A Simplified Form and Improved Type of Photographic Lens.
Year: 1893

page_text: Date of Application, 25th Nov., 1893
Complete Specification Left, 24th Aug., 1894-Accepted, 6th Oct., 1894
PROVISIONAL SPECIFICATION.
A Simplified Form and Improved Type of Photographic Lens.
I, HAROLD DENNIS TAYLOR, of 20 Bootham Terrace, Bootham, York,
Optical Manager to the Firm of T. Cooke & Sons, Engineers and Scientific
Instrument Makers, Buckingham Works, Bishophill, York do hereby declare the
nature of this invention to be as follows:-
The chief object of my invention is the simplification & therefore cheapening of
photographic lenses. At the same time a very considerable improvement in the
optical performance of the lens, as compared with the ordinary forms of lens, is
attained.
My lens is essentially a triplet as substantially shown in the, accompanying
drawings. The two outside lenses are simple positive lenses, while the central
negative leus which I herein call the "corrector," may be either simple or
compound, preferably the former.
The lens has the form of a simple doublet with a diaphragm placed half way
between, such diaphragm being filled in as it were, with the "corrector" lens.
The two positive lenses are made of glass of a comparatively low dispersive
power, in fact the lower the dispersive power the better for the final result, while
the "corrector" lens is made of a flint glass of such a dispersive power as to enable
it to correct the colour aberrations of the two positive lenses consistently with the
other conditions being attained although the form of the whole lens is very simple
yet the nature of the corrections involved in the lens are very complex & inter-
locked together. For the "corrector" lens has to perform the following functions
simultaneously.
1. To correct the colour aberrations of the two positive lenses & thus render the
whole combination achromatic in the sense required by the actual uses to which
the whole lens is put, whether for photographic purposes celestial or terrestrial or
for lantern projection.
2. To correct the spherical aberration of the two positive lenses & render the
whole combination aplanatic.
3. To correct the curvature of image of the two positive lenses & thus render
the final image perfectly flat, while the astigmatism generally incidental to the
oblique pencils in the case of ordinary lenses, is simultaneously eliminated.
This last or third condition is secured by the application of much the same
principle as that which I have involved in a previous application for a Patent for
photographic lenses numbered & dated No. 1991, (1893).
That principle is that two positive lenses on a common optic axis may have their
curvature of field & astigmatism of the oblique rays simultaneously corrected by a
negative lens, made of the same sort of glass, placed anywhere between them so
long as its centre is on the common optic axis and provided that the power of the
negative lens is equal to the sum of the powers of the two positive lenses combined
and provided that the diaphragm corrections of all three lenses are eliminated by
giving to them suitable forms.
If a meniscus positive lens is placed with its convex side towards distant objects
it is well known that the presence of a stop or diaphragm placed axially between
the lens & its focus has the effect of flattening the image of those distant objects,
whereas placing the same stop in front of the same lens has the effect of rendering
the image still more round than that yielded by the naked lens.
On the other hand if the same lens is turned so that the concave side faces the

page_text: distant objects then it is well known that the effect of a stop placed between the
lens & its focus is to render the field of view or image more round or curved than
that yielded by the naked lens, while placing the stop in front of the lens will
flatten the image or field of view. But by these methods of applying diaphragms
or stops to simple lenses it is always impossible to obtain a flat image which is at
the same time free from astigmatism.
Now it can be shown that, given any special case or circumstances in which a
simple lens has to be used, a certain form can always be given to it such as to
render it free from diaphragm corrections; that is, it can be so shaped, agreeably
to calculation, as to render the presence of a stop placed at a certain distance
behind it quite ineffective for modifying the curvature of the image. In the case
of such a lens the curvature of its image is in no appreciable degree modified
by the stop or diaphragm which is virtually formed by the aperture of the
Degative lens. In Fig. 1. I have shown roughly the form of one of these simple
triplet combinations of about 7 inches focal length, which is initially designed
for copying equal size, so that the two positive lenses are precisely similar & the
corrector lens equi-concave. Here all three lenses are so shaped as to be free
from diaphragm corrections, as above defined, when the whole lens is being used
for copying equal size. That condition being attained, then the whole burden of
flattening the final image & correcting the astigmatism of the oblique pencils is
thrown upon the negative corrector lens and the principle of correction, which I
have defined above, has full play; & the power of the negative corrector lens can
be so regulated as to render the final image quite flat and at the same time
practically free from astigmatism. Its focal power will then be about equal to
the combined focal powers of the two positive lenses.
