Improvements in Lenses.
Year: 1895

page_text: Date of Application, 10th Aug., 1895
Complete Specification Left, 8th May, 1896-Accepted, 18th July, 1896
PROVISIONAL SPECIFICATION.
Improvements in Lenses.
.
I, HAROLD DENNIS TAYLOR of Trenfield, Holgate, York, in the County of
York, Optician, do hereby declare the nature of this invention to be as follows :-
The chief object of my invention is the simplification and therefore cheapening
of photographic lenses. At the same time a very considerable improvement in
their optical performance, 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 lens which I herein call the " corrector may be either simple or
compound, preferably the former.
""
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 refractive index is as high as possible. 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 while its refractive index has as low a value as possible. Although the
form of the whole lens is very simple yet the nature of the corrections involved in
" lens has
the lens are very complex and interlocked together; for the "corrector
to perform the following functions simultaneously.
1. To correct the colour aberrations of the two positive lenses and 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 and render the
whole combination aplanatic.
3. To correct the greater part of the curvature of image of the two positive
lenses and thus, in conjunction with a small residue of diaphragm corrections,
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 chiefly by the application of much the
same principle as that which I have involved in a previous application for a patent
for photographie lenses numbered and dated No. 22,607 (1893).
That principle is that two positive lenses on a common optic axis may have their
curvature of field aud astigmatism of the oblique rays simultaneously corrected by
a negative lens, made of approximately 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 and 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 (naked in the sense
that the lenses effective aperture is bounded by its own periphery).

page_text: On the other hand if the same lens is turned so that the concave side faces the
distant objects, then it is well known that the effect of a stop placed between the lens
and 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 negative
lens. In Figure 1, I have shown roughly the form of one of these simple triplet com-
binations baving a compound negative lens. Here all three lenses are so shaped as
to be collectively substantially free from diaphragm corrections, as above defined,
when the whole combination is used for rays of ordinary degrees of divergence.
That condition being attained, then almost the whole burden of flattening the final
image and 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 principal play; and 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 roughly equal to the combined focal
powers of the two positive lenses.
The lenses provisionally described herein differ from the lenses described in my
former Complete Specification No. 22,6073 in that in the present case the refractive
index of the outer positive lenses is equal to or greater than that of the negative
corrector lens, as above alluded to. This device leads to a considerable improvement,
first because it enables more moderate curves to be used for a given focal length and
second because it permits of a slight relaxation of the power of the negative lens,
consistently with a flat and substantially anastigmatic final image being obtained.
If in the first place the refractive indices of the positive lenses and the negative lens
are equal, then the best results are to be obtained, not by carrying out the above
principle of correction too rigidly by eliminating diaphragm corrections entirely
and making the power of the negative lens exactly equal to the combined power of
the two positive lenses, but by eliminating the diaphragm corrections, all but a
small residue of about 2 or 3 per cent., and making the power of the negative lens
equal to 97 or 98 per cent. of the combined powers of the two positive lenses. In
this case 97 to 98 per cent. of the curvature errors of the individual lenses is
corrected by the power of the negative lens, while the small residue of 2 or 3 % of
curvature of image is corrected by the small residue of diaphragm corrections
which is allowed to remain. While this device may lead to a small amount of over
correction for astigmatism in the margins of the final image, yet this defect is so
small as to be more than compensated for by the comparative absence of secondary
aberrations in the way of wings of light etcetera, which immunity follows from the
flatter curves which the above method renders possible.
If in the second place the refractive index of the positive lenses is substantially
higher than that of the negative lens, then this alone permits of a further slight
relaxation of the relative power of the negative lens, consistently with a flat and
anastigmatic final image being obtained. For instance if the refractive index of
the positive lenses is 03 greater than that of the negative lens, then the relative
power of the negative lens may be relaxed about 2 per cent. consistently with
proper correction for astigmatism being preserved. In this case 98 per cent. of the
curvature of image yielded by the positive lenses should theoretically be corrected
by the power of the negative lens and the other 2 per cent. by the residual

