would, without hesitation, have 
this fixed stop opened up to such an extent as upon trial 
would merge the black definition of the lines into grey, 
occasioned by the overlapping rays caused by the intro- 
duction of spherical aberration. It might be necessary, 
in order to have this accomplished, that the fixed aper- 
ture be increased to such an extent as almost to show 
light round the margin of the movable diaphragms, and 
two such lenses in our possession have been opened out 
to that extent. The advantages secured are first, the 
ability to take a photograph with a far briefer exposure 
than was previously possible ; and, secondly, the ability 
to take a portrait in which, while the sharpness is still 
of excellent degree, it is chastened or softened by the 
modicum of aberration so introduced. 

Now the gain thus secured has been obtained with- 
out any loss whatever ; for, if the razor-edge definition 
of the objective in its original state be required at any 
time, it can be immediately secured by the insertion of 
a diaphragm, by which, so far as light and crispness of 
definition are concerned, the lens is returned to its first 
state. We are informed that opticians would with plea- 



FUZZ Y PICTURES. 133 

sure send out their lenses with the fixed stop enlarged 
in the way and to the extent here suggested were it not 
there are many inexperienced photographers who could 
not use aright such a power were it conferred upon 
them, and who, misunderstanding the reason for the 
increased aperture, would be apt to decry the lens as 
being deficient in definition. While we sympathise 
with the opticians in the force of this objection, we 
recommend the propriety of the course suggested to 
those who, being already in possession of objectives of 
the class to which we now refer, are at liberty to alter 
them in their brass work as they see proper. To tamper 
with the glasses themselves would be highly irrational, 
the ability to do so being assumed. 

Mechanical means for producing Fuzzy Pictures. Some 
of the mechanical means employed in the production 
of portraits in which extreme sharpness has no place 
are rather amusing. Among these we may refer to a 
system not long ago patented by one of the most 
eminent photographers of New York City, which con- 
sists in placing between the camera and the sitter a 
gridiron arrangement containing several gas jets, by 
which ascending currents of air of varying densities 
from the flames disturb the sharpness of the definition 
and produce an alleged greater harmony. The pictorial 
results are designated ' vibrotypes.' A similar effect is 
obtained by having a trembling camera-stand, or by 
attaching a string from the camera to the floor and 
causing it to vibrate during exposure. 

Claudet's System. The method employed by M. 



134 DIFFUSION. 

Claudet was much more philosophical. It consisted 
in moving the lens in and out of the camera, within 
certain limits, during the seance^ so that whereas at the 
commencement of the exposure the nose may have been 
sharply in focus and the eyes or ears cut of focus, or 
vice versa, at the conclusion these conditions were 
changed, the nose being then out and the ears in focus. 
The focus was thus distributed over the entire plane of 
the face. M. Claudet made a specialty of very large 
portraits, which necessitated the employment of portrait 
lenses of large dimensions ; and there is no doubt that 
by the means just indicated he secured equalised defini- 
tion over various planes. 

Into the art aspect of diffusion of focus we have 
avoided entering, our attention having been confined 
to considering the question from the optical point of 
view. 

Diffusion by Single Achromatic Lenses. The value of 
a single achromatic lens of plano-convex or meniscus 
form in producing 'diffused' portraits is well known. 
It must be worked with a stop much larger than would 
be employed in landscape work. Portraits of large 
dimensions and great technical excellence have often 
been obtained by such agency. 



CHAPTER XXIII. 

TESTING LENSES. 

Preparation of Camera. In testing a lens it is im- 
portant that the ground glass of the camera be so 
smooth or of such a fine grain as to permit of the 
use of a magnifying glass without the image suffering 
from granularity. The mere masking of this granularity 
by waxing or oiling the surface of the focussing-screen 
will not suffice ; the grain must be fine in itself. 

It is equally important that the surface of the ground 
glass be at precisely the same distance from the lens 
as that of the sensitive plate. This cannot be ascer- 
tained with the requisite accuracy by the usual method 
of pushing a foot-rule through the aperture in the front 
of the camera, observing how far it goes, and then trying 
in the same way a plate in the dark slide. A more 
accurate method consists in laying a straight rule across 
the focussing-glass frame, and inserting between the 
edge of the rule and the surface of the glass a slip 
of card cut in the form of a wedge, and observing the 
distance it can be inserted, making a pencil mark at 
the place where it touches the rule. Next insert a 
plain glass in the camera dark slide, and do likewise. 



; 3 6 TESTING LENSES. 

If the point of contact of the wedge be the same in 
both cases then both planes are coincident. 

In this way a difference of a hundredth part of an 
inch between the plane of the ground glass and of the 
sensitive plate may readily be detected. If the wooden 
adapters in the dark slide be thin and the spring in 
the back be strong, there is a danger of the sensitive 
plate being forced nearer to the lens than it ought to 
be, and the focussing thus disturbed. More than one 
optician of eminence has had lenses of large aperture 
and unmistakable excellence returned for alteration 
owing to an imaginary fault caused by the strength 
of the spring. 

The Points to be Tested. These are various and will be 
treated individually. They comprise covering power 
or area of illumination ; achromatism, actinism, or co- 
incidence of visual and chemical focus ; astigmatism ; 
flatness of field ; surface finish ; striae and air bubbles ; 
purity of glass ; definition ; flare ; focus ; rectilinearity ; 
aplanatism ; and spherical aberration. 

Although these topics are treated in the other 
chapters, yet it may be well here to devote a few 
words to each. 

Covering Power or Area of Illumination. The area of 
illumination is circular, and its diameter determines the 
size of plate that can be got out of such a circle. We 
are not at present referring to the quality of the image 
that may be produced from centre to margin of such 
area, which may be good or bad, but to the mere 
illumination to the corners. The diameter of this circle 



A CHROMA TISM. 137 

equals the diagonal of any plate (that is, measured from 
coiner to corner), which will be lighted to the corners. 
Take the case of a whole-plate, i.e., one of 8J by 6 
inches, the diagonal of this is loj inches, and no lens 
giving a less area of illumination than this latter figure 
will cover the plate. But if a panorama were wanted, 
then by employing a plate of only 3^ inches in height 
and 9! in length, a greater angle, measured on the base, 
could be included. Let the possessor of any lens ascer- 
tain the diameter of its area of illumination and draw 
this on a sheet of paper, he can then by placing any 
plate upon this circle see at a glance whether or not the 
lens will cover it. 

Achromatism or Actinism. The focussing screen 
having been adjusted accurately, it is next desirable 
to ascertain if the lens has a chemical focus, or, in other 
words, whether the actinic and visual foci be so carefully 
adjusted that both shall fall on the same plane. Place 
seven or eight printed cards in a row on edge on 
a slab of wood, the distance between each being six 
inches. In addition to the printed matter, each card 
should be boldly inscribed with a figure in black ink. 
Having placed the slab on a table at a distance of ten 
feet, arrange so that the cards shall be all focussed as 
near the centre of the ground glass as possible, all of 
them being shown. By the aid of a magnifier focus 
sharply, without using a stop, the centre figure of the 
row, which, if seven cards are employed, will be marked 
'4.' Now insert a sensitive plate and take a picture; 
and, if on the subsequent negative the fourth card be 
sharper than the others, it proves the coincidence of the 



138 OVER OR UNDER CORRECTION. 

two foci. Should, however, a card further away than 
that focussed upon be found to be the sharpest in the 
negative, it indicates that the lens is over-corrected for 
colour, or, as expressed by some, it has a back focus. 

Visual Test for Over or Under Correction. At this 
juncture it may be desirable that we give an easy 
method for ascertaining whether a lens has its blue 
and yellow rays brought to the same focus, or is 
'under -corrected' for colour, which is the necessary 
condition in a photographic objective. Bring the lens 
to be examined into a slightly darkened room in which 
there is a gas-light burning, and, retreating several feet 
from it, hold up the lens so as to form an image of this 
light in the eye of the observer. The image must, 
however, be examined through an eyepiece of any 
good construction ; we prefer the * Ramsden ' for this 
purpose. At the point where the image is sharpest 
there is but little colour ; but, by bringing the portrait 
lens a little nearer, the flame, if the lens be properly 
corrected, is seen to be surrounded with a claret fringe, 
while on removing it to a greater distance than distinct 
definition, the light is fringed with green, proving that 
the blue and yellow rays are combined, and, as a con- 
sequence, that the chemical and visual foci coincide. 

When a lens is not properly corrected for colour, 
over -correction is the direction in which the error 
usually lies, especially in foreign Petzval portrait com- 
binations, and in almost every instance which has been 
brought under our observation, the front lens has been 
the defaulter. 



tZSTING FOR ASTIGMATISM. 139 

Astigmatism. Astigmatism is a serious fault for a 
lens to possess in any marked degree. It is closely 
allied with flatness of field that is to say, it is usually 
produced in the endeavour to make a lens which will 
cover a flat field with a large aperture. A lens of this 
class will work quite sharply in the centre, but in pro- 
portion as an object (such as the head of a sitter) is 
made to occupy a position tolerably far from the centre 
of the plate so does the sharpness diminish, and no 
amount of racking the lens in or out will give definition 
equal to that in the centre. To test a lens for astig- 
matism, erect a black cross against a white background. 
What we find most convenient for the purpose are the 
astragals of an ordinary window. At any rate there 
must be a vertical line crossed by a horizontal one. 
Now focus these sharply on the centre of the ground 
glass, and it will be found that both lines, the vertical 
and horizontal, are well delineated and equally distinct. 
Next rotate the camera slightly so as to bring the 
crossed lines to either the side or the top or bottom 
of the focussing-screen, and again examine the image 
very carefully, when the want of sharpness will be most 
apparent. Rack the lens in and out, and a point will 
be found at which the horizontal bars will be sharp, 
while the vertical ones are so far out of focus as to be 
almost invisible, or, at any rate, to have their sharp- 
ness greatly impaired. Now manipulate the rack once 
more, and the vertical lines will become sharp, leaving, 
this time, the horizontal ones as a confused mass 
of indistinctness. 



i 4 o FLATNESS OF FIELD. 

In a similar manner, provide a sheet of brown paper 
with a round hole in it, and fix it on the window. 
Direct the camera to it as before, and observe that 
when the image is thrown on the focussing-screen it 
is quite round, no matter whether the lens be racked 
within or without the point of true focus. Now rotate 
the camera so as to bring the image to the margin, as 
in the previous experiment, and, behold ! it is no longer 
round as before ; for, when the lens is racked in or out, 
it becomes alternately elongated vertically or horizontally, 
according as the lens is nearer to or further from the 
ground glass than the best mean focus. When the 
lens is made to approach the focussing-screen, the 
luminous spot is elongated vertically ; but when, on 
the contrary, the focus is lengthened, the spot expands 
horizontally. 

It is only in lenses corrected for great flatness of 
field that astigmatism is usually to be found in a 
strongly marked degree, although it is present to a 
slight extent in almost every lens. A portrait lens, 
however, having a round field, is more likely to possess 
freedom from it than any other. The skilful optician 
constructs his objectives so as to have as little astig- 
matism with as much flatness of field as possible. 

Flatness of Field, The best lens is that one which, 
giving brilliant definition at the centre with a large 
aperture, shall with the same aperture maintain that 
brilliance and definition farthest away from the centre 
of the plate. Place the camera opposite any row of 
well marked objects not within several yards a row of 



SURFACE FINISH-STRIDE. 141 

brick houses will answer and focus with the greatest 
care on the centre of the ground glass. Note the extent 
of crisp definition, and how near it approaches the side 
of the picture. It will also do to focus on one well- 
marked object in the centre of the field and rotate the 
camera, observing how much it falls away when brought 
to the edge of the ground glass, and how much racking 
in is required to make it sharp there. 

Surface Finish. The quality of this property is ascer- 
tained by holding the lens against the light, gas being 
preferred to daylight, and examining its surfaces with a 
watchmaker's eyeglass or similar powerful glass. In 
this way lenses which have been imperfectly polished, 
or not properly smoothed at the stage prior to receiving 
the final polish, will be discovered to have a slightly 
granular surface. We have known lenses of this sort 
perform well, but it is none the less a defect which 
ought not to exist. 

Striae and Air Bubbles. Striae in the glass is discover- 
able by taking the lens into a room from which daylight 
is excluded, and, turning the gas rather low, examining 
the image by the gas. Step a few feet back from the 
light, and holding up the lens so that the whole surface 
appears one mass of light, move it slightly from side to 
side, and turn it partially around. In this way a very 
slight inequality in the density of the glass, or a want 
of homogeneity in its composition will be discovered, if 
such be present. Air bubbles, if only of small size, are 
not of the same consequence as striae, for they do not 
affect the definition. As no light is radiated from them, 



142 DEFINITION. 

they act only as would so many specks of opaque matter 
of the same dimensions. The testing of glass previous 
to being ground into a lens is spoken of in another 
chapter. 

Purity of the Glass. The quickest acting lenses, 
c&teris paribus, are those the glasses of which are colour- 
less. In some of the oldest combinations the crown 
glass element was of a pronounced green colour, which 
interfered much with their rapidity. To ascertain the 
purity of the glass, as regards colour, place the lens 
upon a sheet of white paper and note the degradation 
of colour, if any, that takes place when looking down 
upon it. If the discoloration be of a brown character, 
the lens will prove slower in action than if it be quite 
colourless. Some who have much work in copying 
paintings or coloured prints assert that they get a truer 
rendering of the value of colours when using a lens of 
dingy colour than with one formed of purer glass. In 
such a case the lens itself enacts the part of the colour- 
screen of pale yellow glass often employed to attain a 
similar end. 

Definition. One of the best test objects for definition 
in a lens is an enamelled watch dial with the seconds 
circle. Let this be placed at a distance of a few yards 
and well lighted, either by lamp or daylight. On focus- 
sing sharply, ascertain that the division between the 
black strokes forming chapters two, three, four, and 
twelve, and also the seconds, are all well made out. 

Flare Spot, Unlike the other tests, this one should 
be applied after a diaphragm has been inserted in the 



FOCUS RECTILINEARITY. 143 

lens. Let the camera be directed against a rather dark 
object, such as a tree in foliage, with a bright sky over- 
head ; an ordinary window will answer, provided the 
lower portion be obscured by a sheet of dark paper. 
If there be a flare spot, it will be seen in the centre of 
the ground glass. This spot, as we have explained in a 
former chapter, is an image of the diaphragm, and single 
lenses as well as combinations are liable to it. For- 
tunately it is easily remedied. This test can also be 
made in a room lighted by gas or lamp. 

Focus, The various methods by which the focus of 
a lens is known are so fully described in Chapter VI. 
that we refer the reader to it, especially as some of the 
systems may from facility of application or otherwise 
be preferred by some rather than others, and to cite 
them here would be but unnecessary repetition. 

Rectilinearity. Place the camera quite level, and 
direct it towards any perfectly straight object, such as 
the wall of a house, the side of a straight window, or, 
in short, to anything that is quite straight, and, having 
focussed it in the centre of the screen, rotate the camera 
until the image is brought close to the margin. Note 
whether the image is now curved or if it preserves its 
straightness. In the latter case the lens is quite 
rectilinear. 

How to Cure Over-Correction. There are three methods 
by which the evils arising from over-correction may be 
cured. The first is that which will prove the most 
effectual and give the least trouble in future. It con- 
sists in removing the lens from its cell, separating its 



1 44 VER- CORRECTION. 

components by immersion in water sufficiently warm to 
soften the Canada balsam by which the lens is cemented, 
and then regrinding the contact surfaces in tools of 
greater radius of curvature. Only few photographers 
are able to execute work of this sort for themselves ; 
for, although many are quite facile in effecting any 
manipulation or original investigation in chemistry, 
others being equally expert in mechanical and artistic 
departments, the number of those who have entered 
the field of optics by way of experiment or amuse- 
ment is very limited. A 'jobbing' optician will be 
more likely to undertake the regrinding of a lens than 
the manufacturing optician, who could scarcely be ex- 
pected to go out of his way to execute a trivial order 
of this nature. 

A method which was much employed when over- 
corrected lenses were more commonly to be met with 
than is now the case consisted in having a graduated 
scale engraved on the sliding tube, so that when a 
visual image was focussed sharply on the ground glass, 
the lens had then to be racked out a certain number 
of degrees in order to ensure the image being sharp 
in the negative. This distance is a constant one only 
for an object situated a definite space from the camera 
or in the major conjugate focus of the lens, and varies 
with every distance of such object. Were this not the 
case it would be easy to sink the ground glass deeper 
in its frame, by which the same end would be achieved. 
If a lens of this class must be employed and it is a 
well- recognised fact that some will produce photographs 



APLANATISM. 145 

as sharp and fine in every respect as those in which 
the actinic and visual foci coincide the best way by 
far to utilise them with a minimum of trouble and with 
freedom from all uncertainty is to adopt the system 
described in Chapter VI., which consists in inserting, in 
the manner of a Waterhouse diaphragm, a very weak 
lens, the power of which shall be such as, when inserted 
in its place, to lengthen the focus of the objective to 
an extent equalling the difference between the visual 
and chemical foci. If, then, the object be focussed 
\vhen this auxiliary lens is inserted, and the lens be 
then withdrawn when the exposure is about to be made, 
the image will be quite sharp. It may, perhaps, be 
scarcely necessary to observe that in all cases when 
purchasing a lens we recommend that one having a 
' chemical focus ' should be avoided. 

Aplanatism and Spherical Aberration. To test a portrait 
combination for spherical aberration, the Shadbolt 
method is as good as, and more convenient than, any 
other. Cut a disc of thick brown paper of the same 
diameter as the front lens of the combination, and from 
the centre of this cut out a smaller disc seven-tenths of 
the entire diameter. There is thus a disc and a ring, 
the areas of which differ only a trifle from one another. 
Now, first insert the ring of brown paper, which will act 
as a diaphragm ; and, having carefully focussed on a 
printed bill, take an impression, which should be clear 
and sharp. Next remove the ring of paper, and with 
a little gum or paste attach the paper disc to the centre 
of the lens, and without altering the focus take another 



146 SPHERICAL ABERRATION. 

picture of the bill. If the spherical aberration be at all 
well corrected, the second picture should be nearly as 
sharp as the first ; but if the correction be insufficient 
the latter picture will be more or less indistinct. 

Again, focus some well-marked test object the 
small bare branches of a tree against the sky will 
answer without any stop, using an eyeglass to ensure 
accuracy, and mark the position on the camera. Next 
insert a rather small diaphragm, and rack the camera 
in and out till the greatest point of sharpness is as- 
certained. Mark the camera again, and then ascertain 
if the two marks quite coincide. If they do, then is the 
combination aplanatic or spherically corrected. 



CHAPTER xxiv. 

THE SHAPE OF THE APERTURE IN THE DIAPHRAGM. 

Fallacies respecting Shapes of Apertures. A popular 
fallacy existed at one time in a greater degree of 
strength than at present to the effect that the shape 
of the aperture in the diaphragm should bear a certain 
relation to the general form of the principal subject in 
the photograph. For example : a vertical slit instead 
of a round hole was believed to be the correct form 
when the subject was tall, such as a church spire or 
other similar vertically elongated subject. 

In the case of portraiture an aperture somewhat like 
a keyhole has been proposed as that best adapted for 
this class of subject, while for landscapes some virtue is 
still by some imagined to be imparted to the illumina- 
tion of the image if the aperture be wide at the base 
and tapered off to a fine point at the top, the imagined 
advantage consisting in a greater volume of light being 
permitted to reach the foreground than that by which 
the sky is produced. 

Circular Apertures Best. The best shape of aperture 
is circular, and the next best such a degree of departure 
from the circular form as shall most nearly confine the 

1.7. 



i4& FANCIFUL APERTURES OBJECTIONABLE. 

transmitted rays to a condensed bundle. This embraces 
a circle (the iris diaphragm) formed of several blades, 
by the motion of which, regulated by a volute, the aper- 
ture may be expanded or contracted to a large extent, 
while a sufficient approximation to the circular form is 
still maintained. Next to this comes a square, which by 
the motion of two plates in opposite directions as first 
described by the late M. Noton is also applicable to 
an easy formation of an expanding and contracting 
aperture. The worst forms of all are those whimsical 
ones shaped sometimes like a bottle, sometimes like a 
pyramid erected on a circle, and, worse than all, like 
a slot. 

It is not difficult to give a reason for such con- 
demnation. Take the case of a sky and foreground as 
an example. For such a subject an aperture of an ex- 
ceedingly tall pyramidal shape has been recommended 
as possessing advantages over others. This recom- 
mendation has been made by individuals who are not 
considered mere ' nobodies ' in photography, otherwise 
it might be allowed to pass without reference ; but it is 
worthy of notice that the recommendation has not been 
backed up by a single argument of a scientific nature. 
They imagine it ought to be so, and think that it really 
is so ; and there the demonstration ends. Let us see 
in what manner this wedge -shape slot or aperture 
affects the foreground as contrasted with the sky of the 
landscape. 

In a previous chapter it has been shown that, in a 
landscape lens, the margin of the picture must be formed 



CIRCULAR APERTURES BEST. 149 

by the margin of the lens, the same conditions pre- 
vailing with the centre of the photograph. Any depar- 
ture from this is attended by disadvantages, such as 
spherical aberration. In order that any photographer 
may satisfy himself that the shape of the diaphragm 
goes for nought in reducing the intensity of light upon 
the sky, it merely suffices that after placing the camera 
in position in front of a landscape he then removes the 
ground glass. Now, having placed his eye where the 
sky on the ground glass was, let him direct his vision 
towards the stop. This will demonstrate to the observer 
that he can see the whole of the aperture in the dia- 
phragm. Let, now, the same thing be done from the 
position occupied by the sky, and precisely the same 
amount of aperture in the diaphragm is seen, showing 
that whimsicality in shape goes for nothing in regard 
to illuminating one portion of the picture more than 
another. 