These corrections being properly adjusted when the triplet is being used for
copying equal size, it can be shown that when the triplet is used for forming an
image of distant objects; although diaphragm corrections will now come into
operation in the case of the front lens (which is exposed to the distant objects)
which tend to round the final image, still this tendency is about neutralised by the
diaphragm corrections simultaneously coming into operation in the case of the
back lens, which have an opposite tendency towards hollowing the final image.
Therefore the final image tends to remain flat in all cases in which the triplet is
likely to be used. But it should be pointed out that, although when copying equal
size the corrector lens should be placed exactly half way between the two positive
lenses in the case of Fig. 1. yet when using the triplet on distant objects this
position for the corrector lens will no longer suffice, for now the image painted by
the blue rays will be a trifle larger than the image of the same objects painted by
the yellow rays, also a slight amount of pincushion distortion may be expected.
These defects are remedied either by causing the corrector lens to approach nearer
to the front lens by a very slight amount or else by screwing the front positive
lens slightly nearer to the corrector lens. And a means of easily making the
necessary adjustment & the amount of the adjustment I will leave to be more
precisely described & ascertained in my Complete Specification. Fig. 2 shows
another form of the simple triplet, so designed that the lenses are severally free
from diaphragm corrections when the rays first entering the front lens F are
parallel. Fig. 3. shows roughly a form in which the front & back lenses are of
unequal powers & the whole lens initially designed for parallel rays. Fig. 4. shows
a form in which, by making the corrector lens compound, its spherical aberrations
can be so reduced as to render a shorter distance between the two positive lenses
feasible, thus obtaining a shorter & more compact lens, more suitable perhaps for
wide angles of view. Fig. 5 shows a form in which the corrector lens is split into
two portions & the stop is inserted between. Altogether it can be shown that
many forms of this lens are possible all alike embodying the same principle of
correcting the field of view; in all cases the corrector lens performing the triple
function of (1) correcting the field of view or rendering the final image flat &
free from astigmatism (2) correcting the combined colour aberrations of the two

page_text: positive lenses, & (3) correcting the spherical aberrations of the two positive lenses
combined.
Thus it will be easily seen that owing to the functions of the corrector lens
being interlocked together in such a way that no single correction can be modified
without altering the amount of the other corrections, at any rate without resorting
to a different sort of glass, therefore the actual planing & perfecting of a triplet of
this type is a highly complex & difficult operation, requiring much calculation &
patience; although when once the proper form & curves necessary to good
performance have been determined, the manufacture of any number of them
presents no serious difficulty. The form which I have sketched in Fig. 1, the
symmetrical triplet promises the greatest practical advantages & ease in working
out & doubtless is the form which I shall first perfect sufficiently for practical use.
I should point out that the bare idea of placing a negative lens between two positive
lenses for the purpose of tending to flatten the final image is not by any means, a
new one; for as long ago as 1858 a certain scientific man named Mr. Sutton
pointed out the advantage of such a combination & possibly applied the idea in
practice; but he placed a negative lens between the two ordinary meniscus like
combinations generally used for doublet lenses & moreover the focal power of the
negative lenses thus applied by himself & others was never more than a mere
fraction of the combined focal powers of the positive lenses with which they were
combined. Thus the flattening of the field was principally due to the diaphragm
corrections of the positive lenses; neither Mr. Sutton nor those working on the
same lines seem to have been conscious of the fact that diaphragm correctious should
be eliminated or that they could be eliminated, or of the principle that the power
of the negative lens should be approximately equal to the sum of the powers of
the positive lenses if a flat field, free from astigmatism, was to be attained. The
same remarks apply to Dallmeyer's & Ross's triplets and to Petzvals' orthoscopic leus.
Dated this 20th day of November 1893.