page_text: diaphragm corrections. Therefore by adding together the slight relaxation of the
power of the negative lens due to compromise as above explained with the further
slight relaxation of the power of the negative lens which legitimately follows from
the excess, if any, of refractive index of the positive lenses over that of the
negative lens, a very appreciable gain in the focal power of the combination is
secured, or in other words the curves and thicknesses of lenses required for a given
aperture and focal length are rendered more moderate, and secondary aberrations
are reduced. This amounts to a very appreciable improvement of the lenses
herein alluded to as compared with those described in my older Specification
No. 22,607 (1893).
Figure 2 is a sketch of a simple triplet consisting of only three simple lenses and
having a full aperture of about I reserve more exact working descriptions
for my Complete Specification.
F
5.6.
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
(I) Correcting the combined colour aberrations of the two positive lenses ;
(II) Correcting the spherical aberration of the two positive lenses combined;
(111) Correcting nearly the whole part of the curvature and astigmatic errors
of the two positive lenses, while the residue of such errors is compensated
by a slight residue of diaphragm corrections, thus securing a flat final image
substantially free from astigmatism.
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 cau be modified
without altering the amount of the other corrections, at any rate without resorting
to a different sort of glass, therefore, the actual planning and perfecting of a
triplet of this type is a highly complex and difficult operation, requiring much
calculation and patience; athough when once the proper form and
necessary to good performance have been determined, the manufacture of any
number of them presents no serious difficulty.
curves
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 and possibly applied
the idea in practice; but he placed a negative lens between the two ordinary
meniscus like combinations generally used for doublet lenses, and moreover the
focal power of the negative lenses thus applied by himself and 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
corrections 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 lens.
A 1005

page_text: Improvements in Lenses.
I, HAROLD DENNIS TAYLOR, of Trenfield, Holgate, York, in the County of
York, Optician, do hereby declare the nature of this invention and in what
manner the same is to be performed, to be particularly described and ascertained
iu and by the following statement :-
The object of this specification is to fully describe certain improvements on the
lenses already fully set forth in the Specification of my Patent No. 22,607 of 1893.
The principle of optical correction herein described also involves a slight
departure from the principle of optical correction explained in my former
specification aforesaid, but before I can explain wherein this new improvement
and departure consists, I shall have to explain the principle of optical correction
carried out in the lenses described in my former specification for this principle is
also the main basis upon which the lenses described herein are constructed.
In order to make clear the essential difference of principle between the photo-
graphic lenses made according to this specification and my earlier Specification
No. 22607 of 1893 and the older forms of photographic lenses manufactured and
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, and 2nd. the new
principle of correction employed by me and embodied in the herein specified lenses
for attaining the same ends in a much simpler manner and in most cases a more
radically perfect degree.
Figure 1 represents the action of the front lens of an ordinary symmetrical
doublet, which is nearly always a double achromatic combination of ordinary
crown and flint glasses having the exterior form of a meniscus lens placed with its
convex surface outwards to receive the parallel rays c, a, b, d, a', b' d¹ from any
distant point.
Supposing these rays to be oblique, then the corresponding focus at C formed
by the naked lens L ("naked" in the sense that its aperture is bounded merely by
its own periphery) 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
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 in the plane of the paper,
which is supposed to cut through the centre of the lens, are said to be rays in the
primary plane or primary rays, to which the following remarks apply.
Any two rays situated extremely closely to the central oblique ray c-C
intercross or come to a focus close to the point C and almost upon the line c-C.
But the two rays a and a¹, falling on the leas 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
slightly below c-C. The more widely separated pair of rays 6 and 1 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-C. This line C-D may be considered to be, roughly speaking, per-
pendicular to the optic axis A-F. Its length is exaggerated in the figure for
the sake of clearness. This successive lateral displacement of foci for symmetrical
pairs of rays such as a & a¹, b & b', 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 (Figure 1 e) or
""
coma as it is
generally termed by opticians. In this case the effect may best be called
66