This applies also to the use of either a vertical or 
horizontal slit instead of a circular hole. If a set 01 
parallel oblique rays fall upon the lens they do not all 
proceed in the same direction after transmission ; but, 
according to the principles of spherical aberration, the 
focus of a pencil transmitted by that side of the lens 
farthest removed from the object whence the rays 
emanate will be much longer than those transmitted 
by the nearer margin of the lens. Hence a slit aperture 
will give confusion ; but if a round aperture be sub- 
stituted, all such confusion will cease to exist. 



CHAPTER XXV. 

EQUALISING THE ILLUMINATION OF SUBJECTS 
SKIES AND FOREGROUNDS. 

How to Obviate the Excessive Light from the Sky. It 
is not only possible but quite easy to arrange a stop so 
that it will admit a much larger volume of light to the 
foreground of a landscape image than to the sky. Not 
only so, but if one side of a subject were in deep shadow 
or of a dark colour such as a dense mass of trees on 
one side placed in contrast with a sunny, well-lighted 
object on the other it is comparatively easy so to 
arrange matters as that one side will receive a more 
intense pencil of light than the other. 

Much ingenuity has of late been displayed in the 
construction of shutters which, in falling, wUl permit of 
a longer exposure being given to the foreground than 
to the sky. But this can be obtained equally well by 
means of a shutter of the ordinary class, or by a pro- 
longed exposure, provided the diaphragm be placed 
oblique to the axis of the lens. 

The Oblique Diaphragm. In demonstration of the 
foregoing we refer to Fig. 48, in which a represents a 
lens of any form ; d is a diaphragm placed at a slope 



OBLIQUE DIAPHRAGM. 151 

instead of the right angle at which it is usually fixed. 
In this position it is directed downwards towards the 
foreground or less-lighted portion of the subject, the 
consequence of this being that the large volume of light 
bounded by the lines r, r' t and which comes from the 




FIG. 48. 

foreground, exceeds by many degrees that coming from 
the sky shown at s, / ; and these arrive at their respec- 
tive foci/,/', the one in a state of great attenuation in 
comparison with the other. 

The principle of the oblique stop is the same whether 
it be applied to a single landscape lens, as in the figure, 
or to a combination. But we have found opticians very 
reluctant to adapt this oblique system to any lens. The 
usual working appliances, we were told, did not embrace 
the easy or effective cutting of a slot obliquely in the 
mount. To describe the several mechanical expedients 
we found it advantageous to adopt in having stops so 
arranged as to be capable of standing at any desired 
angle would be rather out of place in this chapter, espe.- 



152 EQUALISING BY OPAQUE STOP. 

daily as the mere indication of the remedy for under- 
exposed foregrounds is all that is here required. 

Equalising by an Opaque Stop. A system which we 
adopted a few years ago, with exceedingly satisfactory 
results, consists in placing at a little distance in front of 
the diaphragm a small piece of blackened brass of a 
V shape, base upwards. One or two trials will suffice 
to determine its best position. This fulfils the following 
conditions : It gives a proportionately greater illumina- 
tion to the foreground than to the sky, and, while it 
diminishes to any required extent the intensity of the 
light which falls upon the centre of the plate, it gives a 
great increase to that by which the sides are illuminated. 
Added to these, it costs nothing, and can be applied by 
any photographer to his lens without any disfigurement 
of, or tampering with, the brass work ; for the whole 
appliance can easily be made and fixed in a couple of 
minutes by means of a pair of scissors, a bit of stiff 
black paper, and a little mucilage. When making our 
original experiments with this device we actually suc- 
ceeded in turning the tables so that the foreground was 
far better illuminated than the sky, and the margins 
much more so than the centre, a wide angle of subject 
being included. 

The unequal illumination of a negative, especially 
one of wide angle, is due to two causes. The centre 
receives a more intense impact of light than the sides 
on account of the pencil of light transmitted to it being 
both larger and having a shorter distance to travel from 
the lens to the sensitive surface. Not so with the 



BOW'S METHOD. 153 

margins ; for the stop with its circular aperture being 
placed obliquely as regards the margin the aperture is 
not then circular, but oval a matter easily verified by 
looking through a stop, first directly, and then when 
turned in an oblique direction. This renders the oblique 
light less to begin with ; but this attenuated light has 
also got much farther to travel than the stronger central 
bundle, and hence the marginal weakness. 

The Butterfly Stop. In the case of ordinary angles 
of view this difference is so little as not to merit much 
attention ; but this is not so in the case of highly ob- 
lique incidences. To equalise the light by means of the 
stop in his panoramic camera the late Thomas Sutton 
devised a little adjunct of great ingenuity. It was a 
stop which, no matter whether held at right angles and 
looked at directly or at a very oblique angle, always 
presented a perfectly circular aperture. This was 
effected by two thin little wings of brass screwed upon 
the stop in such a manner as to effect the equalisation 
required (see Figure 28, page 66). 

Bow's Method of Equalising. We close this chapter 
by alluding to one other method suggested (by Robert 
H. Bow, C.E.) for causing the iens itself to be the 
equalising medium. It consists in having the crown or 
plate glass element of the lens of a delicate green colour, 
by which the thick centre will stop more actinic rays 
than the thin margin, the other portions of the lens 
acting in an intermediate degree, 



CHAPTER XXVI. 

ADJUSTING DISSIMILAR LENSES. 

Matching Stereoscopic Lenses. In matching a pair of 
lenses for stereoscopic purposes, serious difficulties have 
not unfrequently to be encountered. This difficulty 
scarcely, if ever, prevails when the lenses are of the 
single landscape class, but is most apparent in the case 
of combinations. Indeed, the difficulty of rinding two 
portrait objectives so identical in focus as to produce 
pictures which, as respects dimensions, will be facsimiles 
is well recognised. Even when a number of lenses are 
made out of the same pot of glass, and ground to the same 
curves, marked differences will often exist in their foci. 

Such being the case with lenses of a similar kind 
coming from one optician, the difficulty of obtaining two 
lenses alike, which have been made by different mechanics 
and of different degrees of curvature, is very greatly in- 
creased. It is, however, not only possible to bring two 
dissimilar lenses to absolutely the same focus without 
having to resort to regrinding and polishing their sur- 
faces, but it is a matter which is not attended with so 
much difficulty as to be insurmountable by any reader of 
intelligence who possesses a moderate amount of me- 



CRITERION OF FOCUS. 155 

chanical skill. Let it, however, be understood that the 
two lenses must be of a focus not greatly apart from 
each other, although too much so to warrant their being 
used for binocular purposes. 

Size of Image the Criterion by which Focus is judged. 
The optical tyro must bear in mind the fact that the 
back focus, so called, of a combination, affords no clue 
whatever to the real focus of the lens. In comparing two 
lenses, the size of the image formed by each is the real 
criterion by which they are to be judged. There may 
be two lenses in which the back elements of each are 
precisely the same distance from the ground glass when 
both are sharply focussed, and yet the size of the re- 
spective images on the ground glass be widely different. 
The reason is obvious : the equivalent focus is that by 
which the size of the image is determined, and in a 
portrait lens the point from which the equivalent focus 
is measured or the focal centre has a very wide range 
of position, being in some combinations near the front 
lens, and in others near the back. 

In combining two plano-convex lenses of similar foci 
say of twelve inches each these, if placed with their 
flat sides in contact, will have a focus practically of six 
inches, and it is not possible to make of these two any 
shorter focus than this. But by the mere expedient of 
separating the lenses, the equivalent focus may be 
lengthened to the extent of several inches ; for the 
greater the distance between them or, in other words, 
the longer the tube in which they are mounted the 
longer will be the equivalent focus, 



156 ALTERING LENSES. 

| Bearing this in mind it becomes a very easy matter 
to adjust a pair of compound lenses of dissimilar foci, so 
that both shall produce images absolutely alike in respect 
of size ; for if one give a smaller image than the other, 
and as by separating the lenses the equivalent focus is 
lengthened, a point will be found at which the images 
given by both lenses will be similar. It may here be 
noted that in proportion as the real focus is lengthened 
so is the back focus shortened. 

. In some instances the difference between the size of 
the images is so little that both lenses may be brought 
into coincidence by unscrewing the cell of the back lens 
a few turns. But if this proves insufficient, then should 
there be a short supplemental piece of tube screwed into 
the principal tube, and into which in turn is screwed the 
cell containing the back lens. 

What has been here said of portrait lenses applies 
equally to every kind of combination, e.g., rapid wide- 
angle rectilinears and symmetricals ; and it will be 
obvious that the foci may be assimilated by shortening 
the tube of one lens as well as by increasing the length 
of the other. 

Effect of Altering Lenses on duality of Image. There 
are lenses, especially those of wide angular aperture, so 
delicately adjusted as regards the separation of their 
elementary constituents as would entail a degradation 
of the definitions at the margin of the picture by altering 
them in the way suggested ; but for the purpose here 
suggested it would be so slight as to be scarcely 
appreciable, while it might turn out, as it did in one 



FOCUS RESULTING FROM COMBINING LENSES. 157 

case under our observation, that the performance of 
the lens was greatly improved in every respect ; and, 
at any rate the original mount is all the time un- 
affected. 

Rule for Ascertaining the Focus resulting from Combining 
any two Lenses. It may be well here to give the rule 
by which the focus resulting from the combination of 
any two lenses of known focus may be ascertained. 
Multiply the focus of one lens by the other, and 
divide this product by the focus of both added together, 
less the distance of separation. The quotient is the 
focus sought for. Thus to take an extreme case as an 
example if the two twelve-inch lenses previously spoken 
of, and which when in contact gave a focus of six inches, 
were mounted in a tube so as to be ten inches apart, the 
focus, instead of being six inches as formerly, would now 
be ten inches and (nearly) a quarter. Again, having two 
lenses respectively of twenty and twelve inches focus, 
and mounted two inches apart, what is the equivalent 
focus? The answer may be thus expressed 

20 x 12 = _ 2 = 8 inches. 

The foci when added together give, minus two (the 
separation), 30, the divisor for 240 (the product of the 
multiplication of the foci) giving eight inches as the 
equivalent focus. 



CHAPTER XXVII. 

THE DETERIORATION OF LENSES BY LIGHT. 

THE subject of the deterioration of lenses through 
time or carelessness on the part of assistants is one 
fraught with much interest to the photographer, who 
frequently has a large amount of money invested in 
them. Complaints as to lenses which were at one time 
rapid but have become much slower in action have been 
frequent. In some cases it is possible that imagination 
has to do with such deterioration ; but, for all that, it is 
not less the case that the falling-off in the effective 
performance of a lens is a physical fact which admits of 
no gainsaying. 

Colourless Glass a Factor in Rapidity. The clearer 
and more colourless is a lens the better and more rapidly 
does it act. This may be accepted as an axiom in pho- 
tography, although in astronomical instruments and 
microscopic objectives it is not of like importance. It is 
well known to ourselves and others that of a pair of 
portrait lenses which were selected, and for some time 
noted, for their absolute identity of action, especially as 
regards rapidity, one afterwards, which had been for 
over a year relegated to a different class of work from 



CA USES OF SLO WNESS. i $9 

the other, eventually became so slow by comparison 
with the performance of its twin brother as to prevent 
their ever again being employed in the capacity of 
producing binocular portraits. Seeing that a high-class, 
rapid-working lens involves the expenditure of a con- 
siderable sum, its retention in a state of pristine purity 
is, consequently, an object of importance. 

Causes of Slowness. There are two sources of dete- 
rioration of a photographic objective, and we may here 
explain that by ' deterioration/ in the sense now em- 
ployed by us, slowness is understood. So long ago as 
the second meeting of the London Photographic Society, 
held on the 3rd of March, 1853, ^ was we ^ recognised 
that some lenses worked much slower than others which 
had similar dimensions and working aperture, and some 
attempt was made to elucidate the cause. That the 
yellow colour of the glass of some of the instruments, as 
contrasted with that of others, was a prime factor in the 
slowness was acknowledged ; but it seemed to be a moot 
point as to the part taken in such degradation of work- 
ing by the Canada balsam with which the component 
parts of the front lens were cemented. Mr. Robert 
Hunt, one of the leading spirits of the then young 
Society, went so far on the occasion referred to as to 
say that it had been observed by Daguerre and others 
that by dropping upon the surface of a photographic 
lens a little of the purest oil of almonds, and then wiping 
it off again in as perfect a manner as could be done by 
a silk handkerchief, the attenuated film still left would 
necessitate a great prolongation of the exposure. From 



160 CA USES OF SLO WNESS. 

whatever cause it may have arisen, neither we nor any 
one whom we have known to repeat this experiment 
have found it to yield the result mentioned. But that a 
film of Canada balsam of no great thickness will render 
photographic action sluggish is a fact admitting of no 
question. It has been found by Mr. George Shadbolt 
that in preparing two similar microscopic objects the 
parasites of birds for photographing, one of them being 
mounted in balsam and the other in glycerine, the 
former required an exposure of four minutes, whereas 
an equally good negative was obtained with the latter 
in one minute. 

The great cause of lenses becoming slower is not the 
balsam used in cementing their elementary parts to- 
gether, but the discoloration of the glass itself by the 
action of light. Lenses formed of dense flint glass are 
more liable to become deteriorated by the action of 
light than those of light glass. Why this should be so 
we are unable to say, although it has been surmised that, 
in some instances at any rate, it may have arisen from a 
trace of silver present in the lead which enters into the 
formation of flint glass. We well remember one lens of 
the ' rapid ' type, which was exhibited before the (now) 
Photographic Society of Great Britain several years ago, 
by an eminent optician, as possessing a larger angular 
aperture, and consequently greater intensity of lighting, 
than any lens of a similar class ever previously produced. 
A few years afterwards, when inquiring of the maker 
the reason why a lens of such obvious utility had not 
been commercially manufactured, he said that the glass 



CA USE OF D IS COL OR A TION. T 6 1 

of which that specimen had been composed, and which 
possessed a greater degree of density, had deteriorated 
to such an extent and become so yellow in colour that 
he would not jeopardise his reputation by allowing one 
to be issued from his establishment. He showed us the 
lens in question, and its yellow colour was quite noticeable. 

When Mr. Thomas Gaffield, of Boston, brought the 
subject of the discoloration of glass before the British 
Association, at the Brighton meeting, in 1872, and 
showed examples of glass of a fine quality, which from 
being quite colourless had assumed a very sensible 
degree of deterioration on being exposed to strong 
sunlight under a mask for a brief period, it was felt 
that this deterioration, although of, perhaps, primary 
importance in such a case as the glass roofing of a 
studio, which was constantly exposed to light, would 
also affect photographic lenses, in which a degree of dis- 
coloration far less in amount would produce a greater 
effect in the prolongation of the exposure. To ascertain 
whether optical glass would follow the role of window 
and plate glass, we wrapped a piece of tinfoil round a 
lens in such a manner as to allow one half to be exposed, 
and this we placed where it could receive the beams of 
a September sun for a protracted period. Upon being 
afterwards examined by laying it on a sheet of white 
paper, the exposed half caused the paper to assume a 
decided hue of a character resembling yellow with a 
purplish tinge. 

Cause of Discoloration of Glass. Why glass changes 
it is not altogether easy to say with certainty. In the 

If 



1 62 ACTION OF LIGHT ON BALSAM. 

case of plate-glass it is held to arise from the presence 
of manganese, which is added in the form of its oxide, 
and known as ' glassmakers' soap.' One theory of the 
action of the manganese is that in all kinds of window 
glass, and in some poorer sorts of flint glass, materials 
are used which are not chemically pure. There is 
usually iron present, the protoxide of which imparts a 
green colour to the glass. The addition of the man- 
ganese causes some of its oxygen to fly to the iron and 
convert its protoxide into peroxide, which imparts a 
yellowish colour to the glass ; that, being complementary 
to the natural pink of the manganese, is neutralised and 
the glass rendered of a white colour. By the action of 
sunlight upon this glass the nice equilibrium between 
the oxygen of the iron and the manganese is disturbed, 
and sometimes a yellow and sometimes a pinkish colour 
is produced. Another theory is that the manganese is 
added solely on account of the facility with which it 
parts with oxygen, which consumes any impurities of 
an organic character or any oxidised, opaque, metallic 
particles. A singular fact in connexion with the dis- 
coloration of glass by the action of light is found in 
the further fact that by heating glass thus deteriorated 
it becomes decolorised. 

Action of Light on Canada Balsam. Now at this stage 
an element imagined to be of a conflicting nature has 
to be introduced ; it is the Canada balsam. Painters 
are aware that white oil paint (carbonate of lead) when 
mixed with megilp, although pure enough while it 
remains exposed to light, assumes quite a yellow 



ACTION OF LIGHT ON GLASS. 163 

appearance upon being kept in darkness, or, in the 
case of a painting, in a drawer for a few months or 
even weeks. In like manner it is affirmed that Canada 
balsam becomes bleached and colourless by the action 
of light, resuming its yellow appearance when kept in 
the dark. Here, then, are two antagonistic forces to be 
kept under check. If the lens be exposed to strong 
light the glass has a chance of being discoloured while 
the balsam becomes decolorised ; but if the lens be 
kept in darkness (except when in active use) the glass 
remains pure, while the balsam becomes discoloured. 
Now, while it is true that the discoloured white of the 
megilp oil painting will assume its original purity when 
placed in the sun for a few hours (unless it be a very bad 
case indeed), and, further, that coloured balsam will also 
become colourless, it is not the case that every kind of 
balsam changes colour ; and we believe we speak within 
the mark in saying that for the productions of one 
optician which become deteriorated on this ground, 
those of twenty are unaffected. The balsam scare, 
therefore, need not prove a source of uneasiness to 
photographers, the more especially as by the means 
we recently indicated the old balsam may be readily 
cleaned away and its place supplied with a fresh and 
colourless sample. 

Strong Light Discolours Lenses. Of much greater im- 
portance is it that the lens be not subjected to any 
strong light, as it may cause a discoloration in the 
substance of the glass that cannot be removed. We 
do not hepe allude to surface stains in the form of 



1 64 ACTION OF LIGHT. 

oxidised patches, which are often caused by damp and 
particles of dirt acting as nuclei, and which stains are 
capable of being polished out, but to a discoloration 
existing throughout the entire substance of the glass. 

If an objective be employed for forming an image 
by the direct beams from the sun, such as is used in the 
solar camera, we advise that it be kept for that purpose 
exclusively, because of the facilities which the light has 
for acting injuriously upon it and rendering it slower. 
We would also state that of all classes of lenses which 
should not be employed in the solar camera, or for any 
other purpose associated with the transmission of bright 
light, those of the popular 'rapid' type stand at the 
head ; for being formed of dense glass they are more 
liable than any others to undergo change. It is well, 
therefore, to keep them covered as much as possible 
when not in use. Portrait combinations and ordinary 
single achromatic landscape lenses, being formed of 
glass of less density, are better able to resist the in- 
fluence of light ; but even these should always have 
their caps replaced after being used. 



CHAPTER XXVIII. 

HOW TO ASCERTAIN THE ANGLE OF VIEW INCLUDED 
BY ANY LENS. 

LET it be first of all understood that every class of lens 
having the same focus and covering power embraces the 
same angle on a plate of a given size. It is of no conse- 
quence what is the nature of the lens, or by what name it 
is called, whether single landscape, wide angle or narrow 
angle, rectilinear, symmetrical or portrait lens one thing 
is true of them all that if they be of similar foci the 
angle subtended on a plate of a certain number of 
inches will be alike in all of them. It is the focus of 
the lens and that alone which determines the angle 
of picture depicted on a plate of any given size. Some 
lenses may work sharper or quicker than others ; but, 
though a mere simple spectacle glass be used, or even 
in the absence altogether of a lens, a small hole in front 
of the camera be employed for producing the image, the 
rule holds good. 

In lenses of the distorting kind such as the ordinary 
single combination with a stop in front the compression 
of objects in proportion as they recede from the centre 
of the picture apparently militates against the accuracy 



i66 WIDE-ANGLE LENSES SLOW. 

of the rule here laid down ; but the difference in reality 
is so slight as not to demand attention, and the ad- 
vantage they possess in this respect over the non- 
distorting class of lenses may in practice be ignored. 

Wide-Angle Lens used for Narrow- Angle Views. Let us 
narrow this question and apply it to special cases. Here 
are two landscape objectives of equal foci, but one is a 
wide-angle and the other a narrow-angle lens. If the 
latter cover a 12x10 plate and the former an iSx 15 
plate, but for various reasons the wide-angle one be 
only used for a 12x10, will there be any difference 
between the productions of the wide and the narrow 
angle lenses ? Certainly not. Is there then any ad- 
vantage in having a wide-angle lens for such a purpose 
as that in question ? None whatever. But there is this 
advantage, that although it can do all the work that is 
done by the narrow-angle instrument (at the expense, 
however, of rapidity for every gain is attended by a 
loss in some other direction), it can do more if required. 
It can be used on an 18x15 plate, whereas the other 
cannot. 

Wide -Angle Lenses Necessarily Slow. We have spoken 
of a loss of rapidity in connexion with lenses of wide 
angle. This is inseparable from their method of con- 
struction, their curves being deeper than those of the 
narrow class, and necessitating the employment of a 
smaller stop, for the narrower the angle sought to be 
included by a lens the greater may be its aperture in 
proportion to its focus and vice versa. 