A Simplified Form and Improved Type of Photographic Lens.
I, HAROLD DENNIS TAYLOR of 20 Bootham Terrace, York Optical Manager
to the Firm of Messrs. T. Cooke & Sons, Opticians & Engineers &c. Buckingham
Works, Bishophill, York do hereby declare the nature of this invention and in
what manner the same is to be performed, to be particularly described and
ascertained in and by the following statement :-
In order to make clear the essential difference of principle between the
photographic lenses made according to this Specification and the older forms of
photographic lenses manufactured & used up to this date, it will first be necessary
to shortly explain 1st the older optical devices employed in such lenses for the
purpose of correcting the oblique pencils of light against curvature of field and
astigmatism & 2nd the new principle of correction employed by me, & embodied in
the herein specified lenses, for attaining the same ends in a more simple manner &
more radically perfect degree.
•
Fig. 1. represents the action of the front lens of an ordinary symmetrical doublet,
which is nearly always a double achromatic combination having the exterior form
of a meniscus lens placed with its convex surface outwards to receive the parallel
rays c-a-h-d, a', b', d' from any distant point.
Supposing these rays to be oblique, then the corresponding focus at c formed by
the naked lens I, is of a peculiar form. Let A-F be the optic axis and F the
principal focus. It is well known to opticians that the image formed by a naked

page_text: lens of a distant landscape is not a flat one, but is violently curved and marred also
by a greater and greater amount of astigmatism according to the angular distance
of the image from the optic axis.
Let it be supposed that the oblique rays c, a, b, d a, b, d' are proceeding from a
very distant point of light. All such rays contained either in the plane of the paper
or in planes parallel thereto are said to be rays in primary planes or primary rays.
Any two rays situated extremely closely to the central oblique ray c-c intercross
or come to a focus close to the point c & almost upon the line C-C. But the two
rays a and a falling on the lens at equal distances on either side of C-C do not
intercross or focus on the line C-C but at the point A situated below C-C. The
more widely situated pair of rays b & b' focus at the point B still further below the
point C, while the pair of widely separated rays d and d'intercross or focus at a
point D at a considerable distance below the line C-E. This line C-D may be
considered to be, roughly speaking, perpendicular to the optic axis A-F. Its
length is exaggerated in the figure for the sake of clearness. This lateral displace-
ment of foci for symmetrical pairs of rays such as a & a', b & b¹, d & d', increasing
rapidly according to the distances of each pair of rays from the oblique axis C-C,
brings about at the focus C-D a sort of balloon shaped image (Fig. 1. e) or
coma as it is generally termed by opticians. In this case the effect may best be
called "inward coma because the lateral displacement of foci for symmetrical
pairs of rays is inwards towards the optic axis A-F and moreover the more
diffused and larger end of the coma is also directed inwards towards the optic axis.
The effect of this lateral displacement of foci for symmetrical pairs of rays such
as a & a, b & b', d & d¹ is very important with regard to the consequent effects
upon the foci or intercrossing points of any pairs of rays which are unsymmetrically
situated with regard to C-C, such pairs as a & d c & a' c & b', a¹ & d' &c. for
instance. For their intercrossing points or foci are necessarily displaced either
nearer to or further from the lens L.
"
دو
Thus the two rays a & d focus at the point a + d much nearer to the lens
c & a focus at the point C + a', c & b' at c + b', a' & d' at the point a'd' and
these points are successively further away from the lens. It follows therefore that
if a stop or diaphragm s-s is placed at any distance behind the lens L so as to
limit the pencils effective for forming the image,-only allowing rays to pass which
are refracted eccentrically through the lens such as the eccentric rays c & b' in
Figure 1, then a considerable flattening of the image must take place owing to
those oblique and eccentric pairs of rays only being allowed to pass which have
their foci much further away from the lens and possibly upon or even beyond the
focal plane F-P. Thus it may be said of a lens like that shown in Fig. 1 when
used for parallel
or divergent rays, that it has considerable diaphragm
corrections or that its image is capable of being flattened by the presence of a
smaller circular aperture or diaphragm s-s placed axially behind it.