page_text: For
"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 & a¹,
b&b, d & d' 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 & ', a & d' &c., for instance.
their intercrossing points or foci are necessarily displaced either nearer to or
further from the lens, L. Thus the two rays a & d focus at the point a + d much
nearer to the lens, c & a¹ focus at the point c + a¹, c & b¹ at c + b', a¹ & d' at the
point ad 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 and those included between them, then a
considerable flattening of the image must take place owing to these particular
oblique and eccentric rays having their foci at the point c + b', much nearer the
focal plane F-P if not upon or beyond it. Thus it may be said of a lens like that
shown in Figure 1, when used for parallel or divergent rays, that it is capable of
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.
Figure 2 fairly 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 Figure 1. The foci for symmetrical pairs of rays such as a & a',
& bid & dare 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 and more diffused extremity of
the coma (Figure 2 e) being directed outwards, away from the axis A-F. It is
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 + 6 much nearer to or upon the focal plane F-P.
for
Having now dealt with the rays in the primary plane, that is those rays contained
in the plane of the paper, it will be necessary to consider the case of those rays,
belonging to the same oblique incident pencil, which are contained in the plane
cutting the lens along a chord which is perpendicular to the plane of the paper or
diagram and contains the oblique axis c-C. It would be extremely difficult to
diagrammatically illustrate the course of such rays. I have pointed out that the
image formed by a naked lens is always violently curved. This is the curve F-
rays in primary planes. The radius of this curvature is a fixed quantity for
any given lens and is practically independent of the degree of convergence or
divergence of the rays first incident on the lens, provided the original object is a
flat surface lying normal to the axis of 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 narrow limits according to the refractive indices, etcetera, 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, generally 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 (if, as above, the original object is flat and
normal to the axis) and varies only within small limits according to the refractive
indices, etcetera, of the glass or glasses employed. Thus in the case of lenses made
of crown glass having a refractive index of about 1-50, the image formed by rays
in secondary planes is curved to a radius just 2.2 times the radius of curvature of

page_text: C
the image formed by rays contained in primary sections of the same pencils and the
same is approximately true of ordinary achromatic lenses made of ordinary crown
and flint glasses and it follows that for any given oblique pencil, the focal point
at C formed by rays in the primary plane is situated 2-2 times as far as short of a
plane FP passing through the focus and perpendicular to the optic axis, as the
focal point at g formed by rays in the secondary section. With a properly
fashioned lens and 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 it has been shown that, 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
secondary 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, 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 1½ 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 three 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 of the above principle in combinations made of these exceptional
glasses unfortunately precludes the use of a large relative aperture.
The use of a positive lens having a high index of refraction, tends to accommodate
itself to diaphragm corrections, because it combines a slightly flatter image with
the same amount of astigmatism. For instance, in the case of a lens made of glass,
having a refractive index of 1-6, the ratio between the radii of curvatures of the
images formed by primary and secondary sections of the same oblique pencils,
is 2.23 instead of 2-2 as in the previous case; and this slight difference of 03 is
sufficient, when the lens is properly combined with another negative lens of a lower
refractive index and nearly equal power, to result in a relatively long focussed
combination, whose radii of curvatures for images formed by primary and
secondary sections of the same oblique pencils are in the relation of 3 to 1
and can thus be simultaneously corrected by diaphragm corrections, as in the case
of Ross's concentric lens.
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

page_text: at the outskirts of its field, I bave recourse chiefly to a principle of correction
which formed the sole basis of my earlier Specification No. 22,6073 while at the
same time the above advantages inherent in the use of relatively highly refractive
positive lenses are retained as far as possible. This principle may be explained as
follows:-
Figure 1 shows a case of a lens giving inward coma and consequently capable
of having its image flattened by placing a diaphragm behind it.
Figure 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 Figure 3 shows the action of such a lens. Here the foci of
all symmetrical pairs of rays corresponding to a and a, b and 6', d and din
Figures 1 and 2 fall at the same point C upon the oblique axis c-C if the lens is
compound and free from spherical aberration, as in Figure 3, or if not free from
spherical aberration, as in Figure which represents a simple lens, then the foci of
all such symmetrical pairs of lays 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 Figure 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
air 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 Figure 4, although the oblique focus is likewise
symmetrical, yet it is evident that the placing of a diaphragin s-s either in front
of or behind such a lens must necessarily, owing to the spherical aberration of the
lens, have the tendency to curve its image more than before, since the diaphragm
only admits eccentric sets of rays like c & b which focus at the point c & 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
Figure 4, and the distance of the object from the lens, it is easy to give the lens a
form which is more buiged out towards the left hand, so as to approximate
slightly to the case of Figure 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 ten 1 to further
curve the image, and thus I obtain a simple lens which may as truly be said to
have its diaphragm corrections climinated as in the case of the aplanatic lens
shown in Figure 3. All such simple lenses therefore have their images curved to
the normal degree, uninterfered with by diaphragm corrections for a given distance
of the object. It is exclusively simple lenses, substantially of this type, with their
diaphragm corrections substantially eliminated, that are employed by me for the
two outer positive lenses of all my combinations. Thus, in the case of all simple
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 approximately 2.2 times the
curvature error of the focus for rays in the secondary section of the same oblique
pencil.