How to Measure the Angle of View. The question ma> 



MEASURING ANGLE OF VIEW. 167 

now be asked By what means can it be ascertained 
what angle of view a lens includes on a plate of any 
certain size? Before answering this we may make 
what to the majority of readers will be a self-evident 
statement : if a lens include on a plate of twenty inches 
diagonal an angle of view of 40, on a plate of ten 
inches diagonal the included angle will only be 20. 

Draw on a sheet of paper of sufficient size a straight 
line equal to the diagonal of the plate on which the 
negative is taken, say \^\ inches for a 12x10 plate 
for exarnple, and from the centre of this line erect 
a vertical' line equal in length to the focus of the lens. 
In the diagram the line A B is made the length of the 

c 

\ 



a 



d 

FIG. 49. 



, diagonal, whatever that may be, and the line C D is that 
which in a corresponding manner is made of the length 
of the focus of the lens. Now with a pencil draw lines 
from C (the lens) to A and B (two corners of the plate), 
and the angle thus made with the pencil, or A C B, 
represents the angle of view included, 



168 PROTRACTOR. 

There are few cases of drawing instruments sold in 
which there is not in some form or other a protractor to 
be found by which angles may be measured. But some 
photographers may perhaps not have access to such an 
instrument, so we will now describe how such may make 
for themselves a protractor which if not so elaborate as 
those sold by the dealer in mathematical instruments 
will yet be as useful as the best of them, and possibly 
be more easy to employ. The following diagram repre- 




50. 



sents a protractor which includes an angle of 90, and 
divided into nine equal parts, each part including ten 
degrees. These are further subdivided so as to permit 
five degrees to be read off. See Fig. 50. 

To use this protractor : having laid down a line eo^ual 



PROTRACTOR. 169 

to the diagonal of the plate, having further erected the 
centre perpendicular line equal in length to the focus of 
the lens, and having also drawn the pencil angle lines 
already described, place the protractor down upon the 
lines thus drawn, the point Z being placed exactly on 
the point of the vertical line, and let the line Z S 
coincide with that drawn from C to A in Fig. 49. 
Observe now on what part of the protractor the other 
boundary line (that from C to B) falls, and the figures 
indicate the angle sought for. No calculations are 
required, and the results are obtained in the simplest 
manner. 



CHAPTER XXIX. 

REFINED FOCUSSING BY MEANS OF A TELESCOPE. 

SOMETIMES occasions arise in which it is necessary 
to focus with extreme sharpness, even without a focus- 
ing screen. 

At the Derby Convention in 1886, the author ex- 
hibited a camera to which was attached a pocket tele- 
scope to ensure absolute sharpness, and the conditions 
for the using of which we shall discuss. 

Ground Glass Screens inadequate for absolute Focussing. 
If the acme of perfection in focussing be desired, the 
image should be an aerial one, that is, not broken up by 
being projected upon ground glass which renders it 
difficult, if not altogether impossible, for any one to see 
it distinctly when employing a high magnifying power 
for such purpose. Just imagine the case if in a tele- 
scope a ground glass, no matter how fine its surface, 
were interposed between the eye-piece and the object- 
glass at the point of focus. The system of focussing 
now to be advocated and described permits of the dark 
slide being inserted into its place in the camera, its 
shutter drawn, and everything in readiness for the final 
uncapping of the lens, and all this without having deter- 
mined upon the precise object at which the shot is to be 
made, or its distance from the camera, which in this 



AERIAL IMAGES. 171 

case may have a lens of twenty, thirty, or even forty 
inches focus, and be practically wanting in what is 
known as depth, and which entails the necessity of 
adjusting the focus upon the definite object to be taken, 
and not upon one either nearer or farther away. 

Aerial Images. A well-corrected lens, when directed 
to any scene, produces at its focus an aerial model of 
that scene, each portion of which presents the same 
relative distance to or from any other as do the same 
portions of the original. In a lens of short focus the 
whole of this aerial model is on a scale so diminutive 
and compressed that, except such portions as are close 
at hand, the distance relations between the others is too 
close to enable the eye to distinguish easily between 
them, and hence we say that everything beyond a 
certain distance is in equal sharpness, this ' certain dis- 
tance ' being nearer to the lens the shorter is its focus ; 
but, conversely, the longer is the focus of the lens the 
greater is the separation of the component parts of the 
subject that is being examined, and the farther is that 
distance beyond which everything is practically simul- 
taneously sharp. Five miles is a fairly long distance 
away, and so for that matter is one mile ; but let an 
object at the greater distance be examined through a 
large telescope, the focus of which has been set for 
looking at something only one mile off, and it will be 
seen quite indistinctly until refocussing has been had 
recourse to ; and when the five-mile object is made sharp 
a more distant object still will be blurry and indistinct 
until it in turn has been sharpened by refocussing. 



r 

172 A FOCUSSING TELESCOPE. 

Nature of a Focussing Telescope. Let a little pocket 
telescope be procured, the object-glass of which is the 
same focus as that of the lens on the camera ; such a 
telescope costs but little, and quite apart from the special 
use for it which is about to be described, it forms a 
most useful companion when one is away from home. 
One of such dimensions, with three draws and a leather- 
covered body, as will suit a camera of the average class 
employed in taking views on plates ten or twelve inches 
in size, can readily be obtained at a price under twice as 
many shillings, for high-class workmanship is not neces- 
sary ; what is of importance is that the focus of the 
telescopic object-glass and that of the photographic lens 
must be the same. To prepare this telescope for camera 
use it is only necessary that one of the draws be made 
so easy as to slide in or out by a touch. The one most 
convenient for this is the second from the eye-piece end, 
and the requisite ease in drawing can be imparted by 
unscrewing that particular tube and scraping the interior 
of the short piece into which it travels, or by bending 
out the slots usually made in it to give it a springy 
smoothness of motion. 

How to attach the Telescope to Camera. On the top of 
the camera front to which the lens is attached, or from 
its side (it is immaterial which, so long as it does not 
interfere with other movements), projects a pin, on which 
the telescope fits by means of a small hole cut into the 
leathered-covered portion of the body at any convenient 
distance from the object-glass end. In the one shown 
a,t Derby this distance is three inches from it. The 



FITTING TELESCOPE TO CAMERA. t^ 

same thing must be done with the eye-piece end of the 
telescope, the second sliding tube of which, by preference, 
must be connected in a similar way with the frame of 
the camera which carries the dark slide. It is of no 
consequence whatever how or where the outer end of 
the telescope is attached to the lens end of the camera, 
but care is required in determining the fixing of the 
other. It is effected in this way : Focus the camera 
lens on the ground glass on any moderately distant 
object with the greatest care, using a magnifying glass 
for this purpose, and noting the object that is in the 
centre of the field. Then, stepping the telescope on the 
pin in the front of the camera, direct it to the object 
forming the centre of the scene on the ground glass, 
and, having pulled out the eye-piece tube to its limit, 
focus sharply by means of the second tube, into which 
the second small hole has been drilled, and which will, 
or ought to, fall on some solid portion of the body or 
frame which receives the dark slide. Now insert the 
pin so that the expansion of the telescope is fixed at 
that point, and all the fitting is accomplished. It will 
now be found that, by racking the camera in or out, the 
telescope body will also slide with facility. 

To Use this System. We shall suppose that the ob- 
ject to be photographed is a ship rapidly proceeding out 
to sea, but that, owing to lighting or any other contin- 
gency, the precise moment for effecting the exposure is 
uncertain, and that the distance between ship and 
camera is ever increasing (or lessening). To watch the 
motions of the ship upon the ground glass would be 



174 APPLICA T10N TO LENSES. 

preposterous, because, when the proper moment for 
exposure arrived, the time occupied in removing the 
focussing screen and getting the dark slide inserted 
might cause a delay which would prove fatal to ob- 
taining the right effect at the right instant, whereas, 
without the ground glass examination, one could not be 
quite certain of the object being in correct focus. But 
by the telescopic system, all that is necessary is to in- 
sert the dark slide and let the plate remain open, subject 
to the operation of the instantaneous shutter, watch the 
ship through the telescope, which is kept in sharp focus 
by the rack and pinion of the camera, and at the fitting 
moment press the pneumatic ball of the shutter, when 
the image will be secured with a degree of facility and 
accuracy of focus quite incapable of being attained in 
the usual way. 

Application to Lenses of Various Foci. The real use of 
this system is to be found when employing lenses of 
long focus and rather large aperture, but for experi- 
mental purposes we have also had one attached to a small 
camera in which we use lenses varying in focus from five 
to eight inches, and in accordance with an optical law 
we have made the object-glass of the little telescope 
adaptable for all lenses ranging between these foci. The 
law referred to is treated of in the chapter (page 154) 
' On the Adjustment of Dissimilar Lenses,' but may here 
be summarised as follows : When two lenses of, say, ten 
inches each in focus are placed in contiguity, the focus 
is reduced to five inches approximately, but in pro- 
portion as they are separated so does the focus become 



APPLICATION TO LENSES. 175 

lengthened. Hence, by having two object-glasses of 
ong focus each instead of one in the telescope, the inner 
one being in a small travelling tube moving inside, and 
capable of being run pretty close up towards the eye- 
piece, a considerable range, or rather variety, of foci is 
obtained, the precise amount of focal power being deter* 
mined by graduations at the side of the slot through 
which the button projects by which the inner runner is 
moved. By a camera fitted with a little telescope of the 
nature described, we have, with a lens working with an 
aperture of /-4, selected one or more boys among groups 
which were playing at cricket on a common, and by 
following them with the telescope, focussing all the 
while by the camera rack, we have been able to ' snap ' 
them off in individual sharpness, while, owing to the 
unusually large aperture of the lens, all their surround- 
ings were more or less out of focus. But many applica- 
tions of the system will, doubtless, suggest themselves to 
the ingenious reader. 

Although we have never experienced any difficulty 
in procuring little telescopic object-glasses of any desired 
focus, yet it is conceivable that those at a distance from 
centres of optical industry may not be equally fortunate, 
Such may be interested in learning that in the case of 
an uncemented achromatic object-glass of a cheap tele- 
scope (which are almost invariably uncemented) a dif- 
ference in the focus results by the insertion of a ring 
between the flint and crown lenses so as to separate 
them. The concave lens of the combination being 
nearer to the eye-piece than the convex or crown-glass 



W APPLICATION TO LENSES. 

one, the farther apart they are separated the shorter will 
be their focus, being in this respect contrary to the effect 
produced if both lenses were positive, as previously 
explained. 

If a telescope with two achromatic object-glasses be 
desired so as to permit, as in a case cited, of its being 
made to suit a camera to which more than one lens of a 
certain focus is to be affixed, the rule by which any 
definite focus of such telescopic objective may be accu- 
rately determined or ascertained is the same as that in 
the chapter just referred to, viz. : Knowing the focus of 
each of the two object-glasses, add them together, and 
subtract the distance of their separation ; then multiply 
the two foci together and divide this last quantity by 
the first, which gives the precise focus of the two lenses 
when combined ; the focus thus can be lengthened by 
increasing the separation, and by the above rule this can 
be done with unerring accuracy. 



CHAPTER XXX. 

ANASTIGMATIC LENSES. 

OF the many lenses described in the preceding 
pages, Petzval's Portrait Combination and Dr. Adolf 
Steinheil's Aplanats were, doubtless, the most useful 
types, but their definition of the oblique pencils 
was marred by astigmatism. If the defining power of 
an aplanat be examined, as described on p. 139, it will 
be found impossible to focus simultaneously, near the 
margin of the plate, lines drawn at right angles to each 
other. The images of the two sets lie in different planes, 
which may be seen by altering the distance between lens 
and screen until sharp definition is obtained. At a short 
distance from the centre of the field one set is fuzzy, 
when the other is sharply defined. This is the defect 
known as astigmatism, and the lens from which it has 
been eliminated is called an anastigmat. Many years 
ago opticians came to the conclusion that the error could 
not be corrected until the glass-maker could supply 
glass possessing suitable properties. In a report upon 
the scientific apparatus shown at the International 
Exhibition held in London in 1876, Professor Abbe 
emphasized that there was little hope of progress in 
optical instruments until new varieties of glass were 
manufactured. This document, which was published in 

N 



i ;8 ANASTIGMATIC LENSES. 

1878, came under the notice of Dr. Schott, son of a 
glass manufacturer at Witten, Westphalia. He com- 
municated with Professor Abbe, and it was agreed that 
they should mutually endeavour to solve the problem, 
Dr. Schott making experimental specimens and Pro- 
fessor Abbe determining their optical qualities. Some 
very remarkable samples of glass were made, and 
these results were thought to be so important that the 
Prussian Government was induced to offer a subsidy to 
enable the firm of Schott and Genossen to experiment 
on a scale sufficiently large for commercial purposes. A 
number of new varieties of glass were placed upon the 
market in 1886, and many others have since been added 
to the list. Those which differ materially from the kinds 
previously made are usually called new Jena glasses, 
after the German town where the factory is situate. In 
the older varieties the dispersion of colour increases with 
the refraction, but among the new there are deviations. 
Heaviest Barium Crown, for instance, combines high re- 
fraction with low dispersion. A new era in photographic 
optics has thus been opened, and it is surprising how 
many new combinations of lenses have been invented. 

There are two kinds of cemented achromatic lenses 
which have been found of great use in the construction 
of anastigmats. Otto Lummer has named them Old 
and New 7 Achromats. As the old definitions of Crown 
and Flint do not apply to some of the Jena glasses, Dr. 
Rudolph defines ' Flint,' as the glass with higher relative 
dispersion, and ' Crown ' as that with lower relative dis- 
persion. For the construction of an anastigmat with 



ANASTIGMATTC LENSES. 179 

cemented surfaces it is necessary to combine a new with 
an old Achromat, for the reasons given in the following 
propositions formulated by Dr. Rudolph : 

' To correct spherical aberration in a cemented 
system, a Crown of lower refractive index than the Flint 
must be used ' (Old Achromat). 

* To correct astigmatism in a cemented system, a 
Crown of higher refractive index than the Flint must be 
used ' (New Achromat). 

Dr. Rudolf Steinheil published in Eder's Jahrbuch 
filr Photographic und Reproditctionstechnik, 1897, an 
article on ihe ' Origin and History of the Orthostigmats,' 
which defines in broader terms the necessary conditions 
for correcting spherical aberration and astigmatism. We 
give these dicta also, as they will be found applicable to 
the more recent anastigmats in which air spaces are 
used : 

' An objective can be corrected for spherical aberration 
if two media are separated by a convex 
surface turned towards the medium of 
higher refraction ' (Old Achromat). 

'An objective can be corrected for 
astigmatism if two media are separated 
by a concave surface turned towards the 
medium of higher refraction ' (New 
Achromat). 
FIG ci ^* 5 1 re P resents an Old Achromat, FIG - 5 2 - 

Fig. 52 a New Achromat. 

The flints are indicated by A and C, and the crowns 
by B and D, 



1 80 A NA S TIGMA TIC LENSES. 

As B is a crown of lower refraction than the flint A, 
we may infer that the combination, Fig 5 I, is corrected for 
spherical aberration, also that astigmatism is corrected 
in Pig. 52, because I) is a crown with higher refraction 
than the flint a 

By examining the nature of the contacts, similar 
inferences can be drawn from the propositions laid down 
by Dr. Steinheil. B presents a convex surface to A, the 
medium of higher refraction, and corrects spherical 
aberration. C presents a concave surface to I), the 
medium of higher refraction, and thus corrects astigma- 
tism. 

The use of glass with high refraction and low dis- 
persion permits of many new combinations of lenses. 
The optician's skill is shewn by the invention of new 
combinations, the selection of the most suitable kinds 
of glass for their construction, and the calculation of 
the best curves, thicknesses, and distances of separation. 

Some residual errors had to be remedied in the older 
lenses by using a stop. They were especially apparent 
at the margin of the picture, but have been almost 
eliminated from the best anastigmats, and thus the 
usefulness of the camera has been largely increased. 
This is notably the case with apparatus of small size, as 
the shorter focus of the lens gives the necessary depth 
of definition. 

In the drawings of the lenses described in this 
chapter the kinds of glass are indicated by the shading, 
according to the method used by Dr. Moritz von Rohr 
in his work, Theorie und Geschichte des photographischen 



ANASTIGMATIC LENSES. 181 

Objectivs, to which I am also indebted for many 
particulars. 

Flint glass is indicated by lines leaning to the 

right : x 
53 ^j^ 

Old, low refracting crown, by lines leaning to the 
left : ^ 



New, high refracting crown, by horizontal lines :- 

Air spaces are left blank. 

The light is assumed to pass through the lens from 
left to right. 

The Antiplanet, shown on page 72, may be regarded 
as the forerunner of the anastigmats. It was invented 
by Dr. Adolf Steinheil about 1879, and its great 
originality of construction entitled it to more attention 
than it received in England. Only the old varieties of 
glass could be had, yet, despite this disadvantage, the 
correction for astigmatism is improved, if the lens be 
compared with its antecedent, the Aplanat. The biconvex 
of the front and the biconcave of the back combination 
are of flint. The biconcave of the front, and the bicon- 
vex of the back combination are of ordinary crown m 
His intention evidently was to construct a lens con- 
forming, as closely as circumstances permitted, to the 
conditions subsequently published by his son, Dr. Rudolf 
Steinheil. It will be seen in the front combination that 
the negative crown element presents a concave surface 
to the more refractive flint, and that the positive crown 
in the back combination presents a convex surface to 




1 82 ANASTIGMA TIC LENSES. 

the more refractive flint But with the restricted choice 
of glass, this was only possible by making one com- 
bination negative, the other positive, and compensating 
the errors in one by opposite errors in the other. 

The ' Concentric ' was the next objective, in which 
correction of astigmatism was the leading feature. It 
was patented by Dr. Hugo Schroeder and Mr. John 
Stuart in 1888 and was the first pho- 
tographic lens in which a new Jena 
glass was used. Fig. 53 shows its 
construction. The front and back 
surfaces of each achromatic pair are 
struck from a common centre. The 

HU. 53. 

focus would be negative were only 
one kind of glass used, but as the plano-convex of high 
refraction and low dispersion is cemented to a plano- 
concave of lower refraction and equal or higher disper- 
sion, an achromatic lens of positive focus is formed. To 
obtain a flat image throughout a large field of view, the 
radii of the external surfaces are given a certain ratio to 
each other, dependent upon the refraction and dispersion 
of the two kinds of glass. It is found in practice that the 
limits of the refractive indices for the plano-convex are 
from about 159 to r6i, and for the plano-concave from 
about i'5o to i '5 3, taking the D line of the spectrum. 
Spherical aberration is not corrected for the full aper- 
ture, which is a serious drawback, as the necessary 
stop reduces the intensity of the lens considerably. 
Had the lens been brought out sooner, it might have 
enjoyed more popularity, but the advent of the Zeiss 



ANASTIGMATIC LENSES. 



.83 



Protars, not long afterwards, destroyed any chance of 
its success. 

The triplet planned by Professor Abbe and calculated 
by Dr. Rudolph was the first of the Zeiss photographic 
lenses, the date of its introduction being 1889. Fig. 54 
shows the construction. The outer elements are of 
miniscus form and their spherical and chromatic errors 
are corrected by a compound lens placed in the centre 




FIG. 54- 

of the combination. This corrector is formed of a double 
convex borate crown element, cemented between two 
negatives. The spherical correction is very good, but 
the lens is as astigmatic as the Aplanat. It is included 
in this chapter for convenience, but should not be 
regarded as an anastigmat. Professor Abbe aimed 
rather at apochromatism. The aperture of the lens is 
/-i6. 

Dr. Schroeder put in a claim for prior invention of 
this type, having published, in the AstronomiscJie 
Nachrichten, the description of an objective specially 
adapted for celestial photography, which is practically 



1 84 



ANAS TIG MA TIC 



the same. Dr. Schroeder tried not only triple, but 

double and quadruple combinations, for the construction 

of the central dialyte. 

In 1890 a patent was granted to Dr. P. Rudolph for 

a series of lenses, which are now known as ' Protars.' 
They have been issued in seven 
different degrees of rapidity. 
This was the first successful 
attempt to supply photographers 
with an achromatic objective 
combining spherical correction 
with anastigmatic flatness of 
field. In all the series the front 
combination is an old achromat. 
The back is a new achromat, 
or a more complex construction 

equivalent to a new achromat. Fig. 55 represents the 

simplest form of the Protar. By referring to page 69, it 

will be seen how near Thomas 

Ross and Thomas Grubb came 

to this type, but the crown glass 

necessary for the construction 

of a new achromat did not then 

exist. 

The two combinations of 

which the simple series of Pro- 
tars are constructed will be 6 

recognised by the shading of the 

glass. The front corrects spherical aberration and 

the back astigmatism. Both are of positive focus 





ANASTIGMATIC LENSES. 



,85 



and approximately corrected for colour. Fig. 56 shows 
one of the more complex series of Protars. The back 
combination consists of three elements, of which the 
two outer are positives of higher refraction than the flint 
they enclose. The advantages thus secured are better 
correction of astigmatism and greater flatness of field. 
The relative rapidity of these series \sf-4'$ ) f-6'3 and/- 8, 
but the first and second are not now listed. Two other 
lenses were patented by Dr. Rudolph, prior to the 
Protars. Their construction is shown in figs. 57 and 58. 





FIG. 57. 



FIG. 58. 



These were soon abandoned for those we have just 
described. They were inferior to the corresponding series 
of Aplanats in spherical correction, and though the field 
was flatter, over an angle of about 50, there was greater 
astigmatic difference. 

At the close of 1891 Dr. Rudolph had finished the 
calculation of a series of single lenses resembling in 
construction the half of the well-known Goerz Double 
Anastigmat, for which Emil von Hoegh made an 



1 86 



ANAST1GMATIC LENSES. 




