Improvements in Photographic Lenses.
Year: 1889

page_text: Date of Application, 23rd Jan., 1889
Complete Specification Left, 23rd Oct., 1889-Accepted, 18th Jan., 1890
PROVISIONAL SPECIFICATION.
Improvements in Photographic Lenses.
I, HUGH BLACKWOOD, of Sidney Cottage, Beckenham, in the County of Kent,
Gentleman, do hereby declare the nature of this invention to be as follows:-
In order to take portraits or views by photography in a proper and effective
manner, the essential points to be attained are: the option of rapidity, width of angle,
combined with flatness of field, clearness of definition, and depth of focus (or field),
with a comparatively large stop (or wide orifice).
It has, however, hitherto proved exceedingly difficult to satisfactorily attain all these
points in one lens; an improvement has been effected on one point, with a more or
less contrary effect upon another point.
Even the lenses of Dallmeyer, Ross, Voigtländer, and other well known makers,
which have been produced with the object of obviating these defects, have not entirely
done so, but the improvement is so great, that these lenses are much sought after,
notwithstanding the great cost of the same; and at this very heavy expense (with
added weight when travelling) different lenses have to be used, and have hitherto had
to be purchased separately from these makers, for the various subjects to be photo-
graphed; thus, for instantaneous and general outdoor work, a comparatively narrow
angle lens is used, whilst for indoor work and confined situations, a wide angle lens
is required, and for indoor portraits a portrait lens.
Now the object of my invention is to entirely overcome the difficulties above
mentioned, by simple means, at a small cost.
In carrying out my invention, I employ an extra lens, which I place just in front
of the rear lens, or it may be inserted in the stop itself, different sizes and strengths
being thus optional.
This extra lens by preference consists of a bi-convex and a bi-concave, which may
he cemented together, or arranged with a small air space between. I prefer the latter
arrangement, as I can thereby substitute for one or both a stronger or weaker glass,
according to the effect it is desired to produce, as by the addition of stronger bi-convex
lenses, the focal length may be shortened down to such an extent that the sensitized
plate may almost touch the lens.
The glasses which I employ for my extra lens are ordinary eye glasses that can be
purchased for a small sum almost anywhere.
The strength of glass which I find most suitable varies from No. 5 to No. 16
(French Nos.), according to the class of lens to be used; thus for a rapid rectilinear,
or plate Lancaster meniscus lens, Nos. 7 or S are most suitable, while for a portrait
lens, the strength may vary from Nos. 5 to 16, according to results required. By
the addition of my extra lens, a narrow angle Ross, or other maker's narrow angle
lens is converted into a wide angle; whilst a Dallmeyer, or other maker's wide angle
lens, may be converted into a narrower angle of any required degree, by substitution
of a stronger bi-concave. Also, if objects at a great distance are to be photographed
(say, from the top of a hill), flatness of field and depth of focus (with a large stop
or orifice, enabling instantaneous effects) will be attained. A Voigtländer or other
portrait lens becoines, by addition of my lens, applicable for all known purposes,
as the good qualities previously sacrificed, to get the greatest possible rapidity,
are regained. Similarly, any ordinary leus, with addition of my lens, becomes
increasedly efficient for purposes of enlargement.
I also find that whilst, with the lenses of Dallmeyer, Ross. Lancaster, and other well
known makers, my adaptation produces greatly improved results, it with some French
and other lenses acts as a detective, in case such glasses are badly paired, or the
Jenses be of poor marginal definition, though at the same time it practically removes

page_text: this defect, as it permits of the lens being stopped down to overcome the defect, but
with a larger stop than could be otherwise used.
I have found that with many meniseus lenses, by the mere addition of one bi-convex
glass of No. 8 strength, I obtain rapidity (shortened focal length), width of angle,
clear definition, and flatness of field, but in most cases I prefer to use the compound
leus.