page_text: It can be proved that exactly the same law of curvature of image and rules as
to diaphragm corrections and the elimination of diaphragm corrections apply to
negative lenses as well as to 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. Figure 5 shows a simple positive lens (L) of this type and N 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, but
not further from L than of its focal length 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 fp and this
image must be approximately flat and free from astigmatism, since the curvature
errors of N, being normal and uninterfered with by diaphragm corrections, are
equal and opposite to the curvature errors of the positive lens L, since the
two lenses are of equal focal power. Or the rays may be traced backwards as
follows:-
:-
Supposing f-p to be the original flat object, then oblique rays in a primary
plane originating from the point p, after refraction through the negative lens N,
appear to come from the point 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 about 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 fp 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 neutralise one another. In this way a flat image of considerable angular
extent 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
Figure 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
convergent 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 a triplet combination. The resulting equivalent focal power of the whole

page_text: combination is a function of the separations between the lenses. The above
principle of corrections remains equally in force, as may be gathered from
Figure 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 Figure 6 as though 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
diaphragm corrections under these circumstances. In such a case the negative
lens N would be equiconcave and 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, first the double combination consisting of a
positive lens L' 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 secondly the double combination consisting of a plano-concave
lens, being the right hand half of N, and a positive lens L³, these two also being
equal in principal focal length or focal power. Therefore the curvature errors of
the positive lens L₁ 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 lens N. Such a combination is free from distortion and moreover
approximately compensates itself when used on a distant subject; for if the rays
first entering L¹ 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 L2, diaphragm corrections arise in its case whose tendency is to
flatten its image and these corrections more or less neutralise those operating in
the other lens L₁, while no very 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 N is thin and the diaphragm is placed
very closely to it. In such manner most of the lenses made according to my
principle may be designed to answer as well for copying or enlarging as for distant
Given the fact that each lens used in my combinations is substantially free
from diaphragm corrections when the rays first entering are parallel or have a
certain assigned degree of divergence, then the triplet 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 substantially eliminated from my combinations, then the whole
or nearly 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 shown of substantially 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 for the most part the old
principle of diaphragin corrections hitherto relied upon for flattening the image
of photographic lenses, in favour of the above explained principle, slightly modified
in a manner which I will now explain, which renders possible a much more perfect
correction for astigmatism in conjunction with flat image of considerable angular
work.
extent.
This slight modification of the principle depends almost entirely upon two main
considerations, which may be explained as follows:-
In the first place, in the case of simple triplet combinations in which the
refractive indices of all three lenses are about equal, the strict application of the
above principle theoretically demands that the diaphragm corrections should be
totally eliminated and that the whole burden of correcting the curvature of image
and marginal astigmatism should be thrown upon the negative lens, whose focal
B

