I-NRLF
or PHOTOGRAPHY
AND PHOTOGRAPHIC LENSES
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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.
' It must be said that a more interesting and stimu-
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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.
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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
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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
0
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 late 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.
64
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
78 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<^ al°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\To. 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 p1 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
'37J
21
•849
46
1-427
•389
22
•87
47
i'45
72
•407
23
•89
48
1-48
73
•425
24
•911
49
i '5
74
'443
25
'933
50
i -53
75
•462
26
•954
51
1-55
76
•48
27
'975
52
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)fy
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
23
'i
2
i
4
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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.
;36 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.
i4o 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 0 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 New7 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 ^£* 51 rePresents an Old Achromat, FIG- 52-
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) and 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 'rapid7 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,-
228 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
242 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 dt
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. 263
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,
23
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,
IS2
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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