Similarly, with an addition of one No. 1 or 2 short-sight eye-glass inserted in the
stop, an ordinary plate portrait lens will cover a or 7" x 5" plate at the same foral
length (ir. 67) as before with a large (") stop, yet with flatness of field and depth
of focus, or a Dallmeyer 2. Baby lens will cover from to a 12" x 10" plate,
with a larger orifice, than at present, and attaining increased rapidity and brilliant
effect.
In all cases, a diaphragm will be placed between the extra lens and the rear lens
in the combination.
Dated this 23rd day of Januar 1980
COMPLETE SPECIFICATION.
Improvements in Photographic Lenses.
I, HUGH BLACKWOOD, of Sidney Cottage, Beckenham, in the County of Kent,
Gentleman, 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 take portraits or views by photography in a proper and effective manner,
the essential points to be attained are the option of rapidity, width of angle, com-
bined with flatness of field, clearness of definition, and depth of focus (or field), with
a comparatively large stop (or wide orifice).
It has, hitherto, proved exceedingly difficult to attain all these points in one lens ;
an improvement having been effected on one point, with a more or less contrary effect
upon another point.
The lenses of Dalmeyer, Ross, Voigtländer, and other well known makers, which
have been constructed with the avowed object of obviating these defects, have failed
entirely to do so, but the improvements made therein have been so great that these
lenses are much sought after, notwithstanding the high price of the same. As different
lenses have to be used for the various subjects to be photographed, that is to say, for
instantaneous and general outdoor work, a comparatively narrow angle lens is used;
for indoor work, and confined situations, a wide angled lens is required; while for
indoor portraits, a portrait lens; and a photographic artist, to be properly equipped,
must have all these lenses with him. A very heavy expense is thus entailed upon
the photographer, and the weight of his apparatus for travelling is enormously
increased.
Now the object of my invention is to overcome the difficulties above mentioned, and
my invention consists in the use of simple means whereby any existing photographic
lens may be utilised for the various purposes for which an assortment or series of
lenses have hitherto been required.
In carrying out my invention, I employ sets of cheap glasses, which I combine to
form additional lenses, and which I place between the front and rear ordinary lenses
at the points of foci. These lenses consist of combinations of biconvex and biconcave.
and other classes of glasses (as will be hereafter more clearly defined), which may
be cemented together or arranged with a small air space between, as preferred. I find
the latter arrangement advantageous, as I am thereby enabled to substitute for one or
other, or both, a stronger or weaker glass, according to the effect it is desired to produce.
The glasses which I employ for the additional lenses are ordinary eye glasses, that
can be purchased for a small sum almost anywhere, and I will hereafter describe

page_text: the various conibinations of glasses, and the results that will be obtained from their
use.
In the accompanying drawing I have shewn, by way of example, several modes of
mounting the glasses to effect the object of my invention; thus
Fig. 1 shows in vertical sectional elevation, a doublet, or
with my invention adapted thereto.
"
rapid rectilinear" lens
Fig. 2 is a similar view, illustrating a convenient form of lens tube arranged for
the purposes of my invention.
Fig. 3 shows, also in vertical sectional elevation, a modified form of lens tube.
Figs. 4 shew an arrangement of. mounts suitable for holding or carrying the
additional lenses, the parts of such mount being shown detached and separated.
Fig. 5 shows a modified form of mount, the parts of which are shown detached.
at Figs. G.
Similar letters of reference indicate like parts in the various figures.
A, A are respectively the ordinary front and rear glasses of a photographic lens,
mounted as usual in screw mounts B, B₁ in a lens tube C.
.D, D' are the two additional lenses, the one, D. being mounted behind the front
glass A the other, D', in front of the rear glass A'.
Each additional lens is preferably composed of two glasses, the one, a, being of
bi- or plano-concave form, and the other, b, of bi- or plano-convex form. These glasses
are mounted in a short cylinder e made of vulcanized fibre or equivalent material,
with an interposed annulus or ring & of suitable material and thickness, and the
glasses are secured in place by a ring e, cemented to the cylinder e, and by a ring y
fitting tightly into the said cylinder.