page_text: power would have to be exactly equal to the combined focal powers of the two
positive lenses.
But this results in a relatively long focal length or small focal
power and the smaller the relative separations between the lenses, the longer the
focal length and vice versa. This means that the curves must be deep and the
lenses thick in order to get a relatively large aperture. But deep curves and large
thicknesses tend to produce aberrations of a secondary order, at the final oblique
foci, in the shape of wings of light etcetera, which more or less interfere with
definition. So the best general result, in the shape of a well defined final image,
is therefore to be obtained, not by carrying out the above explained principle of
correction too rigidly, but by eliminating the diaphragm corrections all but a
small residue and making the power of the negative lens fall slightly short of the
sum of the powers of the two positive lenses, so that a residue of correction
against curvature of field and astigmatism is performed by diaphragm corrections,
the main burden of such corrections still falling upon the negative lens. It is by
such a compromise that a very appreciable amount of focal power is gained and
therefore more moderate curves required in relation to aperture, while, if the
compromise results in a little residual negative astigmatism in the margins of the fina!
image, that slight defect is more than neutralised by the comparative absence of
secondary aberrations in the form of wings of light, etcetera.
In the second place, the use of a glass for the positive lenses, whose refractive
index is substantially higher than that of the negative lens, independently permits
of a further slight relaxation of the relative power of the negative lens, consistently
with a flat and an astigmatic field being obtained. In the case of the lens,
Figure 10, Series V, shortly to be described, 96 per cent. of the correction of the
errors of curvature of image and marginal astigmatism of the two positive lenses
should theoretically be corrected by the power of the negative lens alone, leaving
the residual 4 per cent. of curvature and astigmatic errors to be corrected by the
residual diaphragm corrections allowed to remain. Moreover the adoption of a
high index of refraction, whenever possible, for the positive lenses, leads to more
moderate curves and thicknesses for obtaining the same focal length, which is in
itself a practical advantage. A still greater excess of the refractive index of the
positive lenses over the refractive index of the negative lens, should it ever become
possible, would lead to a further small reduction in the relative power of the
negative lens. Thus in the first place I avail myself of the relaxation of the
relative power of the negative lens in the way of compromise, as above cited, and
in the second place of the further strictly legitimate relaxation of the relative focal
power of the negative lens due to the employment of positive lenses of a sub-
stantially higher refractive index than that of the negative lens or element, and
thus I gain a total relaxation of the relative power of the negative lens from
equality equal to about 10 or 11 per cent. in the cases of lenses Series III,
Series IV and Series V hereindescribed.
It is quite possible to make one of these triplets in which the two positive lenses
are made of different glasses. In this case the refractive index of the front or
- most powerful lens should be held as substantially characteristic of the two positive
lenses, as, in most cases, the power of the back lens is relatively small. But such
a device promises no distinct advantage. I will now proceed to give exact
specification of four different triplet combinations, all designed on the modified
principle of correction explained above.
In all cases, the radii of the curves, thicknesses and diameter of the lenses,
separations and diameters of stops or diaphragms are expressed as decimal
fractional parts of the equivalent focal length of the whole combination, which is
invariably supposed to be 1 inch or unity. Therefore, in order to obtain the
correct curves etcetera for a triplet lens of any given focal length, all that is
necessary is to multiply the figures for that type of lens by the focal length
required, the products being the radi of curvatures etcetera required, expressed in
inches.

page_text: SERIES II, FIGURE 7.
PORTRAIT LENS OF APERTURE
F
WHICH IS RENDERED APL
FREE FROM SPHERICAL ABERRATION BY THE ADOPTION OF
COMBINATION FOR THE NEGATIVE LENS.
The two positive lenses L₁ and L. are made of a dense barium sil
ass of the following optical properties.―
Refractive index for the D ray
Difference between the refractive indices for
the D & G rays
-
=
1.57
-
=
01
The refractive index for D minus 1 (or the
index of deviation) divided by the difference
of indices for the C and F rays
.
-
=
57-1
This is the reciprocal of its dispersive power, as usually reckoned, a
all the glasses mentioned in this and the following specifications of l
The negative element L, is made of a light silicate flint glass having
Refractive index for the D ray
Difference between indices for D and G rays
Reciprocal value of the dispersive power
====
1.54
⚫01
===
45.7
=
The meniscus element L, is made of densest baryta crown glass havi
Refractive index for the D ray
Difference of indices for D and G rays
Reciprocal value of the dispersive power
L₁
La
L3
{
{
Radius of 1st surface "₁
2nd
"
"
"
T2
Radius of 3rd surface rs
4th
"
"
وو
Radius of 5th surface r
{
6th
"
"
"
To
LA
Radius of 7th surface
8th
"
"
"
T8
The fourth and fifth surfaces are cemented together.
L₁Finished diameter
{Axial thickness
Finished diameter
La Axial thickness
Is
Finished diameter
{Axial thickness
1.61
01
56.3
+
.29
+ | 1
1-16
46
13
1+
13
23
++
1.42
+55
25
=
.06
+21
.00
=
+21
03

