calcula
Year: 1893

page_text: Date of Application, 30th May, 1893
Complete Specification Left, 10th Mar., 1894-Accepted, 14th Apr., 1894
In other forms of calculati Instruments CIE
been calculated for separately, and a specially divided circular slide rule has been
provided for each factor, and, when the resulting exposure has been shown by the
instrument, it has only given the one exposure for the particular assumed values of
the factors.
My improved calculating instrument is preferably made in two forms-
In one it consists of a cylindrical box, the lid of which is made so that it can be
turned. round upon the body of the box by means of a milled edge or other
suitable device.
The periphery of the box and lid are divided geometrically, preferably into
2 scales, namely a scale of the diaphragm Nos. of plates employed (as hereafter
explained) which are ascertained from actual test, and a scale graduated in the
same ratio of the exposures in seconds. The circular box also serves
actinometer for measuring the actinic value of the rays of light by noting the time
which a strip of sensitive paper contained therem takes to darken to a standard
tint.
as an
In the second form, it comprises a flat slide rule, having an actinometer secured
on the back thereof. This latter is formed preferably of a circular box, having a
rotatable lid with an opening of suitable shape, provided with a disc of standard
tint. The box is furnished with discs of sensitized paper, and these discs are
prevented from rotating in the box (when the lid is turned) by being provided
with lips engaging in slots in the box, the lid being rotated to expose fresh
surfaces to the actinic rays of light. As the actinometer for this arrangement has
not to carry any scales, it is designed in this form and is much more compact than
the box arrangement before described in which a slip of sensitive paper is pulled
through a slot in the usual way. The slide rule aforesaid is divided geometrically
into 2 scales similar to the scales before described upon the circular instrument;
the scales of the diaphragm Nos. of the plates being upon the rule and the scale of
exposures in seconds being upon the brass slide (or vice versa) enables the com-
putation to be made mechanically.
The speed of the plate, instead of being expressed by an arbitrary number, may
be expressed by the size of diaphragm through which a plate of that particular
speed would require the actinometer time for its correct exposure upon a normal
subject. The more rapid the speed of the plate, the smaller would be the size of
its corresponding diaphragm and vice versa.
The unit of plate speed or speed No. 1 is calculated as the speed which takes

page_text: (for a normal subject) the same time for exposure of a photographic plate of this
rapidity through a standard diaphragm the diameter of which is equal to the focal
length of the lens divided by four that the sensitive paper in the actinometer takes
to colour to a standard tint.
It therefore follows that the comparative speeds of any plates of different
diaphragm Nos. are represented by the uniform standard Nos. (of the Photographic
Society of Great Britain) which correspond with the F Nos. of the plates
compared.
F
4
The unit might obviously be calculated with reference to any other diaphragm,
but is preferred, as it is usually the largest stop employed in general photo-
graphy and is also the unit of the uniform standard Nos. of the above named
Society for the comparative areas and duration of exposure of all other stops. It
follows from this that the figure which represents the speed of the plate also
represents the ratio of plate exposure to actinometer exposure for this standard
diaphragm called It is obvious that the uniform standard Nos. might be
engraved upon the scales against the corresponding diaphragm Nos. in order to
show the actual relative speeds of the plates, but, as doing so would not in any
way affect the resulting calculation and would make the instrument more
complicated, it is preferred to use only the F or diaphragm Nos.
The diaphragms on the scale of diaphragms are graduated from the largest to
the smallest in exactly the same proportion as the seconds scale, and in such a
manner that, when the instrument is set correctly for one diaphragm, it is at the
same time set correctly for all diaphragms from the largest to the smallest.
To calculate an exposure therefore for a normal subject, it is only necessary to
set the actinometer time on exposure scale against the diaphragm number of the
particular plate used, when against each diaphragm will be found on the same
scale the correct exposure for that particular stop.
For exceptional subjects which require a longer or shorter exposure than the
normal, instead of having a separate scale for calculating the variation from the
normal exposure due to the subject, it is only necessary to move the slide a certain
number of divisions higher or lower than the normal position according to the
amount of variation required, and calculate as at this higher or lower figure.
It will thus be seen that, by the particular construction of the instrument aud
arrangement of scales, this one scale of diaphragms represents in itself three
factors of exposure, namely, the sizes of diaphragms, the speed of the plate and
the variation of exposure due to special subjects, and that the exposure scale also
represents both the actinometer time and also the exposure in seconds.
Instead of the scales being divided upon the periphery of a box and lid and
upon a rule and slide as described, they might obviously be divided upon two
movable concentric rings or discs.
Among the advantages of my apparatus are the following-
a. The special scale or column of diaphragm numbers being progressively
arranged, the one scale represents three factors of exposure, namely, the sizes of
diaphragms, the speed of the plate, and the variations of exposure due to special
subjects.
b. The special scale or column of seconds and fractions of seconds being also
progressively arranged, the one scale represents two factors, namely, the exposure
in seconds and the actinometer time in seconds.
c. The special division and combination of the scales in such a manner that, in
whatever position the slide is set, the corresponding exposure is shown simul-
taneously against each diaphragm from the largest to the smallest.
Dated this 29th day of May 1893.
WM. P. THOMPSON & Co.,
Of 6, Lord Street, Liverpool, Patent Agents for the Applicant.