The additional lenses D, D', in their mounts, are slidden into the ordinary lens
tube C, which is provided in the centre, as usual, with a diaphragm or stop C*..
The inner end of the lens tube C is formed with suitable flange and screw thread C¹,
by which it is secured to the camera, and, the outer end has the usual hood C³ to
receive the cap or shutter.
In the arrangement shown at Fig. 2, an additional slot is made in the lens tube C,
and through this slot the additional lens D¹ is inserted. There is also in this case
an additional ring f, to form a packing or filling piece when desired, as shewn in
the drawing. The cylinder e, in this case, extends only around a portion of the
internal periphery of the lens tube C, and forms a lining thereto, in order that the
glasses may not be scratched or damaged, during their insertion therein. The lens
tube C is mounted, so as to be free to slide in the direction of its length, in an
outer tube E, provided with a flange and screw thread, by which it is fixed to the
camera.
The motion of the tube C is regulated and controlled by a rack and pinion, or other
equivalent device E, as will be well understood.
In the upper part of the outer tube E is made an opening E*, through which not only
the diaphragm C✶ is inserted, but when the opening in the inner tube C is brought
into coincidence, the rear additional lens D' may also be inserted or removed, so that
the said lens may be changed without unscrewing the leus tube from the camera, or
removing the rear glass therefrom.
Any other telescopie arrangement may be employed, such as that indicated in
diagram at Fig. 7, both ends of the lens tube C drawing out from the centre.
In the arrangement shown at Fig. 3, in place of the compound lenses D D', I
have shown two meniscus lenses, in combination with a biconvex lens De placed in
the diaphragm slot of the tube C. and this glass D³, I am able to readily change,
according to the effect to be produced, without unscrewing either glass of the lens.
It will be obvious that the form and position of mounts for the additional lenses
may be varied considerably; for instance, in Figs. 5 & 6 I have illustrated a modified
form of mount, which consists of a flanged metal ring h, into which are inserted the
glasses a and b, together with the spacing and packing rings d required. Over this
ring is fitted a second flanged ring j, thus securing the glasses in place in a metal

page_text: mount, which may be slipped into the vulcanized fibre cylinder c, as shown at Fig. 5,
& used in place of the other mounts shown.
The strength of glass which I employ varies according to the class of lens to be
used; thus, for a rapid rectilinear or plate Lancaster meniscus lens, Nos. C", 7",
or 8" will be found very suitable, while for a portrait lens, the strength may vary
from Nos. 5" to 16", according to the results required.
I will now proceed to describe various combinations of glasses, and give the results
when used in accordance with my invention.

page_text: It is in combinations such as these, viz.; by the slight increase in either tho
biconcave or in biconvex strength, as necessity may arise, that the marginal definition
of the original lens may be improved as required. This accuracy of marginal
definition is chiefly essential in the case of, say, the reduction of a narrow angle into a
+ 8" on 16" %
wide angle lens; thus +16 on
-
-
8
gives, to a rapid Doublet (of 10 back
focus), a 12" wide angle circle on 8 back focus. With some of the Doublets,
however, an advantageous increase in flatness of field for an instantaneous view, with
wide orifice, would result from placing, say,
alteration, or possibly, with a shallow lens,
+ 8" on
+ 8" on - 15" €
+
+ 15" on
16" on 8"
16%
-
-
8/c
or similar slight
and a flare spot would be
avoided. Another great advantage is being able to substitute for one or both, a
stronger or weaker glass, according to the effect it is desired to produce; as, by the
addition of stronger biconvex lenses, the focal length may be shortened down to such
an extent that the sensitized plate may almost touch the lens. My invention is to
utilize the scientific fact, viz., that biconvex and biconcave eye-glasses of the same
strength, more or less neutralize one another, as a means for the increase or decrease
in the size of the picture which is transmitted by or through them, in combination
with a photographic lens. Therefore, 1 show the graduated scale in which the
increase or decrease in the strengths of the biconvex and biconcave glasses is to be
regulated, and the method of application whereby the desired increase or decrease in
the size of the picture so transmitted is attained; and whereby, in effect a Ross or
other maker's narrow angle lens is converted into a wide angle, and vice versa, or a
Dallmeyer or other maker's wide angle lens may be converted into a narrower angle
of any required degree.