66
It is
Fig. 2 well represents the case of an ordinary achromatic meniscus single
photographic lens or the back lens of a photographic doublet lens. In this case
the meniscus lens, being inverted, has just the opposite effects on parallel rays to
those shown in Fig. 1. The foci for symmetrical pairs of rays such as a & a',
b&b d & d' are displaced to the points A. B and D successively further away
from the optic axis than the point C on the central oblique ray. Instead of inward
coma the lens gives outward coma, the outlying & more diffused extremity of the
coma Fig. 2. e. being directed outwards, away from the axis A-F.
obvious that if the curved image yielded by such a lens is to be flattened then
the stop or diaphragm must be placed in front of the lens, at s-s for instance.
This stop only allows to pass the eccentric rays c & b and those between,
and c and b have their foci at the point c+b much nearer to the focal
plane F-P. Having now dealt with the rays in primary planes, that is those
rays contained either in the plane of the paper or in planes cutting the lens
along chords parallel to the plane of the paper or diagram, it will be necessary to
consider the case of those rays, belonging to the same oblique incident pencil, which

page_text: are contained in planes cutting the lens along chords which are perpendicular to
the plane of the paper or diagram. It would be extremely difficult to diagram-
matically illustrate the course of such rays. n the first place the image formed
by a naked lens is always violently curved along a curve F-h for rays in primary
planes and the radius of this curvature is a fixed quantity for any given lens and
is independent of the degree of convergence or divergence of the rays first incident
on the lens. The radius of curvature of the image formed by primary rays is
generally ths of the principal focal length and only varies within uarrow limits
according to the refractive indices &c. of the glass or glasses employed. On the
other hand the image formed by the naked lens by means of those rays contained
in a secondary plane cutting through the centre of the lens, is formed along a much
flatter curve F-g, and the radius of curvature of this curve is generally ths of
the principal focal length, is a fixed quantity for any given lens and is likewise
independent of the degree of convergence or divergence of the rays first incident
on the lens, and varies only within small limits according to the refractive indices &c.
of the glass or glasses employed. In the case of ordinary glass lenses whether
simple or achromatic the image formed by rays in secondary planes is curved to a
radius just about 2.2 times the radius of curvature of the image formed by rays
contained in primary sections of the same pencils, and it follows that, for any given
oblique pencil, the focal point formed by rays in primary planes is situated
approximately 2.2 times as far short of a plane F-P passing through the focus and
perpendicular to the optic axis, as the focal point at formed by rays in the
secondary section. With a properly fashioned lens & by means of diaphragm
corrections it is quite possible to lengthen out the foci for rays in primary planes to
such a degree as to form a flat image upon the plane F-P, but it can be shown
that it is in nearly all cases impossible to simultaneously flatten the image formed
by rays contained in secondary sections of the same pencils. For it can be
mathematically demonstrated that any diaphragm correction which lengthens out
the focus of rays contained in a primary section of an oblique pencil, by a certain
amount c, is invariably accompanied by a lengthening out of the focus for rays
contained in a secondary section of the same pencil whose amount is just one third
of the correction in primary section or But in the case of the naked lens the
deviation of focus for a primary section of an oblique pencil from the plane F-P
is in the case of ordinary glasses about 2 times the deviation of focus from the
plane F-P for a secondary section of the same pencil. Hence if the diaphragm
corrections are just sufficient to form a flat image for primary sections of the
oblique pencils, it is evident that the corresponding diaphragm corrections for
Fecondary sections of the same pencils will not be sufficient to fully compensate
their curvature of image and therefore the image formed by rays in secondary
sections will remain curved, and to a radius generally equal to somewhat more
than double the focal length. This difference of foci for rays in primary and in
secondary sections of the same oblique pencil constitutes what is known as the
marginal astigmatism of photographic lenses. In the case of all lenses made of
ordinary crown and flint glasses &c. and compensated by the usual diaphragm
corrections, it has been found impossible to obtain an image both flat and free from
astigmatism at the same time; if a flat image is obtained it is astigmatic; if an
image free from astigmatism is obtained then it is considerably curved, (to a radius
equal generally to 1 times the principal focal length). There is however a new
sort of glass now obtainable, combining a very high refractive index with a very
low dispersive power and another sort, combining a low refractive index with a
relatively high dispersive power, which may be so combined together as to form an
achromatic meniscus whose curvature of image for rays in primary planes is just
3 times the curvature for rays contained in the secondary section of the same
oblique pencil and these curvature aberrations are therefore in the right relation
for being simultaneously corrected by means of diaphragm corrections and so a flat
image also free from astigmatism can be obtained. But the thorough carrying out
C

page_text: of the above principle in combinations made of these exceptional glasses
unfortunately precludes the use of a large relative aperture.