FIG. 59. 



application for a patent in December, 1892. The con- 
struction of the latter is shown in fig. 59. Dr. Rudolf 

Steinheil had also been en- 
gaged in calculating a lens 
similar to the Goerz, but as 
his application for patent 
was not made till March of 
the following year, it was 
too late. Dr. Rudolph's ap- 
plication for a German patent ; 
for the single lens, was not 
made till a month later than 
Dr. Steinheil's, and it was 

refused for the same reason. Between the lenses of 
Emil von Hoegh and Dr. Rudolf Steinheil, on the 
one hand, and that of Dr. Rudolph, on the other 
hand, there is a characteristic difference. The former 
made the independent correction of the two halves 
subservient to the correction of the entire objective, but 
the latter aimed at obtaining the best correction of 
the separate combinations, so that they might be used 
to best advantage alone, and yet be available for the 
construction of sets of doublets. The Goere lens was 
issued in two series, one with an aperture of f-J"] 
''subsequently increased to/ 6'8), and another with an 
aperture of/-i I for copying purposes. 

The patent application of Dr. Rudolf Steinheil, to 
which we have referred, specified two types, however, 
and the validity of the claim for the second was admitted 
after considerable delay. The construction of the 



ANASTIGMATIC LENSES. 



187 



Second Series, now known as the Orthostigmat, Type 
II., is shown in fig 60. Here we have another instance 
of a lens construction being worked out independently 
by two opticians. Dr. Kampfer, a director of the firm 
of Voigtlander & Son, of Brunswick, applied for a 
patent for a lens resembling the Orthostigmat, Type II. 




But although his. application was refused, he ap- 
pears to have been more fortunate in the treatment he 
received at the hands of Dr. Rudolf Steinheil than 
the latter received from the Charlottenburg firm. Dr. 
Steinheil granted Messrs. Voigtlander a free licence to 
manufacture Type II., whereas for the right to manu- 
facture Type La royalty was demanded from Dr. Steinheil, 
but declined. The Orthostigmat, Type II., is issued in 
four rapidities: f-6'8, /-8, /-io, and /-I2. Messrs. 
Voigtlander's lens, the Col linear, is made in three series, 
/-5'4,/-6'8, and /-io to/-i2'5. 

In 1895 Dr. Rudolph calculated another series of 
convertible lenses for the firm of Zeiss. Each com- 
bination consists of four elements, and the correction is 
more perfect than in the older series with three elements 



i88 



ANASTIGMATIC LENSES. 



only. The construction of a doublet of the new series 
is shown in fig. 61. These lenses are known as Protars 
also, the single combinations being called Series VII., 





FIG. 6l. 

and the doublets Series VI I A. The construction is a 
further development of Dr. Rudolph's idea of combining 
an old with a new achromat. By referring 
to fig. 62, it will be seen that the front 
combination has been turned the other 
way and cemented to the back, to form 
a compound of four elements. 
It is possible to analyse the 
triple combination similarly, as 
shown in fig. 63, by dividing 
the middle element. But upon examining 
the glasses, it will be found there is a differ- 
ence The analysed triple combination con- 
sists of an old and a new achromat, in which 
both the negative elements are of flint. But 
in Protar VII. , the new achromat is formed of a high 
refracting crown, cemented to a lower refracting crown. 



FIG. 62. 




FIG. 63. 



AN ASTIGMATIC LENSES. 189 

Nevertheless, the gradation of the refractive indices 
maintains the same order as in the simple Protar. 
We find in both a similar arrangement of the contacts, 
a convex and a concave surface being turned to a 
medium of higher refraction. The single lenses forming 
Series VII. have an aperture of f-n in the small sizes 
and f-i 2*5 in the larger. The doublets of Series VIlA. 
have various apertures, viz.: f-$'6 in the small sizes, 
/-6'3 in the larger, and/-/ and/"-/*/ in those combining 
lenses of different foci. Two similar single lenses may 
be used for stereoscopic purposes, or as a doublet. 
Two different lenses give the photographer the choice 
of three different foci, and three different lenses the 
choice of six foci. This is a great convenience, but as 
the lenses are each composed of four elements, they are 
necessarily expensive. 

Shortly after the introduction of the convertible 
Protars, Messrs. Goerz and Von Hoegh also applied for 
a patent for lenses of the convertible kind. The single 
lenses, like the Zeiss Series VII., give fine 
definition at full aperture. They are also 
corrected for astigmatism and flatness of 
field. Fig. 64 represents a single lens of 
this type. It consists of three negative, FIG . 64. 
enclosing two positive, elements. Starting 
at the diaphragm, we find near it a biconcave of 
lowest refraction, connected to a biconvex lens of 
high refraction. Astigmatism is cured by their con- 
tact, and as the curve should have as long a radius 
as possible to flatten the field, the two glasses are 




190 ANA STIC MA TIC LENSES. 

selected for great difference of refractive power. The 
middle, or third lens, is biconcave and of low refraction. 
The light is collected by the contact with the second 
lens, and it exercises a compensating influence upon the 
distortion produced by the other surfaces, without 
adding to the astigmatism. The fourth lens is biconvex, 
and its contact with the fifth is used for correcting 
spherical aberration. The choice of the glasses for these 
two lenses is governed by two considerations. Firstly, 
the last surface, through which the light passes before 
reaching the plate, should refract as much as possible, 
therefore the index of the glass for the fifth lens should 
be high. Secondly, the curve of contact of the fourth and 
fifth lenses must be of definite depth, otherwise the dis- 
tortion would be in excessofthecorrectivepowerofthecon- 
tact of the second and third lenses. A glass of given 
lower refractive power must therefore be selected for the 
fourth lens. A consideration of the following indices of 
refraction, taken from the English Patent Specification, 
will throw further light upon the subject :- 1*51 ; r6i ; 
1*52; 1*54 and i'6i. It will also be noticed that the posi- 
tive elements are of smaller diameter than the negative, 
and it is therefore necessary to cement a positive to a 
negative before the second surface of the positive can be 
finished. The negative elements are thus cemented 
together at the margins, and as they are truly centred, 
the positive elements must also be true in this respect. 
This feature is the essential condition of the German 
patent, and it is unique as the means of protecting by 
patent the construction of a lens. The single lenses 



ANASTIGMAT1C LENSES. 



191 



work at full aperture, /-I I, and maybe combined to 
form symmetricals working at /-5*5, or convertible 
doublets varying in rapidity fromy-5'9 toy-63. 

Another patent was obtained by Dr. Rudolph in 
1897, for a lens known as the Planar. In it we find an 
application of the Gauss method of correcting the 
telescope. Alvan Clark, an American optician, made 
an attempt to use this method of correction for photo- 
graphic lenses in 1889, Dut without marked success. 
The construction of the Planar is shown in fig. 65. The 




FIG. 65. 

cemented pairs are of crown and flint, with the same or 
approximately the same refraction, but different dis- 
persion. This compound lens has the same refractive 
power for a given colour as a similar one made of either 
kind of glass, but the dispersion is different. Moreover, 
the dispersion may be modified by altering the contact 
curve. The va'ue of the Gauss method of correction 
lies in the elimination, for a large aperture, of the 
error known as the chromatic difference of spherical 
aberration. As the cemented lens, to which we have 



1 92 ANASTIGMATIC LENSES. 

referred, gives the optician greater latitude in the 
selection of the glass, the Planar construction is very 
accommodating. The quality of the definition of these 
lenses is such, that they may be used as low-power 
micro^:cpic objectives. In the smaller sizes the intensity 
is/-3*6, which renders them very useful for photomicro- 
graphy. The field is exceptionally flat and free from 
astigmatism. An apochromatic series for three-colour 
work is also supplied. 

In the same year application was made by Dr. 
Rudolph for a patent for the lens known as the Unar, 
shown in fig. 66. Both its com- 
binations are formed by two lenses 
enclosing an air space. The focus 
of the front combination is nega- 
tive, and that of the back positive. 
In this respect it differs from the 

FIG. 66. 

Planar, and other doublets with air 

lenses, which till then had been, without exception, of 
symmetrical construction, or approximately so. The 
development of this lens and that of the Antiplanet, 
constructed by Dr. Adolf Steinheil, is suggestive of a 
parallel. The approximately symmetrical construction 
of the Planar, an objective with air spaces resembling 
one another in their effect, is abandoned for one with air 
spaces of opposite character, and the front lens becomes 
negative in focus. The parallel is not complete, yet a 
striking resemblance exists between the line of thought 
which underlies both constructions. In the patent specifi- 
cation Dr. Rudolph gives the following explanation : 




ANASTIGMATIC LENSES. 193 

' The effect of combining two pairs of facing sur- 
faces ' (those enclosing the air spaces) ' of opposite 
power, is similar to the result obtained in the objective 
described in Specification No. 6028, A.D. 1890 '(fig. 55, 
p. 184), ' by the opposite sign prescribed by the difference 
between the refractive indices of the crown and flint 
lenses in the cemented components of a doublet. The 
pairs of facing surfaces produce, in accordance with the 
signs of their powers, astigmatic differences of opposite 
character, so that, in addition to spherical correction 
of the whole system and flattening of the image, astig- 
matism may be fully corrected.' After pointing out 
that the use of air spaces introduces a greater difference 
between the media through which the light has to pass, 
and increases the number of elements available for 
correcting the objective, Dr. Rudolph adds : ' From 
the foregoing it will be understood that the adoption 
of the new type of objective will result either in larger 
apertures, the spherical correction remaining of the 
same quality, or when the apertures are unaltered in 
improving spherical corrections.' Two series of these 
lenses have been issued : the aperture 
of the first ranges from/~-4'5 to/*- 5 "6. 
according to length of focus ; that of 
the second is/-6'3 f r a ^ tne sizes. 

In 1902 Dr. Rudolph obtained 
a patent for the ' Tessar.' The 
construction of this lens is shown 
in fig. 67. The front combination resembles that of the 
Unar. It has an air space between the lenses, and is 

O 




t94 ANASTIGMATIC L&NS&S. 

of negative focus. The back combination is a new 
achromat of positive focus, and resembles that of the 
Protar. It may thus be looked upon as a cross 
between these k two prior types. The ordinary series 
has an aperture of f-6"$. There is also an apochro- 
matic series for copying and three-colour work, the 
aperture of which ranges from f-io to f-i$, according 
to focal length. This lens is at present the last of the 
photographic lenses invented by Dr. Paul Rudolph. 

In 1900 a patent was granted to C. P. Goerz for a 
lens which is of great value under certain exceptional 
conditions, where an abnormally wide angle has to be 
included in the photograph. Otherwise, the droll effects 
of perspective it produces might 
entitle the lens to rank as a freak. 
Its construction is shown in fig. 68, 
and it is called the Hypergon 
Double Anastigmat. It is un- 
corrected for colour and spherical 
Flr 68 aberration, but it possesses an 

anastigmatic flat field, and covers 

the exceptionally wide angle of about 135. The aperture 
is small, being only /-22. The components are very 
thin, and their surfaces are struck as nearly as possible 
with the same radius to cure astigmatism. The com- 
ponents are nearly hemispherical, to permit the in- 
clusion of the very wide angle referred to. The 
unequal illumination of the objective is compensated 
by a revolving starstop, which cuts off light from centre 
to margin in diminishing degree. The intensity is con- 





ANASTIGMATIC LENSES. 195 

sequently lower than the aperture of the stop indicates. 
On the other hand, there should be little absorption ot 
light by the glass. 

The last series of lenses brought out by C. P. Goerz 
forms the subject of a British patent granted to him 
and Emil von Hoegh in 1898. From 
fig. 69, which shows the construction, 
it will be seen that it resembles the 
Orthostigmat, but with the exception 
that an air space takes the place of 
the middle element in both combina- 
tions. This meniscus, in the Orthos- 

FIG. 69. 

tigmat, has the lowest refractive index, 
but as air is of still lower refractive power, the difference 
in the gradation of the indices, upon which the correc- 
tion depends, is considerably increased. This has been 
used to give the objective a larger aperture. There is a 
rapid series with relative apertures ranging from/^4'5 t 
f-S'S) an d a slower, with the relative aperture f-6'3 for 
each size. 

Dr. Rudolf Steinheil applied in 1901 for a British 
patent for a new lens, which has been called the 
' Unofocal.' Fig. 70 illustrates the construction, which 
is symmetrical. This lens also exhibits a resemblance 
to the Orthostigmat, but there is a remarkable differ- 
ence. The refractive indices of the crown and flint 
are equal. The achromatism is adjusted by a definite 
distance of separation between the elements, and the 
Petzval rule, requiring that the sum of the foci shall 
equal o, is complied with. In the Goerz lens previously 



196 



ANASTIGMATIC LENSES. 



referred to, as in the Orthostigmat, the biconvex front 
lens is a highly refracting crown belonging to the 
series of anomalous glasses introduced by Schott and 
Genossen. In a cemented lens this is necessary for 







FIG. 70. 



curing astigmatism by the combination of a normal 
with an abnormal pair of glasses. But in the Unofocal 
both glasses are of the same refractive index, and 
thus we have neither an abnormal nor a normal pair. 
Yet if we turn to Dr. Rudolf Steinheil's statement of 
the conditions necessary to cure astigmatism and 
spherical aberration, we find that they are satisfied. 
Air being a medium of lower refraction than glass, the 
space between the two elements presents the requisite 
concave and convex surfaces to media of higher re- 
fraction. The series already introduced have the 
apertures f-4'$ and f-6. 

Through Dr. Miethe's acceptance of a professorship 
at the Berlin Technical School, a vacancy occurred in 
the directorate of the firm of Voigtlander & Son. Dr. 
Harting, at that time a member of the staff of the 
firm of Carl Zeiss, was appointed to fill Dr. Miethe's 



AhASTIGMATIC LENSES. 



197 



former position. In December, 1900, Dr. Harting 
made application for a British patent for the lens- 
construction shown in fig. 71. This objective is a 
triplet of symmetrical construction, corrected for astig- 
matism and spherical aberration at large apertures. 
The scheme of correction may be described as fol- 
lows : The crowns of the external combinations must 




FIG. 71. 

have a larger or smaller index of refraction than the 
flints to which they are cemented. The middle lens must 
be of opposite kind of glass to either element facing 
kt, the refractive index of one being smaller, or approxi- 
mately as large, and the dispersion greater than that of 
the other. In the drawing the outer negative lenses 
are of flint, and the positive lenses, to which they are 
cemented, of crown possessing higher refraction and 
lower dispersion. The middle equi-concave lens must 
therefore be of flint, possessing, in comparison with the 
crown, the same or lower refraction combined with 
higher dispersion. Eighteen months later Dr. Harting 



198 ANAST1GMATIC LENSES. 

applied for another patent embodying improvements, 
in the objective we have just described. The difference 
in the construction may be seen by comparing fig. 72 
with the previous one. The deviations do not refer to 
symmetry of arrangement and relative proportionate 
sizes of the indices of refraction and dispersion, but to 




FIG. 72. 

the curves of the lenses and the choice of the indices of 
the glass. The greater freedom thus obtained permits 
of much more effective correction of astigmatism and 
curvature of field, but rectilinearity is affected slightly, 
and likewise the achromatism of the focal lengths. 
Under these patents the * Heliar,' with an aperture of 
/"-4'5, has been brought out. In its construction it re- 
sembles fig. 72. 

More recently Messrs. Voigtlander have introduced 
the * Dynar,' the other alternative construction of the 
patents. In this case, the position of crown and 
flint in the outer combinations has been reversed. It 
follows from the patent specification, under these cir- 
cumstances, that the central negative lens must be of 



ANASTIGMATIC LENSES. 



199 




FIG 73. 



highly refractive crown. The aperture of the Dynar is 
f-6. Fig. 73 shows its construction. 
In November, 1900, a photo- 
graphic objective corrected on the 
Gauss principle was patented in this 
country by Hugo Meyer, of Gorlitz. 
The construction is illustrated in 
fig. 74. The claim provides that 
either or both elements shall have 
one convex and one concave surface. The positive 
element must be of high refraction and low dispersion, 
and the negative of similar or lower 
refraction and greater dispersion 
than the positive. The crown is 
therefore one of the Jena new 
glasses used to form an abnormal 
pair. The lens is also patented in 
Germany, and known there as the Aristostigmat. The 
aperture is/-77. 

Karl Martin, of Rathenow, also applied for a patent 
for a somewhat similar lens in September, 1901, as may 
be seen from fig. 75, which shows 
its construction. In it a negative 
meniscus is combined with a posi- 
tive lens to form an air space be- 
tween them, which has the shape of 
a negative lens. The negative glass 
element must be of higher refraction than the positive, 
consequently the pair of glasses is a normal one. The 
lens is corrected for colour, astigmatism, and spherical 





FIG. 75. 



200 



ANASTIGMATIC LENSES. 



aberration. It is manufactured by the Rathenower 

Optische Industrie Anstalt in two series, with the 

relative apertures /-$'$ and f-7'7- 

In January, 1899, Ernst Leitz, of Wetzlar, obtained 

a British patent for the lens construction shown in 
fig. 76, called the Periplan. The 
front combination resembles in con- 
struction the original Goerz Anas- 
tigmat, whilst the back combination 
is a new achromat. The whole 
burden of spherical correction is 




FIG. 76. 



thrown upon the contact of the meniscus and biconcave 
elements of the front combination. The other two con- 
tacts are used for correcting astigmatism. The relative 
aperture of this objective is/7'8. 

In July of the same year a German patent was 
granted to the same optician for a photographic objec- 
tive of symmetrical character, which 
has been named the ' Summar.' 
The construction is exhibited in 
fig. 77. The biconvex crown of 
high refraction is u^c^j^for correct- 
ing astigmatism. ' The - plano- 
concave flint cemented to the 
meniscus of low refracting crown 
form together a negative lens, and the enclosed contact 
corrects the spherical aberration. The objective has a 
relative aperture of/- 5. 

A German patent was granted to E. Arbeit in 
February, 1901, for a lens constructed as in fig. 78. 




FIG. 77 




ANASTIGMATIC LENSES. 201 

The objective is symmetrical, both combinations being 
similarly formed of a new achromat, separated by an air 
space from a highly refracting crown meniscus. The posi- 
tive elements areof the same kind 
of glass. The lens, the Euryplan, 
is made by Gebruder Schulze, 
of Potsdam, who have introduced 
two series with the relative 
apertures f-6 and f-f$. 

The preceding anastigmatic FIG. 78. 

lenses, without exception, are 

of German origin, but the list, though a long one, 
does not include all. Some are omitted because they 
have not reached the commercial stage, or closely 
resemble others, which have been included. Yet those 
which are described make it apparent how powerful has 
been the impulse given to the optician by the intro- 
duction of new varieties of glass. Even in England two 
opticians, Harold Dennis Taylor and Hugh Lancelot 
Aldis, have made noteworthy efforts to improve the 
photographic lens. The work of the former, which is of 
great originality, dates from 1893, when a patent was 
applied for, which forms the basis of the Cooke lens 
constructions. The meniscus form of the positive lens, 
used in the construction of the Aplanat or Rectilinear, 
is definitely abandoned, because it involves the use of 
diaphragm corrections. In its place a biconvex is used, 
and the radii of its surfaces are given certain relative 
lengths to eliminate coma. The negative lens, used 
for achromatising the combination, is dealt with to 



202 



ANASTIGMATTC LF.N<ES. 



eliminate coma of opposite character. Its focal length 
approximates to that of the positive, a condition laid 
down with precision by Petzval. One of its functions 
is to increase the focus, and as the errors of curvature of 
field are opposite for the two lenses, they tend to 
produce a flattened anastigmatic image, without the use 



FIG. 79. 

of diaphragm corrections. Were the objective con- 
structed of two elements only in the manner described, 
the image would be distorted. This is remedied by 
dividing the positive lens and placing the negative 
between them, thus forming a triplet Various series 
have been issued. Fig. 79 represents a portrait combina- 
. tion of f~4'$ relative aperture. 

Fig. 80 represents another series 
with apertures /-6*5 and /-8. Ano- 
ther patent was applied for in 1899 
to extend the usefulness of these 
FIG. 80 objectives by means of supple- 

mentary lenses formed of glass 

of different dispersive powers. These supplementary 
lenses vary in focal length approximately as their 




ANASTIGMATIC LENSES 



203 



dispersion, and thus considerable variations in the 
focal length of the objective may be secured. Subse- 
quently another patent was obtained for a mechanical 
device by which the objective may be focussed by 
altering the distance of separation between the front 
and middle lenses. 

In September, 1895, Mr. Hugh Lancelot Aldis, then 
with the firm of J. H. Dallmeyer, Ltd., obtained a patent 
for an improvement in photographic lenses. These con- 
structions have been named the Stigmatic, and three 
series have been introduced. Fig. 81 represents the 




FIG. 81. 

portrait lens with a relative aperture of f-^. The 
inventor turned to the method of correction used by 
Dr. Adolf Stcinheil in the Antiplanet. The front 
combination has strong positive spherical aberration, 
which is corrected by sufficient negfltive spherical aber- 
ration in the back combination. All the positive lenses 
are of heavy barium silicate crown. The negative 
lens in the front combination is an ordinary light flint, 



204 



ANASTIGMATIC LENSES. 




FIG. 82. 



and the two negative elements in the back combination 
are of soft silicate crown. The contact of the cemented 
middle pair corrects the astig- 
matism. Fig. 82 represents the 
universal series, which has an 
aperture of f-6. In this the 
cemented elements are of heavy 
barium silicate crown and ordi- 
nary light flint. The detached 
negative meniscus is of soft 

silicate crown. The astigmatism in this series is cor- 
rected by the two cemented contacts. Both com- 
binations may be used as landscape lenses, the front 
being about twice, and the back about one and a 
half times, the focal length of the entire objective. A 
very convenient range of foci is thus obtained in a 
compact form. 