If a single biconvex lens be used alone with any photographic leus, the centre only
is strengthened, leaving the marginal definition still weak, weaker proportionately than
before.
Thus, too great biconvex strength introduces spherical and marginal aberration in
a greater or lesser degree, according to the make and quality of the lens to be
operated with. In like manner, a single biconcave gives great flatness of field, but
reduces depth of focus (or field), so that only with the smallest stops is it possible to
reobtain depth.
At the same time, a "flare spot" is introduced, and the general results are so
unsatisfactory, as hitherto to have effectually prevented such arrangement being
generally used.
-
As an example of the latter arrangement, I may mention a 7% applied to a
plate portrait lens of 4 back focus, gives, on 1917 back focus, a circle of, sny,
24" diameter: but even with a of an inch stop, a circle of 6" diameter would
scarcely be satisfactory.
-
67 %
There would then be great flatness, but without depth of field (or focus). A
or stronger glass, would give neither the one nor the other, having entirely neutralized
the good qualities of the photographic lens acted upon.
By the use of my combinations (say
+ 20" on - 50%
+ 10" on -4%s
which gives, on
19" back
focus a 24" circle), spherical or other aberration, in original plate portrait lens, is
decreased, if not altogether avoided, whilst greater even Hatuess of field, than the leus
had before, is given, depth of field and brilliancy of definition being retained. A
Voigtländer, or other portrait lens, becomes by addition of my improvement,
applicable for all known purposes; as the good qualities previously sacrificed, to get
the greatest possible rapidity, are thereby regained.

page_text: -
-
-
- 4% (or
With the cheaper plate portrait lenses, of 44 back focus, a + 16# on
+16 placed between two Ss), in rear of front lens, and + 20 on 80% in
front of rear lens, gives a 2333 diameter circle on 17" back focus:-Other results
being obtained with + 16" between two S's in front, and either of the following,
vie16" on -64" %*
-
+ 12" on - 48" }, placed at the rear, or dropped into the slot in lens tube.
+ 8" on 32" %)
-
-
-
--
Similarly, with rapid doublet, 10" back focus, + 8" on 5% placed in front,
and the following at rear :-+ 10" on 16", or + 15" on 24" or + 20"
32" or +25" on 40, or + 30″ on - 48" will give circles of diameters
varying between 27" and 32", or even more, on 22" to 24" back focus.
on
-
while
Or, should the operator be working with an 81″ × 6" camera, of 24" draw, with
carriers for the smaller sized plates, then, to produce a picture on the following sized
plates, ris.plate 5" x 4", plate 7" x 5", 81″ × 60″ & 10" x 8" wide angle, he
must employ the respective combinations as follows:-
-
"
30%
30%
"
13+
"
+ 42"
::
"
24"
22" %J
+12 ONT 24%
Which will give circles of the diameters, and on a back focus. previously mentioned
Exact proportions need not now be so sedulously attended to, in order to attain th
required increase of size of picture. A series such as the following may be used
viz.: + 36" ou 79 placed in front, & combinations as follows placed at th
rear:-
-
+ 60" on
+ 52"
or, + 40"
for 12 circle on 9 back focus; or,
"
103
"
"
for 13" circle on 104" back focus
"
"
"
"
"
"
"
"
4 52"
-
22" "
::
"
"
143
ני
"
11"
"
13
+ 52"
"
"
20" %
15
"
??