In order to obtain a lens of a large relative aperture which at the same time
has a flat image or field of view in conjunction with a general freedom from
astigmatism at the outskirts of its field, I have recourse to a principle of correction
which, as far as I know, is entirely novel. This principle may be explained as follows.
Fig. 1 shows a case of a lens giving inward coma and consequently capable of
having its image flattened by placing a diaphragm behind it.
Fig. 2 on the other hand shows a lens having the opposite characteristic of
outward coma and capable of having its image flattened by placing a diaphragm in
front of it. It is obvious then that a form of lens should be possible intermediate in
form between these two, which should give neither outward nor inward coma.
This is actually the fact, and Fig. 3 shows the action of such a lens. Here the
foci of all symmetrical pairs of rays such as a & a, b and b' d and d¹ fall at the
same point Cupon the oblique axis C-C if the lens is free from spherical aberration
as in Fig. 3, or if not free from spherical aberration, as in Fig. 4 which represents
a simple lens, then the foci of all such symmetrical pairs of rays fall upon the
oblique axis C-C although not all at the same point. In either case the oblique
focus is a symmetrical formation. In the case of Fig. 3, since all the rays pass
through the same focal point C, it is plainly evident that diaphragms placed axially
either in front of or behind the lens, can have no effect whatever in modifying the
curvature of its image. Such an aplanatic lens, giving symmetrical oblique
refraction may therefore be said to have its diaphragm corrections eliminated. The
appearances seen at the focus of a distant point of light, formed by such a lens,
pass through the successive phases l, m, n, o, p as the lens is receded from; first the
luminous line which is formed at C vertically to the plane of the diagram and lastly
the luminous line p which is formed at g and lies in the plane of the diagram. These
and the intermediate appearances are quite symmetrical. In the case however of
the uncorrected simple lens in Fig. 4, although the oblique focus is likewise
symmetrical, yet it is evident that the placing of a diaphragm -s either in front
of or behind such a lens must necessarily have the tendency to curve its image
more than before, since the diaphragm only admits eccentric sets of rays like a¹
and d which focus at the point a' d' short of the ultimate focus at C. But it is
possible to eliminate diaphragm corrections from such a simple uncorrected lens
having positive spherical aberration by the following device. Given the position
of the diaphragm behind the simple lens in Fig. 4, it is easy to give the lens a
form which is more hulged out towards the left hand so as to approximate
slightly to the case of Fig. 1. by an amount easily assignable by calculation, and
which will bring about such an amount of inward coma as to give rise to diaphragm
corrections which will just neutralize those diaphragm corrections dependent upon
the spherical aberration. Thus the diaphragm corrections dependent upon inward
coma can be made to flatten the image just as much as the diaphragm corrections
dependent upon positive spherical aberration tend to further curve the image and thus
I obtain a simple lens which miay as truly be said to have its diaphragm corrections
eliminated as in the case of the aplanatic lens shown in Fig. 3. All such lenses
therefore have their images curved to the normal degree, uninterfered with by
diaphragm corrections. In the case of a simple lens whose diaphragm corrections
are thus eliminated formed of glass whose index of refraction is about 152, the
image formed by rays in primary sections of the oblique pencils is curved to a
radius equal to about ths of the principal focal length, and the image formed by
rays in secondary sections of the same oblique pencils is curved to a radius equal
to about 3ths of the principal focal length and thus, in lenses of this type as used
by me, the curvature error of the focus for rays in primary sections of a given
oblique pencil is always about 2-2 times the curvature error of the focus for rays
in the secondary section of the same oblique pencil. This ratio of 2.2 may be
looked upon as practically constant in all lenses used by me.