page_text: COMPLETE SPECIFICATION.
Improvements in Apparatus for Calculating Photographic
Exposures.
I, GEORGE FREDERICK WYNNE of Minera near Wrexham in the County of
Denbigh in the Principality of Wales, 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 :-
This invention relates to an improved apparatus for calculating photographic
exposures of that class in which the light is first tested by an actinometer in the
usual manner and the photographic exposure deduced therefrom.
The duration of exposure required for a photographic plate is affected by four
conditions, namely-
1. The actinic value of the light,
2. The character of the subject,
3. The ratio of the diaphragm of the lens to its focal length, and
4. The sensitiveness of the plate used.
In other forms of calculating instruments hitherto used, these four factors have
been calculated for separately, and a specially divided circular slide rule (with
pointers or with a combination of stops and pointers) has been provided for each
factor, and, when the resulting exposure has been shown by the instrument, it has
only given the one exposure for the particular assumed values of the factors.
My improved calculating instrument is preferably made in two forms set forth
in the accompanying drawings, Figures 1, and 3 respectively.
Figure 1 shows plans of my apparatus arranged in a cylindrical box or watch
case form;
Figure 2, section of same;
Figure 2 plan of disc;
Figures 3 and 4, two slightly varying forms of the slide rule form.
In Figure 1, the lid or glass F upon which the diaphragms and plate speed
numbers scale G is printed is made so that it can be turned round upon the body of
the box J by means of a milled edge j or other suitable device. The periphery of
the box and lid or the part near the edge of the glass and the outer edge of the
exposed portion of the body of the box, as shown in Figure 1, are each divided
geometrically into two scales, as shown, namely, a scale G of diaphragm numbers
of plates employed (as hereafter explained) which are ascertained from actual test,
and a scale H graduated in the same ratio of the exposures in seconds. Either of
these can be on the glass F or lid, the other of them being on the box J. The
circular box also serves as an actinometer for measuring the actinic value of the
rays of light by noting the time which a strip or portion of a disc of sensitive
paper C contained therein takes to darken to a standard tint of disc or part a.
In the second form set forth in Figure 4, it comprises a flat slide rule, having an
actinometer secured on the back thereof. This actinometer is only shown in outline,
as it is fully set forth in Figures 1 and 2; it is formed of a circular box D having
a rotatable upper part or lid J with an opening A (Figures 1 and 2) provided with
a disc or one or more strips of standard tint of any suitable shape and size a and
leaving exposed a part B through which a small segment of the rotatable disc of
sensitised paper C, hereinafter described, can be seen.
The box is furnished with spare discs E of sensitised paper, and these discs can
be prevented from rotating in the box (when the lid is turned) by being provided
with lips I engaging in slots in the box or otherwise, the upper part of the box
being rotated to expose fresh surfaces to the actinic rays of light. Instead,
however, of these lips or slots, I may use any other suitable device for fulfilling the
same object; for instance, by fixing the disc to the box by a suitable cement, or by