"
12"
N
"
"
+ 40"
-
::
"
+ 36"
"
+ 52"
"
"
::
+ 36"
-
"
+ 48"
=
::
"
-
-
18"
10" %
18
15%
16" %
"
10+
"
12
?
"
"
"
"
16"
"
."1
13+
"
"
16
H
"
"
"
133"
"
"
17
13
2
"
"
"
11
"
"
173"
"
22
14}"
"
"
+ 40"
-
";
"
18" "
17"
"
"
32
131"
"
"
+ 14"
-
8"
33
"
19"
15"
"
"
"
"
"1
+ 15"
"2
"
8" %
19
??
""
168"
"
"
+ 36"
-
"7
"
+ 18"
-
"
"
12" %
8'
!!
221"
18"
"
"
"
33
23"
1
"
21+
3
1
here alternate + 40" on
+ 16" on
+ 17"
+ 18"
"
-
-
-
"
or, 48" on
+ 10" on
-
80" %}
"6"
79" %
in front, and
8" %
8" /
in rear, for 24" circle on 20" back focus
"
و"
8" %
"
"
251"
26*"
"
"""
21+
22
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"
او
"
"
26+"
"?
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211
"

page_text: These combinations are chiefly intended for instantaneous work, with a wide orifice,
and for general rapidity combined with clearness of definition. Instead of stopping
down the lens to obtain a larger picture (vide Dallmeyer's large sized plate lens,
special rapidity for babies, the smaller the stop used, the slower the lens becoming),
with my combinations this object is attained on a wider orifice, and only a slightly
longer back focus, thus, for interiors, there is more light in the camera, assisting
greatly the proper adjustment of focus; and objects close to the lens are not dis-
torted; in fact, distortion may be said to be nil.
°
With a plate camera of 24" draw, many combinations could be arranged to give
a good centre equivalent to a 6 x 4 picture.
If small stops be used, the strongest convex glass (of shortest back focal length)
should be placed in front.
At the same time, as the back focal length is shortened, some measure of com-
pensation is afforded for the loss in rapidity.
The combinations arranged according to my system, give perfectly clear definition'
with any respectable maker's lenses, the results as to size of picture, merely varying
acording to the quality or strength of the lens acted upon by them.
That my combinations give correct definition is readily tested by holding up before
the eye, the properly proportioned (as before described) combinations (front & rear)
which it is purposed to use, & the picture seen through them will be perfectly clear
all over.
Further, when carefully examined, it will be seen that the size of the
picture given by them varies with the strengths & proportions of the glasses used in
the combinations; these results again, are, in a greater or lesser degree, affected by
the lens which is being worked with, and this increase or decrease in size can be
regulated by the photographic artist, as herein before pointed out.
My combinations are useful with photographic lenses used for stereoscopic purposes,
supplying a multiplicity of useful pairs as herein before pointed out, whilst any defect
in the original pairing of the stereoscopic lenses themselves, can, on enlargement, be
detected and remedied, increasing or decreasing in however slight a degree central or
marginal definition.
My combinations are useful in pairing doublet lenses, for instance, a
=
9%
+ 5" on
+ 9" on - 5%
(giving, with a rapid doublet of 103" back focus, a 12" circle 9 x 7 wide angle
on back focus) I have found to give, with the front lens placed alone behind it,
an 18 circle on 15" buck focus, but a somewhat larger circle, on 15" back focus,
was given by the rear lens placed alone behind the same combination.
1 #
With an Optimus doublet, any of the before said combinations may be used, and,
for delicate work (interiors &c., with great rapidity). I have found them to give
highly satisfactory results. Also on, say, a 7" x 5" Optimus, where flatness of field is

page_text: VII
+18" on
-
TO
12%
or it
be better rendered a
may
-
-
16 placed between a + 20
12%, or, at option + 17" on 12%
ม
and 22" in front, and at rear + 18 on
Any of these would give excellent portraiture, and are useful (like the other com-
binations with any lens) in photographing flowers, leaves and botanical specimens in
general, where the object is placed at a short distance from the lens, especially so with
short back focus combinations. The combinations are increasedly efficient for purposes
of enlargements; thus, with plate portrait, 4 back focus,
Object.