It can be proved that exactly the same law of curvature of image and rules as

page_text: to diaphragm corrections and the elimination of diaphragm corrections applies to
negative lenses as well as positive lenses, and I will now show how a flat image
almost perfectly free from astigmatism may be obtained by judiciously combining
together positive and negative lenses whose diaphragm corrections have been
eliminated. Fig. 5 shows a simple positive lens (L) of this type and N is a
negative lens of the same type and of the same focal power as L and s-s a
diaphragm or stop placed conveniently between them. The diaphragm corrections
of L are supposed to be eliminated on the condition that the rays first incident on
it are parallel as if coming from distant points.
The lens N, being placed in any desired position between L and its image, must
also have its diaphragm corrections eliminated so that the presence of the
diaphragm s-s between them shall have no effect upon the curvature errors of
either lens. Under these conditions the positive lens L tends to produce an
image F-K, for rays in primary planes, of the normal curvature and an
image F-g for rays in secondary planes also of the normal curvature. But the
negative lens N considerably extends the focal length N-F and tends to throw a
larger image upon the plane f-p and this image must be approximately flat and
free from astigmatism, since the curvature errors of N, being normal and uninter-
fered with by diaphragm corrections, are equal and opposite to the curvature errors
of the positive lens I, since the two lenses are of equal focal power. Or the rays
may be traced backwards as follows:-
Supposing -p to be the original flat object, then oblique rays in primary
planes originating from the point p, after refraction through the negative lens N,
appear to come from the point K while rays originating from f on the axis appear
after refraction through the negative lens, to diverge from the point F and the
virtual image of the original f-p which is formed at F-K is curved to the normal
radius equal to ths of the principal focal length of N. But the principal focal
length of N is equal to the principal focal length of L, and therefore the virtual
image F-K is curved to the same radius as is the real image of distant objects on
the left formed by the positive lens L. Therefore the rays apparently diverging
from the point K will, after being refracted through the positive lens L, emerge on
the left in a condition of parallelism. Conversely, rays in primary planes entering
the lens L from an infinitely distant plane object or landscape on the left hand will
after refraction through both lenses come to focus upon a flat surface f-p
perpendicular to the optic axis. And precisely the same reasoning applies to the
rays contained in secondary sections of the same pencils, in this case also the
curvature errors of the two lenses are equal but opposite and therefore neutralize
one another. In this way a flat image may be obtained which is also free from
astigmatism.
This simple example of only two lenses serves to illustrate the principle, but in
practice two lenses only cannot be employed owing to two defects necessarily
inherent in such a combination one being the presence of an unsightly amount of
negative distortion of marginal straight lines and the other being the failure of
such a lens to adapt itself to copying or enlarging, for it can only give a flat image
for rays of a certain degree of divergence. For instance if the lens shown in
Fig. 5 were used on a near object on the left, then the rays entering L would become
strongly divergent and diaphragm corrections tending to flatten the image formed
by L would ensue. At the same time the rays entering N would grow less con-
vergent and the effect would be to give rise to diaphragm corrections in N whose
tendency would be the same as the diaphragm corrections in L and the final effect
would be that an image would be formed actually curved backwards.
I get over these difficulties by the simple device of dividing my positive lens into
two portions and placing my negative corrector lens between them, thus forming
what is generally a triplet combination. The above principle of corrections remains
equally in force as may be gathered from Fig. 6 which roughly represents such a
triplet lens having its two positive lenses L, and L, alike in power and shape but-
turned in opposite directions. The whole combination is shown in Fig. 6 as though

page_text: being used for copying a diagram to equal size the rays entering L diverging to
the same degree as the rays leaving L, are converging and each lens is in itself
supposed to be free from diaphragin corrections under these circumstances. In
such a case the negative lens N would be equiconcave & since its focal power has
to be approximately equal to the sum of the focal powers of the two positive lenses
it is obvious that if the negative lens is imagined to be split along the dotted line
into two equal plano concave lenses then we should have virtually two double
combinations placed together in reverse order 1st the double combination consisting
of a positive lens L₁ and a negative concavo-plane lens being the left hand half
of N these two being equal in principal focal length or focal power according to
what has preceded and 2nd the double combination consisting of a plano concave
lens, being the right hand half of N, and a positive lens L, these two also being
equal in principal focal length or focal power. Therefore the curvature_errors of
the positive lens L will be compensated by one half of the negative lens N and the
curvature errors of the positive lens L, will be compensated by the other half of
the negative leus N. Such a combination is free from distortion and moreover
compensates itself when used on a distant subject for if the rays first entering L,
become parallel or nearly so instead of divergent then diaphragm corrections arise
in the case of L, whose tendency is to make its image more curved than normal.