After leaving Messrs. J. H. Dallmeyer, Ltd., this 
optician applied for another patent, which was granted 
in February, 1902. The con- 
struction is known as the 
Aldis lens and it is shown 
in fig. 83. Spherical aber- 
ration is corrected by the 
cemented surfaces of the front 
combination, and astigmatism 

is cured by the inner surface of the back lens. The 
leading idea of the construction appears to have been 
economy of means of correction. The number of ele- 
ments has been reduced to the fewest possible, and in 




ANASTIGMATIC LENSES. 



205 



this respect it presents a very marked difference if it 
be compared with the portrait construction by the 
same optician. The aperture of this lens is /-6, but 
another series has been introduced with an aperture 

of/-77- 

Messrs. Watson & Sons have also brought out an 
anastigmatic lens, which may be used in convertible 
sets. Fig. 84 illustrates the 
construction. Two combina- 
tions of similar, o_r dissimilar, 
focus may be united, as in 
the case of the Zeiss Con- 
vertible Protars. The con- 
struction appears to be similar 
to that of one alternative re- 




FIG. 84. 



ferred to in the patent granted to Emil von Hoegh 
in 1892. This lens is called the Holostigmat. 

It may be pointed out, in conclusion, that the 
anastigmats of German origin are broadly divisible 
into two groups. One is constructed of old and new 
achromats, with cemented surfaces, and the other is 
dependent upon the use of air spaces in the combi- 
nations. The fact that the Steinheil Unofocal, the Karl 
Martin, and Cooke lenses may be made without ano- 
malous pairs of glasses shows that an achromatic ob- 
jective, corrected for spherical aberration and anastig- 
matic flatness of field, might have been constructed 
before the introduction of the new Jena glasses. 
Perhaps the prejudice against air spaces in the con- 
struction of a lens may have deterred opticians from 



2o6 ANAST1GMAT1C LENSES. 

experimenting in this direction. Two other novelties also 
deserve special notice the use of the biconvex and 
biconcave elements, approximately free from diaphragm 
corrections, introduced by Mr. H. Dennis Taylor, and 
the hyperchromatic dispersive lens used by Dr. Rudolph 
in the Planar. 



CHAPTER XXXI. 

MOUNTS AND CELLS. 

MANY years ago a French optician, Derogy, intro- 
duced a lens, now disused, but the mechanical features 
of which deserve more general recognition than appears 
to have been accorded to it. It was formed in several 
different sizes (three at any rate), these being the quarter, 
half, and whole-plate portrait lenses. In the normal con- 
dition these were lenses of good quality, suited for the 
different dimensions of plates. 

Bayonet Joints for Lens Cells and Fittings. Upon dis- 
section and examination of these objectives certain 
peculiarities become apparent. First of all, the cells 
containing the lenses are not adapted to the mount by 
screws, but by means of ' bayonet joints,' there being 
two such fastenings fitted to each cell. The workman- 
ship being good there is no chance of anything becoming 
unfastened. On removing the front cell, which is done 
by a quarter of a turn of the hand, it is found to contain 
the means for adapting still another cell, the position of 
which will be nearly midway between the front and back 
lenses. The object of this third cell-receptacle is seen 
when, upon opening a small circular morocco case, which 



io8 FOCtfS. 

is packed in the bond of the lens, two cells, each con- 
taining a supplementary lens one concave and the 
other convex, both being achromatic are disclosed 
neatly fitted in appropriate receptacles. Either of these 
can, with a quarter turn of the hand, as before, be trans- 
ferred to the vacant place in the mount, and thus serve 
to modify the focus. 

Lengthening or Shortening the Focus. The real effi- 
ciency of the system will be seen from the following 
measurements which we have made of the equivalent 
foci of the one such lens in our possession when sub- 
jected seriatim to its several modifying influences : 
Premising that the lens now being described is one 
of the smallest which were made, namely, the quarter- 
plate size, and that the diameter of the front and back 
elements is slightly under one inch and three-quarters, 
in the combined form as a double portrait-lens the 
equivalent focus is seven inches. The insertion of the 
cell containing the concave achromatic, and upon which 
is engraved ' Lentille pour faire plus grandl lengthens 
the equivalent focus to nine inches ; while the sub- 
stitution for it of that containing the convex achromatic, 
and which bears the inscription ' Lentille pour faire plus 
petit,' shortens the focus to five and a quarter inches 
equivalent, or three and a half inches back, focus. But 
the front lens is also adapted for being used alone, for 
which purpose it is transferred to the place of the back 
combination, previously removed from its position, giving 
a focus of eleven inches. This, however, is not all, for 
by employing the front and the concave together a 



DISTANCE BETWEEN LENSES. 209 

focus of seventeen inches is obtained the substitution 
of the convex for the concave in this relation giving a 
focus of eight inches. 

Here, then, are great capabilities condensed in a 
small space. In this one objective we have foci to the 
following extent : Five and a quarter inches, seven 
inches, eight inches, nine inches, eleven inches, and 
seventeen inches. We have an idea that this com- 
bination has long ceased to be manufactured ; but it 
is probable that the causes which led to its having 
fallen into desuetude are now removed, and we describe 
it as containing merits to which manufacturers might 
well pay heed. Incidentally we may state that one of 
the combinations formed is that which, after many years' 
experiment, has been found by Professor Woodward to 
be best adapted for use with his solar camera as an ob- 
jective. The combination alluded to is that in which 
the convex supplementary lens an achromatic meniscus 
is utilised for the purpose of shortening the focus of 
the portrait objective. 

Distance between Lenses should not be Arbitrary. It 
does not follow that in the case of a combination lens 
the distance at which they are set apart in the mount is 
the best for every purpose. The optician has to make a 
compromise, and secure a balance of advantages. That 
distance at which flatness of field is best attained may 
be attended with flare, while an increased angle of view 
may, under certain circumstances, be secured without 
any serious loss by setting the lenses much closer to- 
gether. The most generally useful mount for a lens of 



210 DISTANCE OF STOP IN SINGLE LENSES. 

this class is one in which each lens is set in a short 
supplementary tube, capable of being drawn out from 
the common centre, so as to increase or shorten the dis- 
tance between them at will. When the lenses are sepa- 
rated to the maximum extent, the field will be flat even 
to the verge of astigmatism with a large aperture ; while, 
in proportion as they are made to approach each other, 
so does the area of illumination increase, this, how- 
ever, being attended with roundness of field. Hence, 
by adopting suitable precautions in the separation, a 
doublet lens may be made to act either as a wide or 
narrow angle objective. The expediency of adopting a 
mount of this kind is, however, open to question, as 
there might not be one out of ten who would know 
how to use the power aright were it placed in their 
hands. 

Distance of Stop in Single Lenses. A very sensible 
advantage may frequently be derived by the power of 
adjusting the distance between the stop and the lens in 
the case of a single landscape objective. It is well known 
that with all such lenses, especially those of a plano- 
convex or only slight meniscus form, the farther the 
stop is from the surface of the lens, the wider may be 
the aperture in such stop. This, however, circumscribes 
the field of delineation. By placing the stop nearer to 
the lens, two advantages are secured. First, the lens will 
cover a much larger plate, and, secondly, the distortion 
that is so common to landscape lenses becomes mini- 
mised ; for, as we have shown in a previous chapter, the 
nearer the stop is to the optical centre of a lens, the less 



AL UMINIUM MO UNTS. 2 1 1 

is the distortion : but this approximating of the dia- 
phragm to the lens necessitates a smaller stop being 
employed than when a greater distance intervenes be- 
tween them. 

Cell-bound Lenses. It is of vital consequence that a 
lens be not set in its cell under conditions which give 
great pressure to any part of its substance. A delicate, 
well-constructed lens may have its good qualities dis 
turbed by being forced into a tight cell which is bur- 
nished down upon it, thereby giving considerable pressure. 
The presence of this pressure is readily ascertained by 
placing the lens in a beam of polarised light, and ex- 
amining it by an analyser, by which the strain on the 
glass will be shown. The effect of this is precisely 
as though the lens had been made of badly annealed 
glass. 

Aluminium Mounts. The weight of the brasswork of 
lenses is often far in excess of what is required for 
rigidity. By adopting papier mache, ebonite, or alu- 
minium, an important saving to the wear and tear 
experienced by the photographer would be effected. It 
was at one time objected to aluminium that it was 
expensive. This was true to some extent, although not 
so much so as to render its applicability to a photo- 
graphic lens of great importance in this respect. But it 
is now the case that owing to the demand which has 
arisen for this metal, its price has been reduced to that 
at which copper is now sold. As the specific gravity 
of aluminium is about 2*56, while that of copper is fre- 
quently 8*96, the great gain in lightness will be apparent. 



212 FLANGE APERTURES. 

Some makers, notably in America, have begun to discard 
brass for the diaphragms of their larger lenses, adopting 
ebonite or vulcanite instead, to the great advantage of 
the users. It only remains that this measure of reform 
shall be made to permeate the other portions of the 
mount to have an improvement far exceeding that which 
was inaugurated by the introduction of the leather cap 
in lieu of the heavy brass cap which it supplanted. 

By the apparent paradox of making use of heavy- 
glass the opticians are now able to give us lenses small 
in bulk and comparatively light in weight, so far as 
concerns the mere glasswork of the objective. It now 
devolves upon them to effect a similar measure of 
reform in the mounts of the larger of the portable form, 
such as those exceeding one and a half inches in 
diameter. 

Dimensions of Flange Apertures. It is much to be 
regretted that up to the present time no really 
universal system of diameters of apertures and 
screw threads in lens flanges has yet been adopted, 
notwithstanding the efforts of committees of Con- 
gresses, Conventions, and Societies to bring about so 
desirable an end. A practical outcome of the chaos 
that still prevails is, that three or four lenses may be 
purchased from as many different leading opticians, and 
although the screws on these might have so easily been . 
absolutely as they are nearly identical, not one of them 
will interchange with the others in the flange. This 
for many years has been a sore grievance with users 
of lenses. 



LENS ADAPTERS. 213 

A Universal Lens Adapter. Pending the adoption of 
some system on which all makers will agree, we give 
here a method, originated in France, by which lenses 
having various flange apertures can be quickly adapted 
in succession to any camera. 

A series of discs of ebonite or thin metal, one for 
each lens, is provided. They are all of equal diameter 
outside, but the aperture in each is such as just to 
allow the screw of the mount to pass easily through. 
The flange, which is smaller than the disc, is now screwed 
on the mount and keeps the disc firmly fixed. The hole 
in the camera front is smaller than the disc, which, when 
placed over it, entirely covers the aperture. Three guide 
pins, or, by preference, a round recess in the camera- 
front, ensures the lens being centrally attached, and a 
turn-button at each side secures it firmly to the front. 

This method is equally useful for the studio as for 
the field camera. It permits of one lens being changed 
for another in a very brief period of time, and saves the 
trouble of having a separate camera front for each lens 
that is likely to be used. 

Lens adapters constructed on the iris diaphragm 
system is another French idea. They grip a lens with 
a closeness sufficient to prevent the admission of light, 
but the hold taken of the lens by the thin edges of the 
iris blades is rather too slender to ensure the lens against 
dropping out at an inopportune moment 



CHAPTER XXXlJt. 

LENS GRINDING. 

ALTHOUGH, as we have stated in the Preface, this 
work is intended for users and not manufacturers of 
lenses, yet may there be some among the former who 
desire to know how lenses are ground and finished. 

Selection of the Glass. As all dealers in optical glass 
supply it of the requisite degrees of refractive and dis- 
persive indices, no trouble now arises in procuring it. 
But having been obtained, it is necessary to subject it to 
careful examination for internal defects which would 
otherwise only be discoverable after the labour of grind-- 
ing and finishing the surfaces had been undergone, and 
the labour thus wasted. 

Imperfect annealing demands primary attention. 
This defect, where it exists, is readily discoverable by 
examining the glass by polarised light. Let A (Fig. 85) 
be a lamp, C ten plates of clean glass bound together 
at the edges, and E a Nicol prism. The light from A 
becomes polarised when reflected from C at a suitable 
angle; and when any object, D in this case the slab of 
glass undergoing examination is placed in the path of 
the reflected ray, any heterogeneousness in the glass 



GRINDING. 215 

arising from imperfect annealing is rendered plainly 
visible by rotating the analyser, E. Incidentally we may 
observe that by this means defects in finished lenses can 
also be discovered. For example, an otherwise perfect 





FIG. 85. 

lens, if subjected to undue pressure in its cell, will show 
lines or patches of opacity when subjected to this test. 

To effect the conversion of a piece of plain glass 
into a lens is a class of mechanical work demanding no 
exceptional degree of skill, although care is necessitated. 
Where genius is required is in the determination of 
the curves to suit the special requirement and of the 
glass best adapted to the purpose. An able mathe- [ 
matician can, as the result of his calculations, send to l 
the manager of a lens-grinding establishment a formula 
or specification for a lens as to which he can predicate 
before the work is commenced everything as regards 
its capabilities and performance. This, however, belongs 
to the 'fine-art' department of the business and to the 
higher mathematics. We must here confine ourselves to 
the more material aspects of the construction of a lens. 



216 RULES FOR FINDING PRINCIPAL FOCUS. 

Density influences Curvatures. The density of the glass 
determines the curvature requisite in making a lens of 
a definite focus; but the following rules and expla- 
nations will serve to afford an average or general idea 
of the relation between focus and curvature. On the 
supposition that we are dealing with crown glass : if a 
circle be made on a sheet of paper with any opening of 
the compasses say three inches and a portion of this 
circle be cut off by a straight line, such portion will 
represent a plano-convex lens of three inches radius, 
and its focus for parallel rays will (assuming the convex 
surface to be directed outwards) be nearly upon the line 
of the circle opposite to the lenticular slice. This is 
more tersely expressed in treatises on mathematical 
optics as follows : 

Eule for Finding the Principal Focus of a Piano -Convex 
Lens. When the convex side is exposed to parallel rays 
the focal distance will be equal to twice the radius of 
its convex surface, diminished by two-thirds of the 
thickness of the lens. 

Rule for Finding the Principal Focus of an Equally 
Double -Convex Lens. The focal distance is equal to the 
radius. In the drawing which we have imagined above 
if, instead of the portion of the circle having been 
separated by a straight line, a curved line of the same 
radius as the circle had been employed, the lens 
formed would have come under this category, namely, 
equally double-convex, and its focus would have been 
approximately in the centre of the circle, or three 
inches. 



GRINDING TOOLS. ^1 

Rule for Finding the Focus of a Double-Convex Lens of 
Unequal Curvatures. Multiply the radius of one surface 
by the radius of the other, and divide twice this product 
by the sum of the same radii. This last lens is usually 
designated a ' crossed ' lens. 

In the case of a meniscus with parallel rays we must 
divide twice the product of the two radii by their 
difference, and the quotient will be the focal distance 
required. 

These rules must not be considered absolute, for 
with every different sample of glass there may be a 
departure from them, and, in some cases e.g., dense 
flint the departure will be very considerable ; but with 
ordinary crown or plate glass they are, probably, as 
near as can be framed in popular language. 

Grinding Tools. Having determined upon thediameter 
and curvature of the lens to be made, the first thing to 
do is to obtain grinding tools of the radius of curvative 
required. They consist of a pair namely, a convex 
and a concave and can be purchased of any radius 
from those who make a speciality of this department of 
business ; but an amateur will, doubtless, prefer to make 
them f6r himself. To do this he must make two tem- 
plates of thin sheet brass or zinc (Fig. 86), by turning one 
piece to exactly three inches in diameter, assuming that 
a radius of three inches is to be employed ; the other 
piece to have a hole of this diameter cut in it, and after- 
wards divided into two pieces. To make a concave 
grinding tool : provide a thick and substantial piece of 
brass or gun-metal in the form of a chuck, and with a 



TOOLS. 



suitable turning-tool hollow out the end so as to fit the 
curvature of the round template. This may necesskatc 
several trials if the amateur be inexperienced in the use 
of lathe tools. A second piece of brass is now turned in 
the same way, but so as to be the exact counterpart of 





FIG. 86. 

the preceding ; that is, its outer end must be rounded, 
and this curve must be gauged by the hollow template. 
Both tools having been finished by the 
lathe tool as well as possible, they are 
next ground one upon the other by 
friction, with the interposition of a little 
fine flour emery and water until they fit 
each other with great nicety. 

To prevent waste and save labour, 
glass-makers now supply the material 
moulded appropriately to the form the 
lens ultimately assumes ; but where the 
raw material is in the form of a flat slab, 
it is cut into squares, each of which is 
' shanked' to a circular form by means FIG. 87. 
of a pair of shanks, as shown in Fig. 87, these being 




&&1NDTNG TOOLS. 219 

made of soft iron, and procurable from all dealers in 
opticians' requirements. 

The glass having been nibbled or ' shanked ' to a 
round form is cemented by pitch or sealing-wax to a 
suitable handle, and is rough ground, either on a grind- 
stone or in an iron mould with coarse sand, until it is 

nearly the shape required. 
For small lenses this may 
be effected in the turning 
lathe with a sharp steel 
cutting tool, which must 
be kept constantly wet 
with spirit of turpentine, benzoline, or one or other of 
several liquids of a similar kind, which have been found 
to answer the purpose equally well. It is desirable that 
a flat piece of glass, B, be interposed between the handle 
A and the lens C to prevent marginal errors in grinding. 
In either this or the rough-grinding method the tem- 
plate must be occasionally applied as a means of ascer- 
taining progress. It being desirable to save the brass 
tools as much as possible, the more effectively the first 




FIG. 88. 



I 



FIG. 89. 

grinding is done in the coarser tool the better will it be 
for the chances of the finishing tool preserving its form 
unimpaired for a long period. Fig. 89 shows a convex 



220 GRINDING THE SURFACES. 

and a concave tool ready for insertion in the turning 
lathe. 

Grinding the Surfaces. Grinding proper is effected by 
means of emery, of which several grades arc employed 
In large cities opticians' emery is a commercial article. 
Those who prefer to make it for themselves may do so 
by taking a quantity of flour emery say a pound and 
placing it in a clean jar. To this add water and stir it 
about until it is all wet and of a pasty consistence. 
Now add water to fill up the jar, stir the whole contents 
well round, and, after waiting for a little till the heavier 
particles subside, pour off the water, in which is mixed 
up the lighter portions, into a second jar, which fill up 
with water and stir vigorously as before, pouring off the 
water, after five minutes, into a third jar. This is 
repeated, a longer time for settling in each case being 
given. The result of this washing process is that 
while in the first jar the deposit consists of the coarsest 
portions of the emery, the deposit becomes finer and 
finer as the washing is allowed to proceed, till at last the 
water holds in suspension only the very smallest atoms 
of the emery, which, when precipitated, forms the finest 
emery capable of being procured. 

To smooth the roughly ground surface of the lens the 
coarsest of these deposits of emery is first employed, 
mixed, of course, with water. When upon examination 
with a magnifier the surface is homogeneous, the grind- 
ing is repeated with a finer, succeeded by a still finer, 
grade of emery, until at last the convex surface of the 
glass is so fine as to present the appearance of being 



POLISHING THE LENS. 221 

ready to burst into a black gloss. At this stage the 
operation with the emery terminates. It need scarcely 
be said that in the grinding with the various grades of 
emery careful washing must be resorted to between each, 
and that the grinding with any one class of emery must 
be continued until every mark made by its predecessor 
has been removed. Also, in course of the grinding it is 
well that the counterpart of the tool be applied, with a little 
emery and water, so as to ensure its being kept in shape. 
Polishing the Lens. To impart a final polish we have 
seen several methods adopted. One, and the most 
primitive, is to cement on the face of the grinding tool a 
piece of textile fabric of a fine nature from which the 
nap has been removed by a hot iron. Some employ 
woollen cloth, others fine linen, and in some instances 
paper. It is cemented on the face of the tool by pitch 
or other cement, the counter tool being employed to 
preserve the curve. Rouge, a mixture of rouge and 
putty powder, or, not unfrequently, putty powder alone 
moistened with water, is employed to give the final 
polish. When the finest surface possible to be obtained 
is desired, instead of polishing upon cloth or linen the 
tool is faced with pitch. This is applied by warming 
the tool and then rubbing over it a piece of pitch, which 
melts and coats the surface in a uniform manner. It is 
spread more evenly by the application of the counterpart 
tool. A little rouge or putty powder is spread over the 
surface and moistened with water. On applying the 
surface of the lens to this with rapid friction it imme- 
diately receives a fine black polish. 



222 BLOCKING LENSES. 

Putty Powder. The best way to make putty powder 
for this purpose is to dissolve tin in aqua regia and 
precipitate by diluted ammonia. Wash the peroxide in 
several changes of water, and, after drying, expose in a 
crucible to a low white heat, by which the particles 
acquire the property of polishing quicker and better. 
Owing to the white colour of the putty powder many 
prefer to mix with it a little rouge or crocus not alone 
to modify its polishing properties, but also to enable 
it to be seen when on the cloth. The polishing 
powder must not be too wet, but sufficiently so to 
take a partially glazed appearance from the action of 
polishing. 

Edging and Centering. The edging of the lens is 
effected by cementing it upon a chuck, and while 
rotating in the lathe the reflection of the flame of a 
candle is observed. If it remain quite steady all is right ; 
but, if not, it must be shifted slightly before the cement 
hardens until it do so. A piece of copper or brass well 
supplied with emery and water is then applied to the 
lower edge, an even pressure being given until the edge 
is smooth and the lens quite round. 