Distance
from lens.
Back focus,
+
with +
5" on
7" on
7% in front
5%, in rear,
enlarges
1"
to 2"
at 7"
on 17"
"
or, + 14" on
-
10%, in rear,
enlarges
1"
to-21"
"
64"
on 17"
or
42" on
30%, in rear,
enlarges
1"
to 3"
+
4" oll
8%, in front,
enlarges
1"
to 4"
"5"
4"
on 17"
נן
on 17"
40%, in rear,
"3
on
-
6%
30%, in rear,
in front,
enlarges
1"
to 4
31"
on 17"
"
"
enlarges 1"
to 3
"
33"
"
on 12",
1+ 80" on
+ 3 on
+60
-
-
or the same combination
and so
on in varying strengths and proportions, giving most pleasing results
resembling soft crayon drawing, but, being so faithful and searching, that the object
to be copied, if, say, carte de visite, should be specially got up beforehand, as even
the pores in the photographic paper are shown in an enlarged form.
The strength of glass which I find most suitable for the purposes of my invention,
varies from less than 1" back focus, up to 79" back focal length, and even up to
120 inches back focal length, both of the bi-concaves and bi-convexes. The higher
numbers or strengths of long back focus, such as + or - 10", being of a plano-convexo
or plano concavo form respectively, enables any and every advantage, say, in rapidity.
arising therefrom to be taken advantage of at comparatively small cost. The lower
or stronger numbers, such as + or 6", offer, by reason of the double nature (bicon-
vexity or bi-concavity) of their strength or structure (which some might otherwise
seek to get rid of by cutting them, as it were, respectively in half), affords a means
whereby any required degree of density in convexity may be given to the centre of a
lens, say, when it is intended to apply the smaller stops, while, at the same time, any
desirable proportionate increase in marginal definition may be given by a more or
less corresponding increase of bi-concave strength.
-
My single combination lens may consist of, say,
or +40 or 60, and + 79% giving to a
ī
-
79% on 10", or +8″, + 20,
rapid doublet a circular picture of
from 6" up to 14" diameter, or + 79 or
-
GCC
-
79'
-
40% -20% or 15
-
giving a picture of from 14 to over 26" diameter circle, the back focus throughout
varying from 5" to 224" inches. The plano concave and plano-convex glasses may be
placed in the stop itself, or in a suitable slot just between the stop and the rear lens.
They affect, and are applicable to any and all lenses, doublets or single, and may be
placed in front of the front lens of a portrait combination when the rear portion of
the portrait combination, having been removed, the front lens has been screwed into
its place.

page_text: My double combination leus, adapted to high class lenses for brilliant and effective
work, may consist of + 6" on placed in the space between the stop of, say,
8
I rapid doublet and the rear lens, and
0%
+ 12 on - 16%
+24 on
32%
+ 48 on
64%
+30 on
40%
+ 60 on 79%
and so on, placed in the space in rear of front lens of doublet, will give circles
increasing in size from 12" up to 1937-This last being equivalent to a 15 x 12 plate
on 14 back focus. Another series of combinations, giving most excellent results,
arise out of
+ 40 on
+ 80 on
-
-
80% similar combinations of multiples or proportionates
40%
having been alternately applied.
Of the more than 10,000 combinations which can thus be formed, it would not be
convenient, if desirable, to enumerate more than those already mentioned, which I
apprehend to be sufficient to lay down or point out the general principle on which
my invention is worked. As also my slight difference in the length of the tube in
which the doublet or single lens may be set would make an alteration in the size of
the picture resulting from the application to them of any of my combinations or com-
pound lenses, a inch increase in length giving a narrower diameter circle than with
the original lens tube. A further increase in size of picture results from any of my
lenses when, instead of heing separated, and placed part in rear of the front lens,
and part in front of the rear lens, they are placed together at the point of foci just in
front of the rear lens of a doublet, or in front of a single lens, varying moreover with
the extent of the back focal length.