But at the same time owing to the much lessened divergence of the rays entering L
diaphragm corrections arise in its case whose tendency is to flatten its image and
these corrections more or less perfectly neutralise those operating in the other
lens L₁ while no appreciable diaphragm corrections can arise in the case of the
negative lens so long as the oblique pencils pass centrally or almost centrally
through it, as they must do if the diaphragm is placed very closely to N. In such
manner all the lenses made according to my principle may be designed to answer as
well for copying or enlarging as for distant work. Given the fact that each lens
used in my combinations is individually free from diaphragm correction when the
rays first entering have a certain assigned degree of divergence or parallelism, then
the combination as a whole will remain sufficiently free from diaphragm corrections,
supposing the combination is used for rays having any other likely degree of
divergence and thus, diaphragm corrections being practically eliminated from my
combinations, then the whole burden of flattening the final image and compensating
its marginal astigmatism is thrown upon the negative lens whose errors of curvature
of image are, as I have stated, of the same character and amount but opposite in
sign to the curvature errors of the positive lenses, provided that the focal power of
the negative lens is approximately equal to the sum of the focal powers of the two
positive lenses. I thus discard altogether the old principle of diaphragm corrections
hitherto relied upon for flattening the image of photographic lenses, in favor of the
above explained principle which renders possible a much more perfect correction
for astigmatism in conjunction with a flat image of considerable angular extent.
I do not claim priority for the mere expedient of mounting a negative lens behind
a positive lens or between two positive lenses, for the purpose of merely helping
to flatten the final image, for these devices were long ago carried out in
Prof. Petzvals orthoscopic lens also in Sutton's, Dallmeyer's and Ross's triplet
lenses. But it can be shown that in none of these lenses does the focal power of
the negative lens amount to inore than a fraction, four tenths or so, of the sum of
the focal powers of the positive lenses, while a large part of the flattening of the
final image is due to the influence of the usual diaphragm corrections, the idea of
eliminating the diaphragm corrections and throwing the whole burden of flattening
the final image and correcting marginal astigmatism apparently not having
occurred to these inventors. So far I have dealt with the principle adopted in my
lenses for correcting the curvature and astigmatic errors of the oblique rays.
I will now, before giving the curves and other particulars relating to my lenses,
touch upon the other important corrections against spherical aberration and
chromatic aberration which must be more or less perfectly carried out in any lens
that is to be practically useful.

page_text: Difference between refractive indices for the D & }
G rays or ua – MD
== -02086
I will now give the radii of curvatures of the surfaces, the central or axial
thicknesses of the lenses and their finished diameters and the axial air spaces
between them, expressing them in fractions of the equivalent focal length of the
whole combination, so that if any leus of a certain focal length is required, all that
is necessary is to multiply the following figures or fractions by the focal length
required and the proper radii &c. will then be obtained. Convex surfaces have
a+sign before their radii and concave surfaces a sign before their radii.
-

page_text: The corrections against axial spherical and chromatic aberrations in this com-
bination depend on the total reparation between the two outside lenses. If too
wide apart there will arise positive spherical aberration and undercorrection for
colour. At the same time the relation between the two air spaces must be correct
in order to make the combination rectilinear and achromatic towards the edge of
the image.