Blocking Lenses. When lenses are not large and are 
to be ground to shallow curves, a considerable number 
may be cemented on a block and operated upon 
simultaneously. In this way upwards of two dozen 
may be ground and polished in the same time that one 
would take. For grinding the commoner class of lenses, 
such as spectacle glasses, machinery is employed in 
connexion with the block system, 



SPECIMEN LENS CURVES. 223 

Specimen Lens Curves. It would be foreign to the 
object of a work like this to give formulae by which the 
curves of lenses formed of the many different kinds of 
glass now procurable may be ascertained, but it may 
not be out of place to give the curves (supplied through 
the courtesy of the present head of the firm), of a fine 
specimen of the No. 2 wide-angle lens of Dallmeyer, of 
the form shown on page 44, Fig. 18, made for us in 
1865. Measuring from the diaphragm, the radii are 

1. - 5*253- 4- + 4'3o6. 

2. + 1*46. 5. 4*306. 

3. 1*46. 6. + 2 '2. 

The diameter is 2 inches, and the focus 8J inches. It 
is made of Chance's glass. Soon after receiving it we 
found that it would bear a working aperture very greatly 
in excess of that intended by the optician, and for over 
twenty years we have used it for portraiture, with an 
opening of f-8. When stopped down it covers 10 x 8 
easily. 



CHAPTER XXXIII. 

OPTICAL CONTACT CEMENTING LENSES. 

THE fewer the reflections in or connected with a 
lens the better, because the invariable tendency of these 
is the fogging of the plate. Some lenses distribute the 
reflections all over the plate ; in the case of others a 
concentration takes place upon the centre of the 
negative. The former is not good, and the latter is 
highly objectionable. 

What we here mean by reflections will be better 
explained by a demonstration. Take a portrait lens 
and step with it into a darkened room. Light a candle 
and place it at a distance of a few feet ; then hold up 
the lens in the line of the candle light, when a repeated 
duplication of the image of the flame will be seen, some 
of these images being erect, others inverted. 

Reflections Reduced by Cementing. Now, seeing that 
the fewer reflecting surfaces there are in an objective 
the fewer will be the number of these reflected images, 
of course, it follows that the multiplicity of such surfaces 
is an evil, and for this reason opticians have sought to 
make the inner surfaces of achromatic lenses ' contact 
curves ' as far as possible. The reason for this is 



CEMENTING B Y BALSAM. 22 ; 

obvious : if these inner surfaces be concentric as regards 
curvature, it is only necessary that they be placed in 
optical contact to ensure a nearly total elimination of the 
reflections that would inevitably arise were the contact 
between them merely mechanical instead of being 
optical. To secure the latter, all that is necessary is 
to interpose between the two concentric surfaces any 
clear fluid such as water, oil, or varnish when the 
interior surfaces that could previously be seen by looking 
down upon them immediately disappear, and the lens 
appears to be formed of one homogeneous piece of 
glass. 

Cementing by Balsam. Of the various substances 
employed in the cementing of achromatic lenses, that 
which is most generally preferred is Canada balsam ; 
for it is easy of application, possesses the requisite 
degree of transparence^ and dries quite hard. There 
is a well-grounded objection to the employment of this 
substance for large telescopic object-glasses, because 
the expanding ratio of flint and crown glasses being 
different, they will be affected by thermal influences, 
which would cause a strain owing to the two unequally 
expanding bodies being securely cemented together. 
To obviate this a permanently fluid body e.g., castor 
oil is recommended in preference to balsam for lenses 
of this class. 

The photographer who wishes for ocular demon- 
stration as to the advantages arising from cementing 
a lens can obtain it in the following manner : Provide 
two clean pieces of glass, such as quarter-plates, and, 



226 ARBORESCENT MARKINGS. 

holding one of them in a level position, allow a drop of 
oil to fall upon it. Now lay the second plate on the 
top of the other so as to cover and flatten out the 
drop of oil. Observe how transparent the glasses have 
become by the cementing of the inner surfaces in 
the manner described. Wherever the oil touches both 
surfaces optical contact is secured. The experiment 
just described serves to demonstrate the difference 
between optical and mechanical contact, and also to 
show the brilliancy arising from the cementing of two 
surfaces of glass. 

Almost without exception the front lens of the 
portrait combination and both lenses of the ' rapid ' 
class of objectives are cemented; but the cement not 
unfrequently undergoes changes and vicissitudes by 
which the performance of the objective is seriously 
damaged. We shall here describe the nature of some 
of these changes and the means of cure. 

Arborescent Markings in Balsamed Lenses. Occa- 
sionally, after a portrait combination has been some 
time in use, an arborescent growth, commencing with 
a single, delicate, leaf-like form, appears at one side 
of the front lens, and gradually spreads inwards. If 
the balsam has been very thin when applied, this 
arborescence spreads over a large portion of the sur- 
face. One of the finest examples of this defect occurred 
in the back lens of one of our 10 x 8 'rapid 7 objectives 
which remained good for about four years after being 
made, and then had a beautiful mass of shrubbery 
growing all round the margin. This increased to such 



DISCOLORATION OF TH'E CEMENT. 227 

an extent as to leave only a small clear spot the size 
of a threepenny piece in the centre. This is, perhaps, 
the most prevalent form of defect in the cement of a 
lens. 

Discoloration of the Cement. Another, which also makes 
its appearance after the lens has been in use for a few 
years, consists in a discoloration of the cement. All 
round the margin the lens is found to have become of 
a yellow colour, which, although at first pale, afterwards 
becomes more decided, and not unfrequently assumes a 
green hue. Eventually the lens becomes so slow in its 
action as to be cast aside, and to have its place supplied 
by the instrument of another maker. In all cases of 
this character which we have had an opportunity of 
examining, the defect in question invariably arose from 
the lens having been burnished (or screwed) into its cell 
before the balsam had been allowed to harden, in con- 
sequence of which an action had set up between the 
balsam and the brass cell surrounding the lens, resulting 
in a slow decomposition of the latter, which eventually 
coloured the balsam. 

There are some kinds of balsam which acquire a 
yellow colour through age ; but we are not aware, in 
our own experience, of any thin film such as that 
which forms the cementing stratum of two lenses ever 
having become discoloured by light to an extent that 
could be appreciated. On the contrary, the tendency 
of light is to bleach it. Time, however, and exposure 
to the atmosphere certainly imparts a yellow colour 
a fact well known to those who have prepared trans,- 



2 2 8 to REMEDY DEFECTIVE CEMENTING. 

parent paper by the agency of Canada balsam. It is 
also known to microscopists that sometimes slides which 
have been prepared with balsam have, after a few years, 
acquired a yellow tint somewhat similar to that which 
results if an excess of heat be applied in the preparation 
of the slide. 

To Remedy Defective Cementing. When a defect in 
the cementing of the lens is observed, or when a dis- 
coloration is suspected owing to a lens working more 
slowly than it did originally, and which discoloration 
may be detected by laying the lens upon a sheet of 
white paper and noting its appearance, the first stage 
in the remedying of the defect supposing the photo- 
grapher elects to cure it himself instead of sending it 
to an optician consists in removing the lens from its 
cell into which it is fixed, either by the edge of the cell 
being turned over its margin or by a screwed ring. 

On its removal from the cell, the lens is placed in a 
saucepan on the bottom of which is laid a small piece 
of wood to prevent the contact of the glass with the 
metallic bottom. Slightly lukewarm water is now 
poured in to a height more than sufficient to cover 
the lens, and heat is gently applied until the balsam 
has become so soft as to permit the lenses, when ma- 
nipulated by the fingers, to be slidden one from the 
top of the other. When this has been done, the water 
is wiped off and the lenses allowed to become cold. 
Ether or collodion is now poured over each surface, and 
gentle friction with a soft cloth applied. By this means 
the old balsam is dissolved and entirely removed. Oil 



CEMENTING THE SURFACES. 229 

of turpentine or benzole answer a similar purpose as a 
solvent. The cleaning of the surfaces is finally com- 
pleted by means of soap and water. 

Some have recommended the use of the carbonates 
of potash or soda as a solvent for the balsam ; but these 
are bad, on account of their action on the glass. 

Cementing the Surfaces. When quite clean, and wiped 
dry by means of wash-leather, lay the flint glass on a 
sheet of paper, concave side up, and deftly apply a large 
drop of the finest quality of Canada balsam to the centre, 
taking care that it is free from air bubbles. Arrange- 
ments must be made for keeping the lens quite warm 
during this operation. Now lower down upon it the 
contact surface of the crown glass, and by gentle pressure 
guide it so as to cause the drop of balsam to expand 
equally outwards until it oozes slightly out at the margin. 
Next lift it up, and by means of a long piece of soft 
string tie the two together, crossing and recrossing the 
string in every direction. This ensures their being kept 
in a central position. Heat is now gently applied by 
laying it on the hot plate of a warm but not superheated 
oven, until upon removing the lens and testing the 
balsam which has oozed out at the edges it is found 
to be hard. Then, having allowed the lens to cool 
slowly, remove the string, and clean thoroughly with 
ether or benzole. The lens will now be found to have 
become rejuvenated. 



CHAPTER XXXIV. 

SELECTION OF LENSES. 

Form of Lenses for Enlarging. For an enlarging 
objective with the solar camera, in which the source of 
light partakes more of the nature of a point than what 
we have been considering, the construction of the 
objective may partake of a far wider range and be of a 
more diversified character than any of the others. We 
have seen images similar in dimensions produced from a 
test negative in which the objective was composed 
respectively of a portrait lens, a ' rapid ' combination, 
and an achromatised meniscus. It was not only a 
difficult matter to adjudicate upon the respective merits 
of these pictures, but experts present at the time and 
having before them examples produced by each system 
of objective were found to have arrived at varying 
conclusions respecting their relative merits. In conver- 
sation with the late Dr. van Monckhoven, who had 
bestowed much attention upon the subject, that gentle- 
man gave it as his opinion that the best of all objectives 
for the solar camera would yet prove to be a single 
achromatic. Previous to that time he had, in his work 
on Photographic Optics, in 1866, in the portion in which 



LANDSCAPES. 231 

, he describes his enlarging solar camera, spoken of its 
; objective as having the ' external form of Ramsden's 
' eye-pieces placed on pocket telescopes, but constructed 
on the principles of M. Petzval's doublet.' That the 
doctor had altered his opinion subsequently to writing 
this is apparent from the fact that to none of the 
objectives manufactured by him at a later period does 
this description apply, and we have seen several. 
Woodward, of Baltimore, who has constructed more 
solar cameras than any other, makes the objectives of 
best ' solars ' of three achromatic lenses, the front and 
back being similar to those of the ordinary portrait com- 
bination, but having the focus shortened by the insertion 
of a third meniscus achromatic lens between them. 

Enlarging Portrait*. If the subject to be enlarged is 
a single portrait, say of carte size or a little larger, then 
will a carte or other good quarter-plate portrait lens be 
found to be the most suitable. It is of the greatest con- 
sequence, however, that the back lens of the combination 
be placed next to the negative, otherwise will the de- 
finition and flatness of field be inferior. There will be 
little or no necessity for using a diaphragm in the lens, 
as the area of sharpness when employing full apertures 
will be quite sufficient for the intended purpose. 
[ Landscapes. But in the case of a landscape or a 
group, some members of which are near to the margin 
of the plate, it will be requisite either to make use of 
a diaphragm, so as to ensure marginal definition of the 
highest class, or to employ a lens of longer focus. The 
solar focus of a lens is not its focus when used for 



232 LANDSCAPES. 

enlarging-, more especially to the extent of only a few 
diameters, and hence it should be borne in mind that 
the focus being longer when thus employed its covering 
power is extended. A combination lens of the ' rapid ' 
doublet type will be found excellent in the case of a 
landscape, in which, unlike a portrait, the marginal 
definition must equal that of the centre. 

If time of exposure be of no consequence, then will an 
achromatic of plano-convex or slight meniscus form 
answer well the purpose of an enlarging lens. But it is 
necessary that a rather small top be employed, that it be 
situated at not less, but preferably more, than the 
diameter of the lens from its flat surface, and that the 
convex surface of the lens be placed next to the nega- 
tive. There will be a residuum of distortion when 
employing such a lens, but in the case of a landscape or 
group it will not be discoverable in the large picture 
which results from the operation, and this being so, nice 
theoretical considerations concerning rectilinearity may 
be placed to one side. But if any curvature of a 
marginal vertical straight line as in the case of a 
building be discoverable, this may be reduced by 
removing the diaphragm and placing it closer to the 
lens. This applies only to single landscape lenses, and 
only then if the subject be an architectural one, the 
vertical lines of which extend to the margin and show 
indications of being curved. 

In an objective employed in enlarging one is apt to 
be deceived as to its focus. This may be illustrated by 
an example. Suppose that the solar focus (equivalent) 



STEREOSCOPIC LENSES. 233 

of the enlarging objective be six inches, the distance 
between the centre of the lens and the negative to be 
enlarged would be six inches practically, were the screen 
on which the enlargement is projected at an infinite 
distance. These two, the negative and the screen, 
represent the anterior and posterior conjugate foci ofthe 
lens. But as such a position ofthe screen is impracti- 
cable it must be brought nearer, and as there is a strict 
relationship between the conjugate foci, the nearer the 
screen is made to approach the objective the further 
must the negative be removed from it. When the screen 
has been brought so near as to show the image of the 
same dimensions as the negative, then if a careful 
measurement be made, it will be found that the lens has 
now a focus of twelve inches, or double that it possesses 
for distant objects. The anterior focus of the lens, 
represented by its distance from the screen, is now found 
to have been reduced from infinity to twelve inches also. 
Stereoscopic Lenses. For securing instantaneous 
stereoscopic pictures of a well-lighted outdoor scene the 
great majority of subjects will be amenable to the action 
of a single lens of about six inches focus. This admits 
of the employment of a diaphragm sufficiently large to 
permit the usual class of subjects, including seaside 
groups, boats, &c., to be taken in a quasi-instantaneous 
manner. But if the very best effects as regards rapidity 
of exposure are desired, it then becomes necessary to 
employ a pair of portrait combinations, used without any 
diaphragm. Of these the finest effects will probably be 
obtained with a back focus of from five and a half to six 



234 LENSES FOR DIRECT PORTRAITURE. 

and a half inches, as this gives a more uniformly lighted 
picture than when an objective of short focus, such as 
three and three-quarters inches back focus, is employed, 
as was frequently the case in those days when instan- 
taneous stereoscopic photography was prevalent. 

For indoor groups and scenes it is probable that the 
regular stereoscopic portrait combination of short focus 
cannot be surpassed, or even equalled, for general utility. 
Its small diameter and short focus give it a great 
penetrative range, while its large * angular aperture ' 
enables it to be worked with great rapidity. Although 
on this account we advise its employment in preference 
to any other in a room in which it is desired that a 
scene or group be taken with a short exposure, we are 
strongly of opinion that for outdoor purposes it is not to 
be commended, unless the subject to be taken be at no 
considerable distance from the camera. 

Lenses for Direct Portraiture. For children's portraits 
it is necessary that the lens has a large aperture, seeing 
that they must be taken with the briefest of exposures. 
To this end a Petzval portrait combination will form the 
most useful lens, this being employed with a concealed 
pneumatic shutter. For adults, or where the same 
rapidity is not necessary, either a portrait lens of as long 
a focus as possible, or a cemented rapid doublet may be 
used. If the studio is badly lighted, the former will 
prove the more useful. If a large head and bust be 
wanted, and the light permit, a single landscape lens, 
working with a large aperture, will give soft and 
harmonious pictures. 



COPYING PORTRAITS. 235 

Lenses for Pure Landscape. Ordinary landscapes in 
which architectural subjects do not form a chief feature 
are best taken with a landscape lens that is, a single 
achromatic. This class gives bold, crisp definition, this 
brilliancy of the image being due to the simplicity of the 
form ; for, as we have shown in a previous chapter, the 
presence of a second lens in a photographic objective 
causes the formation of flare, which, when not confined 
to one central spot, becomes diffused over the negative, 
thus leading to a want of vigour in the shadows. This 
class of lenses, when employed for pictures of medium 
dimensions in which the included subject is of a some- 
what small angle, is capable of being used with a stop 
sufficiently large to permit good negatives being obtained 
with an exposure of a fractional part of a second. 

Groups. When a group is to be taken, either in a 
studio or in a dull light out of doors, one of the * rapid ' 
class of lens with an intensity of from f-'j to/- 13, 
according to circumstances, will be found to be the most 
useful. This lens also forms the best objective for large 
portraits in the studio, which it produces of more harmo- 
nious quality than a large portrait lens could possibly do. 

Copying Portraits. In the copying of a portrait it is 
probable that photographers will invariably use the 
portrait lens they commonly employ. This class of 
work falls quite within its scope ; but, as the majority of 
operators first focus the picture and then insert a small 
stop to work with, we caution them that several other- 
wise good and useful portrait lenses, as well as some 
specially constructed for copying, have their focus altered 



236 ARCHITECTURE. 

by the insertion of a smaller stop to work with than that 
by which the focussing was effected. This is not always 
the case ; but, as it is sometimes so, it is a wise pre- 
caution to use the full aperture of the lens for making 
the general arrangements and having the focussing 
effected, and then, after inserting the working stop, to 
take a final look at the image on the ground glass and 
ascertain the state of its sharpness by means of a magni- 
fying-glass, observing whether by slightly turning the 
pinion the definition is not capable of being improved. 
Observe this : that when the copy is required to be 
larger, or on a larger scale, than the original it is 
necessary that the lens be turned ' end for end ' so as to 
have its back lens nearest to the picture to be copied. 

Maps or Charts. When the subject to be copied is a 
map or chart it is absolutely necessary that a non- 
distorting objective be employed. A lens of the 'rapid' 
class is most advantageous for this kind of work. 

Large Micro Objects. If the class of work required to 
be reproduced on a scale of magnification be flies or 
insects of moderately large dimensions, a quick-acting 
locket lens will answer the purpose better than a 
properly constructed miscroscopic objective of the same 
focus, as f .he former has its chemical and visual foci 
coincident whereas the latter has not. 

Architecture. Architectural work can be produced 
equally with * rapid ' as with wide-angle lenses, provided 
these be of a non-distorting class. Distortion, as here 
meant, implies the curving of lines near the margin of 
the picture which are straight in the original. 



CHAPTER XXXV. 

ON THE CURE OF EXISTING DISTORTION. 

Distortion of Curvature. A single landscape lens, more 
especially when made to take in a wide angle of view, 
gives to all straight lines near the margin of the view an 
offensive curvature to the image, which becomes in- 
creasingly great as these recede from the centre. In 
ordinary landscapes this is quite immaterial, unless when 
such happen to be bounded at either side by a very tall 
building which extends considerably up the margin of 
the plate ; in portraits or groups the distortion of curva- 
ture is also not of a nature that can be discovered. But 
quite different is it when the subject of the photograph 
is architecture or a map or plan. For these a non- 
distorting compound lens should be used, but we are 
now dealing with the fact that such has not been 
employed. Here, then, we are confronted with a nega- 
tive in which what should be straight lines towards the 
margins are bent like the sides of a barrel, and the 
question is, How to cure it ? 

The first thing to be done is to make from the 
distorted negative a transparency by superposition on 
the same sized plate. If care be taken to have negative? 



238 THE DISTORTION OF CONVERGENCE. 

and plate in perfect contact throughout, and, moreover, 
if the light by which the impression is to be made be 
made to fall upon the printing frame from one direction 
only, there will not be any loss of sharpness. A full 
exposure, with proper development, will ensure every 
detail in the one to be seen in the other. Too great 
intensity or contrast between the lights and shadows is 
to be avoided. We have now got a transparency as 
sharp as the negative, having all its detail and also all 
its distortion. 

Now, by means of the camera, and with a single 
lens, preferably of shorter focus than that originally 
employed, having its stop next to the sensitive plate, 
make a negative from the transparency. As the negative 
thus obtained is distorted as regards the exact reproduc- 
tion of the transparency, and as that distortion is of the 
opposite character, the result will be that the lines which 
were curved in the original negative are straight in the 
reproduced one. To ensure sharpness the diaphragm 
must be a very small one, any remaining traces of cur- 
vature, should such be perceived on the ground glass, 
being removed by placing the stop nearer to or 
farther from the lens, for the more the margin of 
the lens is brought into requisition the greater is 
its power of producing or, in this case, correcting 
distortion. 

The Distortion of Convergence. Every one knows that 
if a camera be pointed upwards at a building, so as to 
get its upper part into the picture, the perpendiculars 
will converge, being narrower at the top than at the 



THE DISTORTION O& CONVERGENCE. 2^9 

bottom. Every one also knows that this convergence 
of the perpendiculars may be altogether prevented by 
swinging the back of the camera so as to cause the 
ground glass to stand in a vertical position. It is not 
here a question of this or that lens, for no lens has been, 
or can be, made by which such convergence will not 
result if the sensitive plate be not vertical. All the best 
cameras are now provided with a swing-back, but in 
those of the hand or detective class it is seldom, if ever, 
to be found. 

Let us suppose, then, that we have got a negative of 
a tall building, in which, from tilting up the camera 
without having brought the ground glass or sensitive 
plate into the vertical position, the sides converge and 
lean towards each other. How is this to be cured, or 
more correctly, how is another negative to be produced 
from it in which no details shall be lost, but in which 
the converging building shall be restored to its original 
perpendicular position ? 