page_text: only must the optical centres of the lenses lie exactly upon a common axis but
the lenses themselves must be accurately perpendicular or square with respect
to that axis, and since errors so commonly arise in the turning of a series of
metal cells held in alignment by a tube with screwed surfaces, I have therefore
found it in the highest degree conducive to expeditious and certain adjustment
of my combinations to mount the negative lens in a separate cell which can
be moved laterally across the optic axis and also tilted with respect to that
axis by means of small screws. It is also advisable to fix the negative lens if
compound, in its cell by a black cement instead of bezelling it in. This
cement may be made of hard balsam and lamp black or balsam and black
sealing wax. Figure 11 gives a section of a simple form of this arrangement.
Here pare three (or four) conical pointed screws which are threaded through the
tube and bear upon the chamfered edge of the negative lens cell. These three
screws have the effect of pushing the cell in any direction laterally, when the
opposing ones are relaxed and also have the effect of clamping the cell down in its
place when all these are tightened. The other 3 (or four) screws t-t-t-are for
tilting the negative lens. They are threaded through the flange of the negative
lens cell and push against the fixed ring or flange and raise the former if the
corresponding pushing screw p is first relaxed. The procedure in adjusting all of
the above described lenses is generally as follows:
1st. Shorten up air space between Li and L, until the combination shows just a
little positive spherical aberration along the axis when tried telescopically with an
eyepiece, the object being an artificial star placed at some distance away in a
darkened room.
2nd. If, when examined along the axis the disc of light seen when inside of
focus, is unsymmetrical and shows a stronger and brighter edge inclined to redness
upon one side, say the right hand, then this points to the negative lens being
laterally out of centre and in this case towards the right hand. By means of the
3 screws p, p & p the negative leus in its cell must be moved a trifle towards the
left hand. This operation must be repeated until the disc of light is perfectly
symmetrical both inside and outside of focus. Yet it may appear oval, owing to
the negative lens being out of square.
3rd. The combination should next be examined for symmetry of field. Dividing
the field or ground glass, as viewed from behind the camera, as usual, into left
hand, right hand, top and bottom parts for reference, if the oblique rays are found
to focus. longer on the right hand than ou the left and show an overcorrected
astigmatism on the right hand relatively to the left it shows that the negative lens
is out of square and in this case the fault will be cured by pushing the negative
lens cell further out, away from the screen, or the right hand side, and so on.
If
this adjustment is carried out approximately until the next test has been applied
it will in most cases be sufficient.
4th. The field should next be examined for curvature. A distant weathercock
is a very good test object especially if it present both upright and horizontal
straight lines. Such an object is first carefully focussed upon the centre of the
screen and the position of the camera slide marked. Then the camera is rotated
so as to permit the weathercock to be focussed at the two right and left edges of
the plate which the lens is intended to cover and the mean position of these lateral
foci compared with the position of the central focus. There should be, no
difference between the central and oblique foci if the lens is correctly adjusted.
If the oblique focus is shorter than the central focus by a very small amount the
error may often be corrected by shortening the distance between L₁ and L, if
there is sufficient excess of positive spherical aberration to permit of this. If this
cannot be done, then the 5th surface rs must be slightly flattened.
If, on the other hand, the oblique foci are longer than the central focus. and the
field is thus overcorrected, then, if the distance between L₁ and L, cannot be
increased, because of the consequent spherical aberration, recourse must be had to
deepening the 5th surface. r a trifle. Such alterations, however, should not be