If the image is at all round or concave towards lens and at the same time the
spherical and colour aberrations are more or less undercorrected and there is also
outward coma towards sides of image then the combination requires shortening up,
at both ends if the lens is rectilinear, or only at the front end L₁ if the lens gives
negative or pincushion distortion, or only at the back end L, if the lens gives
positive or barrel shaped distortion of straight lines. But if the above specifi-
cation is rigidly carried out there should not be much occasion for further serious
adjustments.
is
It should be pointed out that the correction for spherical aberration in this
portrait. lens is not perfect, there being a zone of rays (focussing shorter than the
F
rest) at about half way between the edge and centre of the aperture when
used. But this zonal aberration is not so great as to interfere with as much
sharpness of definition as is most suitable for portrait work.
P
4
The diaphragms for regulating the aperture are placed as closely as possible
behind the negative lens L, and the aperture of such diaphragm necessary to give
a working aperture of should be 177, expressed in the same manner as the
previous figures. It should be understood that this lens should be tested upon
distant, or moderately distant objects.
4
I will now give the figures, expressed in the same manner as the last, for a rapid
lens giving a wider angle of view than the preceding.
F
5-65
This lens is shown in Fig. 8. in about correct proportions. Its full working
aperture is with that aperture it will give a well defined flat image up to the
corners of a plate whose longer side is equal to ths of the equivalent focal length,
while with stop it will about cover a plate whose greater side is equal to the
F
8
8
equivalent focal length, while the aperture of full is in operation up to the
extreme margins of the plate. Thus its illumination is remarkably even in its
character.
The two positive lenses L, and L, are made of a borosilicate crown glass having
the following optical properties.
Refractive index for the D ray -
- =
1.5101

page_text: The freedom from or presence of distortion in the image thrown by this lens
depends upon L, being adjusted to the right distance from L. If there is
positive or barrelshaped distortion present, then L, should be screwed nearer up
to Ls and vice versa.
The above are specifications of the only two lenses on my principle which I have
yet had the time or opportunity to work out; it being a work of great theoretical
and practical difficulty, owing to all the various corrections performed by the
negative lens being so interlocked together and interdependent. I have yet to
work out a wider angle lens than the one shown in Fig. 8, among others. These
lenses may be easily made on a scale small enough for microscopic work with the
lantern.
It is not at all necessary to the embodiment of my principle that the negative
lens should be a cemented combination, if made of two lenses. For instance
Fig. 9 sketches out a perfectly symmetrical combination in which the negative lens
consists of two equal lenses turned opposite ways, with the diaphragm placed
halfway between them. Such an arrangement has certain features much in its
favour and I hope to be able to work it out more exactly. I should here point out
that the deep curves and comparatively large thicknesses required in my lenses
render necessary an apparent departure from the rule that the focal power of the
negative lens should be equal to the combined focal powers of the two positive
lenses. In the lenses I have herein specified the focal power of the negative lens
amounts to appreciably less than the combined focal powers of the two positive
lenses when reckoned by the usual approximate formula; but such formulæ
become less and less accurate and reliable as the curves are deepened and the
relative thicknesses increased.
The most perfect and approved types of modern photographic lenses, the only
ones which can claim to give results at all comparable with those obtainable with
lenses made on my principle, rarely consist of so few as four separate lenses, five
or six separate lenses being very commonly employed in such combinations and
therefore, relatively speaking, I may describe my invention as involving a
simplification in form as well as an improvement in type.
Having now particularly described and ascertained the nature of my said
invention and in what manner the-same is to be performed, I declare that what I
claim is :-
A photographic lens, lantern or microscope projecting lens, composed of two
simple positive lenses and a simple or compound negative lens placed between the
two positive lenses; all three lenses being severally so designed as to be
simultaneously free from diaphragm corrections or have their diaphragm
corrections eliminated when the rays first incident on the front lens are either
parallel or else have a certain assigned degree of divergence; while the focal
power of the negative lens is made as closely equal to the combined focal powers
of the two positive lenses as is found necessary to the complete flattening of the
final image or field of view; the whole burden of correcting the oblique pencils
of light against curvature of image and astigmatism thus falling entirely upon the
negative lens, by which device a flat image characterised by a substantial freedom
from marginal astigmatism, is secured.
Dated this 10th day of A uœust 1894
H. DENNIS TAYLOR.