First of all, make from it a transparency by super- 
position in a printing frame, as before, and, having 
erected this transparency in front of a plate of opal 
glass (by preference), and, by means of a camera fitted 
with a short-focus, non-distorting lens, and a swing-back, 
focus as sharply as possible with the largest aperture, 
and swing back the ground glass until the convergence 
of the building is seen to be neutralised, and the vertical 
lines rendered parallel. Now insert the smallest stop, 
so as to ensure top and bottom being equally sharp, and 
expose. The negative which results from this treatment 



240 CURVATURE AND CONVERGENCE. 

Vvill be rectilinear, and in every respect perfect so far as 
drawing is concerned. 

The Distortion of Curvature and Convergence Combined. 
This compound distortion is one of an intensely offensive 
nature. In it not only are the marginal straight lines 
curved, but they also converge. It is produced in its 
most perfect degree by having a camera without a swing- 
back fitted with a wide-angle, single-landscape lens, 
and pointing it well upwards, to take in the upper part 
of an architectural subject. But, provided it only be 
sharply defined, it is amenable to being perfectly cured 
equally as in the former cases. 

The treatment is precisely the same as in the case of 
the distortion of convergence, subject to this difference, 
that the lens by which the cure is to be effected must 
not be rectilinear, as in the former instance, but must 
be single, as in that necessary for curing the distortion 
of curvature. The swinging of the camera back ensures 
the converging lines being rendered vertical, while the 
counter distortion of the lens equally ensures their being 
made quite straight. 



CHAPTER XXXVI. 

LANTERN OPTICS ENLARGING AND PROJECTING. 

IN this subject are embraced the radiant or light, the 
condenser, and the object-glass or projecting lens. 

The Light. This, for enlarging, must be small in 
dimensions and intense in quality ; the former in order 
to obtain sharpness, the latter to obviate the necessity 
of giving a protracted exposure. 

Mineral Oil Lamps. These, when well selected, are 
extremely convenient. It is probable that on account 
of its smallness an Argand flame possesses greater ad- 
vantages than any other. It will undoubtedly serve the 
purpose equally as well as the limelight, provided a 
diaphragm be interposed, close to the flame, to cut off 
the top and bottom, as magnitude of the radiant in an 
optical lantern conduces to impaired definition. The 
theoretically perfect light is one in which dimensions 
scarcely find a place ; but its attainment being imprac- 
ticable we must do the next best thing. Something 
much higher is aimed at in the optical requirements of 
enlarging than those which obtain when a luminous 
image is thrown upon a screen for the illustration of a 
lecture or the delectation of juveniles at an evening 
gathering. Parallax in the flame must be avoided as 

R 



24 2 LIMELIGHT. 

much as possible, else will definition of a high class be 
sought for in vain. 

The Marcy Lamp. The Marcy lamp is also a good 
one. By the Marcy lamp we mean all those in which 
the burner consists of more than one flat wick turned 
endwise to the condenser. Marcy, of Philadelphia, used 
two, others three, four, and five. The principle is the 
same. Various names are now given to this system of 
lighting according as it finds development in the lamps 
of the numerous manufacturers by whom it is issued. 
For projections it is powerful, and has superseded the 
Argand burner, but we have now to look at something 
else than mere power. The fact of the edges of the 
various flames which are contracted being axial in the 
optical system implies a fulfilment of the condition of 
sharpness in its highest form axially ; but when oblique 
incidences are considered then are we met by magnitude 
of flame, and consequent parallax. Nevertheless, when 
the relative positions of the flames each to the other 
are such as to prevent vertical lines of varying luminous- 
ness being apparent in the centre, enlargements of fairly 
good sharpness and equal illumination throughout may 
be obtained by its agency. 

Limelight. One or other forms of the limelight may 
be employed with unvarying success. The blow-through 
jet is the safest and simplest when carburetted hydrogen 
or common house gas is used. Where this gas is not acces- 
sible then will the flame of a spirit lamp answer quite well. 
The blowpipe from the cylinder or bag of oxygen play- 
ing on this flame causes it to impinge upon a cylinder of 



ALBO-CARBON. 243 

lime, which, becoming incandescent under the great heat, 
emits a powerful light. The most intense form of this light 
is when the hydrogen and oxygen, both under high pres- 
sure, are brought into mixture just before they issue from 
the orifice of the burner. The light, when the gases are 
properly regulated, is not large, but exceedingly intense. 
Albo-Carbon. A flame of common gas enriched by 
' albo-carbon,' or any other suitable hydro-carbon, has in 
our hands as well as in those of others proved to be a 
very suitable light for enlargements. Its best form is 
that which we introduced at the 1887 Conference of the 
Camera Club, and consists of two fishtail burners, sepa- 
rated from each other by the extent of an inch, both 
flames having their flat sides towards the condenser, 
there being an opaque disc, with a circular aperture in 
it of a little over half an inch in diameter, placed as 
close as possible up against the foremost flame so as to 
reduce its effective area. The position of this aperture 
must be such as to be opposite to the most luminous 
part of the flame. The second flame behind the anterior 
one serves to confer intensity, and is of great utility ; 
but nothing seems to be gained by a third burner. The 
gas flame, when thus enriched by the vapours of the albo- 
carbon, becomes very intense. An Argand flame from 
gas thus enriched ought to yield a light of great excel- 
lence, provided it has a smaller flame ascending through 
its centre, and that provision is made to condense it by 
diminishing its diameter either by a brass solar cap to 
cause a strong air current to impinge upon the flame 
a little above the burner, or by a contraction in the glass 



244 



THE CONDENSER. 



chimney. Whiteness and intensity in such a case are 
increased by a judicious lengthening of the chimney to 
increase the draught. The area of the flame must, how-- 
ever, be reduced by the expedient already pointed out. 

The Condenser. The use of a condenser is to gather 
together rays from the lamp which would otherwise be 




Fig. 90. 




Fig. 91. 

lost, and bend them in such a way as to pass through 
the negative and on towards the objective. Let a 



THE DOUBLE CONDENSER. 245 

(Fig. 90) be the radiant, b the negative, and c the ob- 
jective. Notice that while the rays are transmitted 
through the negative only those that are nearly central 
or axial reach the objective. Observe now what takes 
place when a lens intercepts the rays ere they reach the 
negative. They are refracted, and instead of getting 
lost as before (as shown by the diverging dotted lines), 
they have become deflected in the direction of the objec- 
tive, and were the eye placed at the diaphragm of this 
objective it would see every part of the negative brightly 
illuminated. One condenser, however, does not answer 
properly on account of the spherical aberration that 
arises when the focus is short, as it must necessarily be. 
The condensers most commonly employed in optical 
lanterns consist of two plano-convex lenses mounted 




Fig. 92. 

close together, the convex surfaces towards each other, 
as shown in Fig. 92, in which L is the light, C the con- 
densers, and A the apex of the projected luminous cone. 
This form answers fairly well when the flame is large, 
butj. unless made with a long focus, it will not perform 
in a satisfactory manner when the flame or radiant is 



246 



THE DOUBLE CONDENSER. 



small. A better form, if only two lenses must be em- 
ployed, is to have a plano-convex, or a lens of slightly 
meniscus form (this being according to the nature 
of the glass employed) working in conjunction with a 
bi-convex lens (Fig. 93). When the curvatures are such 
that the rays after transmission through the former of 
these fall upon the latter in a 
parallel direction, then must the 
curves of the latter not be of 
equal radius, but such as to 
make it a 'crossed lens,' in 
which the radii are as one to 
six, or nearly so, this being well 
known to be a form that is 
fairly conducive to the reduc- 
tion of spherical aberration. But 
even here, unless the focus of 
this combination be long, per- 
fection of illumination is un- 
attainable. Why, then, not make it long? For this 
reason, that the loss of light would be too serious. It 
is of primary importance that the light in a lantern, 
whether for projecting a picture for examination or 
for enlarging, be powerful. How is this to be done ? 
We answer it by requesting attention to the preceding 
diagram, in which, although the lenses are not so 
correctly figured as they ought to be, the principle is 
plainly enough shown. In this diagram an angle of 
illumination, say of ninety degrees, might be obtained, 
but the condensers are unable to grasp more than c and d t 




Fig. 93- 



DOUBLE AND TRIPLE CONDENSERS. 247 

and even a little less than this, the large volume from 
a to c and from b to d being left outstanding. Now 
when it is considered that this represents a loss of about 
one-half of the light, its reclamation is evidently worthy 
of attempting. By intercepting these lost rays by a 
plano-convex or a meniscus lens, which need not be of 
so great a diameter as the others, they are by it secured 
and made to impinge. The deduction from this is that 
a triple condenser is better than a double one. 

The best form of a double condenser is that shown 
in Fig. 94), which consists of a plano-convex and a 




Fig. 94- 

crossed lens, the flat side of the former being next to the 
light. It does not, however, include such a wide angle 
as those of triple form, although when well made its 

spherical aberration is but small. 
Triple C ndensers. One of this 
class which we devised many 
years ago, was composed of three 
piano - convex lenses (Fig. 95), 
the centre one of which was 

achromatised. and that farthest 
Fig. 95- 




248 



ACHROMATIC CONDENSERS. 



from the light of colourless crown of a high refractive 
index. This gave excellent illumination, but is expensive 
to construct. 

The achromatic condenser of Thomas Grubb, figured 
below (Fig. 96), in which A is a piece of plain glass 
to act as a protection to the condensers ; B is a 
plano-convex sample lens ; c a plano-convex achro- 
matised ; and D a combination very much over- corrected 




Fig. 96. 

for colour, and of slight negative power, although the 
externals are plane. From c to D the rays are nearly 
parallel. Passing through D, they diverge until they are 
received by a large lens by which they are rendered 
convergent. 

Achromatic Condensers. Although we have just de- 
scribed two achromatic condensers, we do not consider 
that these are necessary for lantern work. Chromatic 
aberration is reduced to some extent by the behaviour 
of a ray which, when it passes through the first lens 
(unduly separated from the second for the sake of 
illustration), is decomposed as shown at r and b in the 
figure (97), the second lens having a tendency to bring 



WIDE-ANGLE TRIPLE CONDENSERS. 



249 



these coloured rays together again. For this reason 
the compound condenser was formerly designated as 
achromatic. 




Fig. 97. 

Wide-Angle Triple Condensers- When lecturing before 
the Camera Club on the principles which underlay the 
construction of a condenser that would transmit a larger 
amount of light than was usual, and assuming the lime- 
light to be the source of illumination, we inquired the 
greatest angle of light possible to be got advantageously 
through a condensing system, as this lay at the root of 
the whole matter, and in doing so had to ascertain how 
near can the light be approximated to the first surface 
with safety. From innumerable trials with lenses of a 
thickness not too great, and set with such a degree of 
looseness in the brass-work as not to be cell-bound, we 
find that two inches may be considered as quite safe. 
When condensers crack, it is usually the result of their 
being too tightly burnished in their cells, brought too 
suddenly under the influence of the heat of the radiant, 
or being subjected to currents of cold air. We assume, 
of course, the perfect annealing of the glass of which 
they are formed. 



250 FUNCTIONS OF THE CONDENSER. 

Functions of the Condenser. At this stage we proceed 
to analyse the functions of a lantern condenser, so-called. 
We find that these are (i) the collecting and (2) the 
condensing of the light. Of these, the former is much 
the more important. What we wish done is the collec- 
tion of so many rays as to form a large angle, and their 
projection forward in as near an approach to parallelism 
as possible. Absolute parallelism cannot be obtained 
unless the flame were a point, instead of being, as it is, a 
disc or patch having sensible dimensions of, say, a quarter 
of an inch upwards. 

Some of the cheap French condensers (of which we 
would not speak disparagingly, for they render excellent 
service, and are marvels at their price) transmit an angle 
of light of from 40 to 50, and a superior class of London- 
made articles claims to embrace 60. But, by a slightly 
increased expenditure of optical means, it is possible to 
increase this angle to 95, which somewhat more than 
doubles the intensity of the illumination. Let us see in 
what way this is to be accomplished. 

Kepler's law is that the focus of a plano-convex lens 
equals the diameter of the sphere of convexity. This is, 
of course, for parallel rays, and it is those we are dealing 
with at present ; and we are also dealing with plano- 
convex lenses, these being the best for condensers, sub- 
ject, perhaps, to a slight hollowing of the flat surface. 
Well, it is very evident that, if we desire a large angle of 
light, the single Kepler won't do much for us, unless, 
indeed, it were made enormously thick even hemi- 
spherical when we would encounter two evils. First, 



COLLECTING SYSTEM. 251 

the enormous spherical aberration consequent upon 
transmitting light through a bull's-eye, and, secondly, 
the proximity of the said bull's-eye to the radiant, which 
not only emits light but heat a heat which would 
quickly cause our bull's-eye to be fractured. How, then, 
is it to be accomplished ? By borrowing the ideas of the 
microscopist. Who ever heard of a microscopic objective 
of even the most distant pretensions to wide angle being 
composed of one lens ? Well, no more is it possible in 
our collecting system, which is analogous. 

Collecting System. We must have, at least, two lenses 
for our purpose. One of them that nearest to the light 
must be 4^ inches in diameter in order to catch up the 
95 spoken of. But this cannot render the rays parallel ; 
still, it transmits them to its colleague under such cir- 
cumstances that it does so, the two lenses thus doing 
what no one singly could effect. The first lens of the 
collecting system is comparatively thin, which, apart 
from any optical advantage, is useful in this respect, that 
it has to bear the first impact of the heat, and this lessens 
the liability to fracture. It is only sixteen mm. (f inch) 
thick in the centre, is eight to nine inches focus, and is 
formed by preference of flint glass. The second element 
is five inches in diameter, and, the radius of curvature 
being rather shorter, this, combined with its greater 
diameter, causes it to be proportionally thicker, being 
twenty-eight mm. (i^g- inch) at its centre, and seven inches 
focus. This lens, too, should be made of colourless glass. 
The loss of light from absorption is trivial, and that from 
oblique incidence is really so little as to be unworthy of 



252 COLLECTING SYSTEM. 

notice, but it carries with it its compensation, for it occurs 
most at the thinnest portions of the lens, where there is 
the least absorption, and thus aids in ensuring uniformity 
of illumination throughout the entire beam. But it may 
be reduced by rendering the first surface concave instead 
of plane, and retaining the balance of power by grinding 
the back surface on a tool of shorter radius. At one 
time we were much in love with the meniscus form of 
lens for this purpose, but, after many trials with lenses 
both piano, meniscus, and plano-convex, and formed of 
different kinds of glass, from St. Gobain's crown to 
English flint, we arrived at the conclusion that the plane 
surface answered every purpose. 

If the radiant were infinitesimally small, a parallel 
beam of a large collected angle could be transmitted 
with a singular degree of perfection for several yards. 
With a triple collecting system (that worke 1 out by Dr. 
Charles Cresson, in which the first lens is a plano-convex 
4^ inches radius, the second a meniscus, respectively 30 
inches and 6 inches ; and the third a crossed lens of 
52 inches and 8f inches radii) we projected a very tiny 
gaslight on to the dial of a French clock several yards 
distant, which was thus illuminated a whole season. But 
such extreme nicety is not required in the practical 
working of the optical lantern, as, owing to the magni- 
tude of the flame, two elements answer every purpose. 
The two described should be mounted together as closely 
as possible, fixed permanently in the lantern, and must 
always be used together, and not separate. Until a 
compound collecting system of this nature is tried, one 



THE LANTERN OBJECTIVE. ^53 

can form no idea of the capabilities of the lantern for 
certain scientific purposes, such as polarising. 

Condensing System. We now direct attention to the 
condensing element of this optical system. We have seen 
that the two elements of the collecting portion must be 
fixed and inseparable. This, on the contrary, should be 
variable, and selected to suit the special end in view. 
Its form may be plano-convex, more especially if for 
use with long-focus objectives ; but if the latter is to be 
short-focus, and the condenser of crown glass, then is 
the crossed form, in which the curves are as one to six 
or two to thirteen, open to be preferred. 

But dealing, as we now are, with immergent parallel 
rays, it were folly to imagine that a condenser properly 
adapted for an objective of 12 inches focus will answer 
equally well for one of 6 inches. Bearing in mind 
Kepler's law, which, however, applies only to ojie kind 
of glass, and must not be held as applicable equally to 
the flint glasses, especially those of the denser sort pro- 
curable at the present day, we would say that for long- 
projection lenses of 12 to 1 5-inch focus a plano-convex 
having a radius of curvature of 7 inches will serve every, 
purpose; for an objective of 8 to 10 inches the radius 
may be 4^ inches, while for one of 6 to 8 inches 4 inches 
will suffice. But, as we have said, this latter may with 
advantage be a crossed lens, in which case the radius of 
the more convex side will be longer. 

The Lantern Objective. The requirements of the 
lantern objective are that it shall receive and transmit 
all the light that passes through the condensers, ancj 



254 THE LANTERN OBJECTIVE. 

that it shall give a flat field with good definition 
throughout. Its diameter, especially that of its posterior 
combination, must be sufficiently large to take in not 
merely the whole of the cone of rays emerging from the 
condenser, but by preference a little more. This permits 
of the utilisation of a small portion of light radiated 
from the substance of the image itself. 

A large back lens also permits it to be brought nearer 
to the picture, and this is advantageous, especially with 
the condensers of the common order, as it acts in con- 
densing the scattered rays from those of this class, 
enabling also the light to be approached nearer to the 
condenser. The lens tube should be longer than in the 
case of its application to photography, for, unlike this 
all it is required to cover is the very limited area com-' 
prised in a plate three and a quarter inches square, minus 
the portion occupied by the mat. For the highest class 
of objective, it suffices that it be achromatic in the sense 
different from actinic, for, so long as the visual image is 
perfect, it matters not what becomes of the violet or 
chemical rays, or what relations they have to the 
luminous ones. 

It is in the construction of a lantern objective of 
short focus that the skill of the optician is taxed, as it 
has to cover sharply to the margin with its full aperture, 
and under circumstances in which the slightest inequality 
in the definition is instantly detected. To a cultivated 
eye it is extremely unpleasant to see an image quite 
sharp in the centre of the disc, and falling off rapidly 
towards the margin, or by racking in securing marginal 



THE LANTERN OBJECTIVE. 255 

sharpness at the expense of the centre. Of the various 
forms of objective to be met with, at any rate for those 
of medium short focus, we incline to give preference to 
that introduced ten or twelve years ago by J. H. Dall- 
meyer, judging by a comparison of the performance of 
one of this class, with several others in our possession. 
In it the mount is longer and the elements of the back 
lens (see Fig. 39, page 78) are separated to an extent 
which would prove fatal to sharpness in the case of 
one employed in producing a photographic image in the 
camera. If photographic lenses are to be employed in 
the lantern, those of the carte-de-visite (Petzval form), 
that is, those corrected for flatness of field, even to the 
extent of there being slight astigmatism, are advan- 
tageous. One of the most satisfactory short- focus objec- 
tives we ever used had a back lens two and a quarter 
inches in diameter, the front lens being one and three- 
quarter inches. We gave a very great excess of negative 
spherical aberration to the back lens, and the front was 
a nearly plano-convex achromatic of short focus. This 
gave a field which was singularly flat, the definition at 
the margin quite equalling that in the centre ; but, 
owing to the excess of aberration spoken of, the image 
did not quite equal in sharpness that obtained by the 
ordinary carte-de-visite lens with rounder field. Still, 
spectators seated at a distance of five yards from the 
screen were unable readily to appreciate that the 
definition was imperfect, for, as is well known, even the 
crude brushwork of the scene-painter seems sharp 
when viewed from a distance. 



256 THE LANTERN POLARISCOPE. 

For objectives of long focus there does not appear to 
be the same tax on the skill of the optician. Poor, 
indeed, must be the lens of ten, twelve, or fourteen 
inches focus that will not cover sharply and uniformly a 
plate three inches in dimensions. 

In the foregoing, double combinations of lenses are 
implied, but single achromatic lenses ,of plano-convex 
or slightly meniscus form also answer as objectives. 
Whether they are used singly or two placed close 
together, their convex sides must be next the slide, and 
a diaphragm must be placed outside. 

The Lantern Polariscope- With respect to polarisation 
of light by the lantern, the method which we described 
at the Nottingham meeting of the British Association 
fulfils the requirements of giving, with the usual lantern, 
a much more intense volume of polarised light than is 
otherwise obtainable. 

Without going into too great detail, it may suffice to 
say that when the cone of light from the condenser is 
made to fall upon the bundle of glass plates by which it 
is polarised, only a portion of the light is thus affected, 
for as the angle of polarisation is an exact one, none 
but the axial rays are polarised in a perfect manner, all 
the others impinging upon the plates at other than the 
polarising angle. The expedient we adopt is to receive 
the cone upon a concave lens, by which the cone is trans- 
formed into a cylindrical bundle of rays, every one of 
which becomes amenable to the polarising influence of the 
plates, and after undergoing the change they are brought 
into a state of convergence by means of a, convex lens, 



THE LANTERN POLARlSCOPE. 257 

This system applies equally to the analyser or Nicol 
prism, as to the polariser ; in either case a considerable 
gain in the light accrues. 

In certain scientific institutions in America, where 
lantern condensers have some pretensions to be called 
perfect, the polarising of a large volume of light is 
effected in a simple and most excellent manner. By 
means of the two collecting lenses previously described, 
the light from the lime is reduced to a large parallel 
beam, which falls upon a bundle of glass plates placed 
at the usual polarising angle of 56, and after reflection 
is received by another lens of the same diameter, by 
which it is condensed. This lens is of either long or 
short focus, to suit its special requirement. 