page_text: required if the glasses are true to this specification and if the curves and
thicknesses are carefully followed out.
5th. After the image has thus been adjusted for flatness and any residual errors
in squaring on or laterally adjusting the negative lens eliminated, the combinations
should then be examined for distortion. If the air spaces prescribed herein are
exactly followed out there should be no distortion in the image, but if slight
deviations of the glasses from the normal optical characteristics render necessary
slight alterations in the curves as above indicated, then it is advisable to re-examine
the combination for distortion. A vertically stretched wire is the best test object.
The lateral image of this wire should be rigidly straight and of course accurately
coincide with straight lines ruled on the screen parallel and close to the edges of
the plate the lens is intended to cover. If the lens shows any trace of barrel
shaped distortion then the back air space A, must be decreased by shortening the
tube, and if any trace of pincushion distortion is shown, then the back air space A,
must be increased.
6th. After all the optical adjustments have been completed, the three screws ppp
should be filed down flush with the tube or else be concealed; according to the
fashion of the mounting.
In explaining the principle of correction employed in all the above lenses, I
have shown that the theorem that one or two positive lenses may have their
curvature of field and marginal astigmatism simultaneously corrected by a negative
lens on the same axis and made of similar or nearly similar glass provided that
diaphragm corrections are substantially eliminated, is the principal basis upon
which all these lenses are constructed.
But the theorem, if strictly applied, requires that the lenses concerned should
be very thin. But I could not get the required apertures if I were limited as to
thicknesses, and the more the thickness of a lens is increased, the greater becomes
the discrepancy between its actual equivalent focal power and the focal power of
an infinitely thin lens of the same curves and material as calculated by the usual
refraction, the radius of 1st surface and s = the radius of 2nd surface.
While this formula correctly expresses the focal power of a very thin lens, yet,
when the thickness becomes relatively substantial, as in the case of the front
positive lenses of all my combinations, the same formula becomes somewhat inaccurate,
inasmuch as it overrates the equivalent focal power of a thick lens and therefore a
substantial correction for thickness bas to be deducted.
Hence if the powers of the positive lenses in some of my combinations are
calculated by the above approximate formula and their sum compared with the
power of the negative lens, the sum of the powers of the two positive lenses will
come out relatively in excess of the value obtained after the corrections for
thickness are applied, the true or corrected sum of the powers of the two positive
lenses in the cases of the combinations herein described being about 11 per cent. in
excess of the power of the negative lens and more appreciably in excess in
proportion as the refractive index of the positive lenses is substantially greater
than that of the negative lens. A large number of various combinations of 3 or 4
lenses are possible besides the few actually specified herein all involving the same
principle of correction. For instance, the most powerful of the two positive lenses
may be placed at the back of the combination instead of at the front. Possibly I
may have occasion for working out further varieties, perhaps realising slight
improvements upon those already perfected.
So far I have only dealt with the principle adopted in my lenses for correcting
the curvature and astigmatic errors of the oblique rays. I will now touch upon the

page_text: other important corrections against spherical aberration and ohromatic aberration
which are embodied in the above specified lenses and must be more or less perfectly
carried out in any lens that is to be practically useful.
It has almost always been the practice to make each lens of a doublet or triplet
lens in itself compound and self-corrected against spherical and chromatic
aberrations. However Prof. Abbe of Jena, in a triplet lens patented a few years
ago in this country, practically demonstrated that it was possible to make a triplet
lens in which the two outside positive lenses were perfectly simple, while the
chromatic and spherical aberrations both of the central direct pencil and the
oblique pencils were simultaneously corrected by means of a compound lens, of
scarcely any focal power or even positive, placed half way between the two
positive lenses. But this central correcting lens was not designed to contribute
anything towards the flattening of the image and eliminating marginal astigmatism,
whereas in my lenses I make my negative correcting lens perform the triple
function of
1st. Correcting the final image against curvature and astigmatism, thus permitting
an unusually flat and perfect image of considerable angular extent to be attained.
2nd. Correcting both the direct and oblique pencils against chromatic aberration.
3rd. Correcting both the direct and oblique pencils sufficiently against spherical
aberration.
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,
Figure 12 sketches out a perfectly symmetrical combination in which the negative
lens consists of two equal lenscs 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 have not given a drawing of a metal lens mount in this specification because a
great variety in mounts is possible and each manufacturer prefers a mount to suit
his own requirements. Moreover when these lenses are designed for lantern or
microscopic projection, a special mounting must be devised.
Having now particularly described and ascertained the nature of my said inven-
tion, and in what manner the same is to be performed, I declare that what I
claim is :-
:-
1. 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, the negative lens, if simple, or the negative element, if the
negative lens is compound, having a refractive index for the D ray lower than and
never greater than the refractive index for the D ray of the positive lenses; all
three lenses being severally so designed as to be collectively substantially free from
diaphragm corrections or have their diaphragm corrections for much the greater
part eliminated, when the rays first incident on the front lens are either parallel
or divergent in normal degrees; while the focal power of the negative lens is made
nearly equal to the combined focal powers of the two positive lenses, in order to
counteract all but a residuum of the curvature and astigmatic errors of the two
positive lenses, which residuum is in a condition to be approximately corrected by
and is corrected by the residual diaphragm corrections allowed to remain in the
system; by which device a flat final image, characterised by a substantial freedom
from marginal astigmatism, is secured.
•
2. The improved constructions of lenses substantially as herein described and
illustrated.
WM. P. THOMPSON & Co.,
Of Liverpool, Manchester, Birmingham, and London, Patent Agents.