By the use of my combinations, the effect of a bellows tube" to the lens is
produced, and the back focal length is thereby shortened, and the lens thus becomes
an excellent adjunct to the bellows camera. Instantos of express trains, steamboats,
river scenes, &c., invariably give clear definition and flatness of field, combined with
depth.
There is obtained, by means of this system, a series of cheap, efficient, and simple
compound lenses, any one of which conveys, perfectly intact, a picture of any
dimensions as herein described, and whereby one lens, by any maker of repute, may
do the work of that battery of lenses which otherwise, at very heavy expense,
with added weight when travelling, would be required by the photographic operator
to produce pictures of exteriors, interiors, portraits, or views in general, of a size
varying from less than 24 to more than 48" in diameter.
A doublet having a wide angle on a short back focal length, and with only room
for the stop or iris diaphragm to work between the front and rear lens. may be treated
as a single lens, and any of my combinations may be placed in front at point of

page_text: foci, the metal tube or box containing them being screw threaded on the outside, so
as to screw into the place of the hood of the lens, the hood being taken off.
For larger lenses, such as 10" x 8", 12" × 10", plate portrait and upwards, the
glasses of my combinations, if made to size. would be more or less expensive in pro-
portion, so a system of packing round the metal box or tube containing the "
glasses may be resorted to.
For a series of combinations, such as
arrangement shown at Fig. 2 is preferred..
+ 80" on
+ 4 on
40%
8%
hereinbefore described, the
+ rapid rectilinear and a ¦
Τ
->
In the form shown at Fig. 3, used with a
camera, I
divide, say, an 18" biconcave (its equivalent being a pair of 86" meniscus glasses),
which may be placed one each at the point of the foci of front and rear lens of the
doublet, concavity of eye glasses being turned towards concavity of lenses. Bi- or
plano-convex glasses, of strengths varying from + 79" down to + 5" back focal
length, are then, one at a time, inserted in the slot between the stop diaphragms in
centre of lens tube. On a back focus varying from about 22" down to 3", the
diameter of resulting picture circles will be from about 28" down to 5", definition
being good throughout. The effect of this combination is, that the picture from the
centre, or best portion of the front lens, is conveyed to the meniscus, where an
enlargement takes place. The enlarged picture is then conveyed to the second
meniscus, where a further enlargement takes place, and, in this enlarged form, the
picture is conveyed to the rear lens of the doublet, which transfers it to the sensitized
plate. The picture now may be too much enlarged for the purposes of the operator,
and he will therefore have to reduce it more or less to suit his plate. This will be
effected by inserting between the two meniscus glasses, bi- or plano-convex glasses
of suitable strength.
To illustrate my meaning, I would give a kind of formula as follows:-
Let A & B represent front and rear glasses of doublet; C and D meniscus glasses,
and E bi- or plano-convex.
11 may be said that the same result would arise by using two meniscus glasses, CD,
of the strength to make the enlargement 24".
This may be true, but unless the lenses A and B are correctly paired and ground,
central and marginal definition will be defective. This defect will, however, he
remedied by the addition of E.

page_text: 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
1. The means herein described whereby any existing photographic lens may be
utilised for the various purposes for which an assortment or series of lenses has been
hitherto required.
2. The use in or with a photographic lens, of one or more sets or combinations of
ordinary eye glasses, constituting an additional lens or lenses, as and for the purpose
set forth.
3. The combination with the ordinary glasses of a photographic lens, of sets of
meniscus, bi- or plano-concave or convex glasses, as and for the purpose set
forth.
4. The combination with a single or divided lens tube, of separate boxes or mounts
for holding sets or combinations of ordinary glasses, as and for the purpose set
forth.