Such a world of wonder and beauty is opened up by 
the polarising attachment to the lantern, that it is 
matter of surprise it is not more common than it is. 
To polarise the light, all that is necessary is to take a 
packet of eight or ten clean and rather thin quarter-plate 
size glass plates of the best quality, and having bound 
them all tightly together by the edges, place them in 
front of the condenser, at a little distance from it, and at 
an angle of fifty-six degrees. With the light reflected 
from this parcel of plates, all the phenomena incident to 
noiarised light may be obtained. 



CHAPTER XXXVII. 

PHOTO-TELESCOPIC LENSES. 

BY telescopic effects is here understood the produc- 
tion of an image necessitating, under ordinary conditions, 
either a telescope of moderate dimensions having its 
systems of eye-pieces to magnify the aerial image, or an 
objective of unusually long focus. What is required is 
a combination that magnifies in itself while permitting 
the employment of a camera of no unusual length. 

A telescope of the ordinary kind, having its eye- 
piece in sitti, gives an image the size of which depends 
upon the dimensions of the telescope and the distance 
of the ground glass from the eye-piece. This image, 
however, is only sharp visually, and the adjustment for 
photography necessitates a number of trials in order to 
ascertain the position of the chemical focus. Dr. Dick, 
in his Practical Astronomer (1845), describes how the 
telescope may be used for throwing an image of the sun 
up to thirty inches in diameter upon a screen in a 
camera obscura consisting of the room in which the 
spectators are seated. This was for the purpose of 
exhibiting the solar spots to a number of persons at a 
time. 

In 1870 we published a simple way of obtaining a 
sharp telescopic view of the sun or other distant object. 



TELEO-PHOTO OBJECTIVES. 259 

An aerial image is formed by a lens corrected for 
photography, and this is magnified by a similar lens of 
short focus placed the requisite distance in front of the 
aerial image. It is simple and answers well. 

In the chapter on the orthoscopic lens (page 60), 
we have spoken of the property possessed by it of 
giving a larger image in a given extent of camera than 
that obtainable by any other objective, and it is also 
known to many that a greatly enlarged view of a scene 
can be obtained by employing an ordinary opera glass 
as the objective, the large lens to the outside of course. 
We long ago used one of the barrels of a ' twelve-lens ' 
opera glass, that is, one in which each lens was achro- 
matised by being formed of three elements ; but felt 
dissatisfied on account of the very small field covered. 
What was covered, however, showed an image of greatly 
enlarged dimensions. 

Dallmeyer's Teleo-Photo Objective. It is gratifying to 
find that the optician named has been directing his 
attention to the Galilean method of forming an image, 
so as to adapt it for photographic purposes. The image 
by the outer or object-glass, which may be either a 
plano-convex or a crossed achromatic, is, previous to 
arriving at its focus, intercepted by an actinically cor- 
rected negative lens of greater negative power than the 
positive power of the other. This negative lens is formed 
of two or three elements, but the field capable of being 
sharply covered is limited. The degree of enlargement 
obtainable is determined by the separation of the lenses, 
coupled with the distance of the focussing screens. 



CHAPTER XXXViil. 

EXCEPTIONAL RAPIDITY WITH HIGH DEFINITION. 

Piazzi Smyth's Corrector. We have previously pointed 
out that when a portrait combination is corrected for 
flatness of field, this is attainable at the expense of 
marginal astigmatism, unless the field to be covered be 
very narrow. One on the contrary that is corrected to 
give the best definition at the margin does so at the 
expense of roundness of field, so that when the centre is 
in focus the ^ides are out, and vice versa. 

When <C. Piazzi Smyth was Astronomer Royal for 
Scotland, he, knowing well the highest requirements of 
a photographic lens, devised an ingenious means by 
which the oblique pencils of a round field lens could be 
so lengthened as to eventually render the whole field 
flat. 

The corrector employed for this purpose consists o* 
a rather thick plate of glass the size of the "sensitive 
plate, one side, that towards the front, being ground to 
a hollow or concave curve, the other side being flat. 
This must be mounted in front of and as close to the 
sensitive plate as possible. 

The action of the corrector is as follows : The axial 
rays from the lens fall upon its centre, where it is very 



PIAZZI SMYTfTS CORRECTOR. 261 

thin, and although the convergence is affected, it is only 
so in a slight degree, and the rays come to a focus upon 
the plane of representation. But at the margin the rays 
have to pass through a considerable body of glass, which 
they do in a degree more nearly approaching parallelism 
than previous to their entrance, and upon emerging from 
the flat surface they have still to travel a little farther 
before being brought to a focus on the sensitive plate. 

As the curved surface of the corrector stands so 
nearly normal to the rays there is scarcely any loss 
from oblique incidence, while there is a very decided 
gain in rapidity in consequence of the large aperture 
that can be given to the lens. 



CHAPTER XXXTX. 

MISCELLANEOUS. 

To Remove Lacquer from Mounts. By immersing the 
brass work in boiling water in which washing soda or 
potash is dissolved, the lacquer will be immediately 
removed. 

A better way, and one by which the necessity for 
heating is obviated, consists in applying, by means of a 
tuft of cotton wool, a mixture of equal parts of alcohol 
and ammonia. 

Lacquering. Ordinary lacquering necessitates the 
heating of the mount in order to its close adhesion, and 
not drying with a chilled surface. But if the lacquer be 
rendered alkaline by the addition pf a small proportion 
of ammonia, it will dry bright without heat. This 
applies in a special manner to lacquers of which shellac 
forms the main constituent. 

An excellent tough transparent coating for brass is 
obtained by coating it cold with a solution of celluloid 
in acetate of amyl (or acetone). It takes several hours 
to become quite dry, but is then hard and durable. 

Staining Brass Black. This system consists in stain- 
ing the surface of the metal in contradistinction to 
applying an opaque black varnish. 

A black which penetrates the surface well, consists 



STAINING BRASS VARIOUS COLOURS. 26 3 

in immersing the article, previously made clean and 
freed from greasiness, in a weak solution of a mixture of 
the nitrates of copper and silver, and then exposing to 
heat till the colour was well developed, afterwards 
plunging into water. In this mixture the copper should 
largely predominate. 

A method of staining, without applying heat, con- 
sists in suspending the article for a short time in a 
solution composed of one ounce of carbonate of copper, 
dissolved in eight ounces of ammonia, to which is 
then added sixteen ounces of water. The carbonate of 
copper is obtained by dissolving sulphate of copper in 
water, and carbonate of potash in another quantity of 
water, and pouring one into the other. Decomposition 
immediately takes place, the carbonate of copper being 
precipated. Pour off the supernatant liquid and wash 
in two or three changes of water. 

Staining Brass various Colours. Although the author 
cannot conceive of lens fittings being stained other than 
black, yet such a variety of really beautiful colours was 
obtained by the following process before the brass took 
on a black stain, that it may be well to record it. Every 
photographer knows that a solution of hyposulphite of 
soda is immediately decomposed by the addition of a 
variety of salts and by acids. Dissolve three-quarters 
of an ounce of hyposulphite in a half-pint of water, and 
in another half-pint of water dissolve three-quarters of 
an ounce of acetate of lead. Mix the two solutions and 
warm them, then at once immerse the articles to be 
coloured, 



264 DEAD BLACK VARNISH. 

Instead of the acetate of lead we have employed 
sulphuric acid in very small quantity to effect the de- 
composition of the hyposulphite. Scarcely a colour can 
be named which the brass (which must be scrupulously 
clean) will not assume in successive stages of the 
immersion. 

Some stain brass to a good black colour by brushing 
it with a dilute solution of nitrate of mercury, followed 
by two or more applications of a solution of sulphide of 
potassium (liver of sulphur). 

Dead Black Varnish. This may be made in several 
ways, among others by stirring lamp black intimately 
with a rather thin Brunswick black, the quantity of the 
former being such as to ensure its drying with a dead 
surface. 

The best opticians employ a mixture of vegetable 
black and lacquer, the proportions being determined by 
trial. The quantity of black must be just such as to 
cause it to dry dead and no more. A good way of 
mixing them is to place an ounce of ordinary gun-shot 
with it in the bottle, and shake well up. The article 
must be heated ere this varnish is applied. 

One of the toughest dead black varnishes we have 
ever tried is obtained from importers of American 
lacquers. It is sold under the name of enameloid, 
and, so far as we can see, is composed of celluloid 
dissolved in acetone, with the requisite quantity of 
vegetable or other black added to ensure deadness. It 
is applied cold, and dries in one or two hours. 

I 'ocussirg Screen for Lens Testing. If an extremely 



FOCUSSING SCREEN. 265 

fine grey glass surface be required for receiving a small 
and delicate image, as in testing lenses or for photo- 
microscopic focussing, the ground glass obtained in 
commerce may not unfrequently be found to be too 
coarse. A very fine grain can be obtained by exposing 
a sheet of scrupulously cleaned patent plate to the fumes 
of fluoric acid generated in the following manner : 
Having obtained a moderately deep vessel formed of 
sheet lead, gutta percha, or vulcanite, sprinkle the 
bottom with some finely crushed fluor spar, and over 
this pour a little sulphuric acid. Acid fumes will be 
immediately generated, and by allowing them to act 
upon the surface of the glass this becomes corroded, the 
grain at first being exceedingly fine and yet capable of 
arresting an image thrown upon it by a lens. 



INDEX. 



Aberration What is it ? 6 
Abbe, Professor, 177 
Abbe's lens, 183 
Achromatic condensers, 248 
Achromatism, Testing for, 137 
Achromats, New, 179 
Old, 179 
Actinism, 3 
Adjusting dissimilar lenses, 154 

,, uncorrected lenses, 29 
Advantage of large diaphragm, 131 

,, single lenses, 48 

Aerial images, 171 
Albo-carbon light, 243 
Aldis, H. L., 201, 203 

Lens, 204 
Aluminium mounts, 211 
Angle of view included by lens, 165 

,, measuring, 166 

Angular aperture, 75 
Antiplanet, Dr. A. Steinheil's, 72, 

181, 192, 203 

Aperture in diaphragm, 147 
Aplanat, Dr. A. Steinheil's, 177 
Aplanatism, Meaning of, 129 
Arbeit, E., 200 

Arborescent marks in lenses, 226 
Architecture, Lens for, 236 
Aristostigmat, Dr. Hugo Meyer's, 

199 
Ascertaining angle of view, 165 

,, equivalent focus, 101 

Astigmatism, How to test for, 139, 

177 

Astigmatism, to correct, 179 
Axial versus oblique rays, 3 

Back combinations, Properties of, 76 



Balsaming lenses, 229 

Barrel distortion, 52 

Bayonet joints for lens fittings, 207 

Beck on aberration, 18 

Blackening brass, 262 

Blocking lenses, 222 

Bow's compensating method for 

single lenses, 29 
Bow's method for equalising light, 

153 

Brewster's graduated eye tube, 123 
Busch Anastigmat, Karl Martin's, 

199, 208 
Butterfly stop for equalising light, 

153 

Camera Club focimeter, 119 
,, for lens testing, 135 

Canada balsam, 159 

Casket lenses, 88 

Cause of discolouration of glass, 161 
,, distortion, 51 

Cell-bound lenses, 21 1 

Cementing lenses, 224 

Central rays, Aberration of, 15 

Chemical and visual foci, 2 

Chromatic aberration, 9 

Circular diaphragms, Reasons for, 
148 

Clark, Alvan, 191 

Claudet on conjugate foci, 107 

Collecting system, 251 

Collinear lens, 187 

Coloured light, its properties, 4 

Colouring glass, Theories of, 162 

Colourless glass, 158 

Combination landscape lens, 62 

Combining lenses, Rule for, 157 



INDEX. 



267 



Compound lenses, Flare in, 96 
Concave lens, Effect produced by, 

209 

Concentric lens, 182 
Condenser, its uses, 219 
Conjugate foci, 106 

,, focus, formulae for, 114 
Cooke lens, 201, 205 
Copying maps, Lens for, 236 

,, portraits, 235 
Covering power, 136 
Cresson's condenser, 252 
Crossed lens, Centre of, 36 
' Crown,' 178 
Cundell's lens, 54 
Curing distortion, 54 

,, existing distortion, 237 

,, over-correction, 144 
Curvature of straight lines, 50 

Dallmeyer's diffusion objective, 130 
,, non - distorting land- 

scape lens, 47 

Dallmeyer's portrait combination, 78 
, , triple achromatic, 63 
,, . wide-angle rectilinear, 

71 

Davidson's combination, 70 
Dead black varnish, 264 
Deep meniscus lenses, 27 

The, 31 

Defective cementing, 228 
Dense flint glass, Deterioration of, 

1 60 

Dense glass, Advantage of, 82 
Density influences curvatures, 216 
Depth increased by small aperture, 

125 

Depth of focus, 123 

,, produced by a stop, 23 
Diaphragm, Advantage of large, 131 

,, Apertures in, 147 
Diaphragms standing at angle, 151 

,, their functions, 20 

Diffusion by single lenses, 134 
,, of focus, 128 
,, Claudet's method, 

133 



Discolouration of cement, 227 
Dissimilar lenses, Adjustment of, 154 
Distortion, 49 

of convergence, 49, 238 
, , of curvature, its cure, 237 
of curvature and con- 
vergence combined, 240 
Double condensers, 245 

,, best form, 247 

Doublets of Grubb and Ross, 68 

single lenses, 31 
Dynar, Dr. Har ting's, 198 

Edging and centering, 222 

Elasticity of focus, 32 

Elements of combination as land- 
scape lenses, 91 

Emery preparing for lens grinding, 
220 

Enameloid, 264 

Enlarging and reducing, 113 
,, landscapes, 231 

Equalising theskiesand foregrounds, 
150 

Equi-double convex, Rule for focus, 
216 

Equivalent focus, 100 

, , Grubb's method 

of ascertaining, 102 

Euryplan, E. Arbeit's, 200 

Fashion in lenses, 56 

First diffusion of focus lens, 130 

Fitting focussing telescope to camera, 

172 

Fixed focus for landscapes, 126 
Flange apertures, 212 
Flare in rectilinears, 97 

spot, 93 

,, ,, How to discover, 142 
Flatness of Field, Testing for, 140 
' Flint,' 178 
Focal centre of a combination, 38 

,, range, Table showing, 127 
Focus adjuster, 89 

Diffusion of, 128 
Focussing by telescope, 170 
screens, 170, 265 



INDEX. 



Focussing with working stop, 26 
Form of lens for enlarging, 230 
Functions of a condenser, 250 
Fuzzy pictures, Means for producing, 
133 

Gauss correction, 191, 199 
Genesis of doublet, 69 
Glass affected by light, 161 

,, selecting by polariscope, 214 
Globe lens, 66 
Goddard's double periscopic, 61 

,, triple lens, 62 
Goerz Double Anastigmat, I , 195 
Goerz Double Anastigmat, HA. 

(Convertible), 189 
Goerz Double Anastigmat, III., 186 
,, Hypergon, 194 

Grinding curved margins of lenses, 4 5 

,, lenses, 220 
Groups, Lenses for, 235 
Grubb, Sir Howard, on conjugate 

foci, 116 

Grubb's aplanatic, 44 
,, condensers, 248 
,, T., method for conjugate 
foci, 109 
Grubb, T., 184 

Hand cameras and conjugate foci, 

112 

Harting, Dr., 196 
Helivar, Dr. Harting's, 196 
Hoegh, Emil von, 186, 189, 205 
Holostigmat, Watson & Sons', 205 
How to find focal centre, 39 
Hypergon Double Anastigmat, 194 

Ideal definition, 7 
Images in telescope, 171 
Imperfect mounting causes flare, 93 
Inverted image, Cause of, 12 
Iris diaphragm, 148 

Jena glasses, 178, 205 

Kampfer, Dr., 187 
Kepler's law, 250 



Lacquering, 262 

Lacquer, to remove from mounts, 262 
Landscape lenses, Flare in, 96 
Lenses for, 235 
,, lens, Mounting, 46 
Lantern objectives, 253 
,, optics, 241 
,, Polariscope, 256 
Large micro objects, 236 

,, portrait lenses without depth, 

25 

Laws governing conjugate foci, 107 
Leitz, Ernst, 200 
Lens grinding, 214 

,, tools, 217 

Lenses, Forms of, 10 

,, interchanging their element, 

33 
Lenses of unequal curvature, Foci of, 

217 
Light, action of, on Canada balsam, 

162 
Light, action of, on glass, 160 

,, causes deterioration of lenses, 

.158 
Light, Decomposition of, 3 

,, for enlarging or projecting, 

241 
Limelight, 242 

Manganese in glass, 162 

Marcey lamp, 242 

Martin, Karl, 199, 205 

Matching lenses, 154 

Mechanical means of estimating con- 
jugate foci, 117 

Mechanical not the focal centre, 40 

Mineral oil lamps, 241 

Misconceptions regarding dia- 
phragms, 22 

Morrison's rapid doublet, 84 
,, wide angle, 67 

Mounts and cells, 207 

Nature of focussing telescope, 172 
Negative aberration, Lenses giving, 

17 
Non-achromatic lenses, 28 



INDEX. 



269 



Noton's diaphragm, 148 



Object glasses for focussing tele- 
scope, 173 
Oblique diaphragms, 150 

,, rays, Aberration of, 16 
Opaque stop, Equalising by, 152 
Optical centre of single lens, 34 

,, centre, Properties of, 37 

,, contact, 199 

,, flare spot, 94 

,, perfection not necessary, 

131 

Orthoscopic lens, 55 
Orthostigmats, Dr. R. Steinheil's, 

187, 195 
Over and under correction, 138 

Panoramic lens, 65 

Pebble lenses, 6 

Periplan, Ernst Leit/.'s, 200 

Petzval lenses for lantern, 255 

Petzval's portrait combination, 76, 
177 

Photographic correction a compro- 
mise, 16 

Photographic definition, 8 

Piazzi Smyth's corrector, 260 

Pincushion distortion, 53 

Pinhole aperture for testing equiva- 
lent focus, 103 

Pinhole apertures, 13 

Planar Lens, Dr. Rudolph's, 191, 
206 

Plano-convex lens, Rule for focus, 
216 

Polariscope for lantern, 256 

Polishing lens, 221 

Portrait lenses, History of, 73 

Portraiture, Lenses for, 234 

Positive and negative aberration, 17 

Protar Lenses, Dr. Rudolph's, 184 
,, VII., 188 
VIlA. (Convertible), 
1 88 

Protractors, Use of, 168 

Purity of glass, 142 

Putty powder, 222 



Quality of image by altering lenses, 
156 

Rapid lenses, Nature of, 80 

,, rectilinears, 81 
Rathenower Optische Industrie An- 

stalt, 200 

Razor-edge definition, 132 
Rectilinearity, Testing for, 143 
Rectilinear landscape lenses, 46 
Refraction influenced by density of 

glass, 5 

Remedy for flare, 94 
Removing lacquer, 262 
Rohr, Dr. M. von, 180 
Ross, T., 184 

Rudolph, Dr. P., 179, 184, 185, 206 
Rudolph's wide-angle anastigmats, 

185 
Rule for estimating foci, in 

,, focus of equi-double convex, 

216 

Rule for focus of plano-convex, 216 
of three applied to focus, 104 

Schott, Dr., 178 
Schroeder, Dr. H., 182, 183 
Schroeder and Stuart's lens, 182 
Schulze Gebriider, 201 
Secondary image, Cause of, 95 
Selection of lenses, 230 
Separating lenses, Effect of, 155 
Separation of lenses not arbitrary, 

209 

Shanks, 218 
Sharpness conferred by diaphragms, 

2 3 

Single lenses compensated, 27 
,, achromatic lenses, 42 

Size of image, 155 

,, image determined by focus, 13 

Slit apertures in diaphragms, 149 

Slowness of lenses, Causes of, 159 
,, wide-angle lenses, 166 

Specimen lens curves, 223 

Spherical aberration, 15, 179 

Staining brass black, 262 

,, various colours, 263 



270 



INDEX. 



Steinheil, Dr. A., 181, 192, 203 

Dr. R., 179, 186 
Steinheil's periskop, 68 

,, wide-angle aplanat, 71 
Stereoscopic lenses, 154, 233 
Stigmatic Lens, 203 
Stop, position of, in single lenses, 

210 

Stop reducing, and its object, 125 
Strise in lenses, How to find, 141 
Summar, Ernst Leitz's, 200 
Surface finish of lenses, 141 
Sutton's triplet, 63 
Symmetry, 83 

Taylor, Harold Dennis, 201, 206 
Teleo-photo. objectives, 258 
Telescope for focussing, 170 

,, with two object-glasses, 

176 
Telescopic definition, 8 

,, effects without a tele- 
scope, 258 
Templates, 218 

Tessar Lens, Dr. Rudolph's, 193 
Testing focus by single glass, 104 

,, for aplanatism, 145 

,, ,, definition, 142 

,, lenses, 135 

,, points to be noted, 136 



Treatment of lenses for solar en- 
largements, 164 

Triple condensers, 247 
lens, 63 

Unar Lens, Dr. Rudolph's, 192 
Unequal illumination, Cause of, 60 
,, illumination of negatives, 

IS 2 

Universal flange adapters, 213 
,, landscape lenses, 87 
,, lens on new system, 88 
Unofocal Lens, Dr. R. Steinheil's, 
I95 205 

Voigtlander's orthoscopic lens, 58, 
187 

Waterhouse diaphragms, 79 

Watson Sons, 205 

What constitutes photographic op- 
tics, i 

Whimsical diaphragms, 148 

Wide-angle lenses, 65 

,, lenses for narrow views, 
1 66 

Wide-angle triple condensers, 249 

Woodman's table of view angles, 1 15 

Zeiss' lenses, 183 
Zentmayer's lens, 68 



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