page_text: placing it in contact with such a surface attached to the box that the friction of
this surface acting upon the disc is sufficient to turn it, or in any other suitable
way.
The disc of sensitive paper may be kept pressed up to the opening in the lid by
any suitable device, but I preferably employ a disc of thick felt D', which possesses
a considerable amount of elasticity and exerts sufficient friction to turn the disc of
sensitive paper, especially if lips or projections I are formed upon it as before
described.
In the drawing, C is the disc of sensitive paper; D', the felt pad; and E, a series
of spare discs to be used in succession in place of C; L, a pin holding the pad from
turning; K, a plate enclosing between it and box D a space for spare discs;
M, milled edge to box D. The slide rule, Figure 3, is divided geometrically into
2 scales similar to the scales before described upon the circular instrument; the
scales of the diaphragm Nos. of the plates being upon the rule and the scale of
exposures in seconds being upon the brass slide (or vice versa) enables the computation
to be made mechanically.
In the modification of this arrangement shown in Figure 4, a third scale of the
relative or comparative speeds of the plates as hereafter explained is shown, but
the presence or absence of this scale does not affect the working of the meter,
and, although I have sold a large number with it, I intend in future to omit it as a
rule.
The speed of the plate, instead of being expressed by an arbitrary number, may
be, and is in the drawings, expressed by the size of diaphragm through which a
plate of that particular speed would require the actinometer time for its correct
exposure upon a normal subject. The more rapid the speed of the plate, the
smaller would be the size of its corresponding diaphragm, and vice versa.
Where the scale of relative or comparative speeds is used, the unit of plate speed
or speed No. 1 may be calculated as the speed which takes (for a normal subject)
the same time for exposure of a photographic plate of this rapidity through a
standark diaphragm (the diameter of which is equal to the focal length of the lens
divided by four) that the sensitive paper in the actinometer takes to colour to a
atandard tint.
It therefore follows that the comparative speeds of any plates of different
diaphragm Nos. are represented by the uniform standard Nos. (of the Photographic
Society of Great Britain) which correspond with the F Nos. of the plates
compared.
-
-
-
The unit might obviously be calculated with reference to any other diaphragm,
and, if the unit is taken with reference to F 45, then, when the scales of the
instrument are placed in their normal position as shown, the comparative or relative
speed of any plate will be simultaneously found against its diaphragm No. 3
thus F 451 F 64 2 F 90 = 4 F 28=-F8=3 and so on.
If F 4 is taken as the unit for the comparative speeds of the plates, the figure
which represents the speed of the plate will also represent the ratio of plate
F
exposure to actinometer exposure for this standard diaphragm called 4'
obvious that the uniform standard Nos. or any other comparative Nos. might be
engraved upon the scales against the corresponding diaphragm Nos. (as shown in
one plan of the slide rule form of instrument) in order to show the actual relative
speeds of the plates, but, as doing so would not in any way affect the resulting
calculation and would make the instrument more complicated, it is preferred to use
only the For diaphragm Nos.
It is
The diaphragms on the scale of diaphragms are graduated from the largest to
the smallest in exactly the same proportion as the time scale, and in such a manner
that, when the instrument is set correctly for one diaphragm, it is at the same time
set correctly for all diaphragms from the largest to the smallest.
To calculate an exposure, therefore, for a normal subject, it is only necessary to
set the actinometer time on exposure scale against the diaphragm number of the

page_text: particular plate used, when against each diaphragm will be found on the same
scale the correct exposure for that particular stop.
For exceptional subjects which require a longer or shorter exposure than the
normal, instead of having a separate scale as hitherto employed for calculating the
variation from the normal exposure due to the subject, it is only necessary to move
the slide a certain number of divisions higher or lower than the normal position
according to the amount of variation required, and calculate as at this higher or
lower figure.
It will thus be seen that, by the particular construction of the instrument and
arrangement of scales, this one scale of diaphragms represents in itself three
factors of exposure, namely, the sizes of diaphragms, the speed of the plate and
the variation of exposure due to special subjects, and that the exposure scale also
represents both the actinometer time and also the exposure in seconds.
It will be seen that the scales can be divided either upon the periphery of a box
and lid, upon two movable concentric rings or discs, or upon a rule and slide.
Describing further Figure 2, F is the glass to which is securely cemented the
diaphragm number scale as shown; J, upper part of body; D, lower part of body
having milled edge for turning the same or for holding it while the top is turned.
It will thus be seen that, by moving the glass relatively to the upper part of the
body, the scales are adjusted to each other, while, by moving the lower part round
slightly, a fresh piece of the sensitised paper disc is exposed.
It is obvious that, instead of having one of the scales fixed to the glass, it might
be fixed to an extra and specially made part of the case, but the plan shown
being the simplest is the one I prefer to employ.
The mode of working is as follows:-
Instead of expressing the sensitiveness of a plate by an arbitrary number, it is
expressed by the size of diaphragm through which the plate would require the
actinometer time for its correct exposure upon a normal subject. Thus Ilford
ordinary plates, No. F 39, means that an ordinary subject upon these plates,
through stop F 39, would require the actinometer time for its correct exposure.
TO CALCULATE THE CORRECT EXPOSURE.
Turn the movable scale of the instrument until the actinometer time in seconds
upon the exposure scale is opposite the diaphragm number of the plate. Then the
correct exposure in seconds and fractions of seconds will be found simultaneously
against each stop from the largest to the smallest.
Stop
Put 3 seconds against F 90. Then against F 5-6 will be found 5 of a second,
and also against every other stop, the correct relative exposure. If, in an interior
exposure or an exposure in a shaded situation, the sensitive paper takes minutes
instead of seconds to colour to the standard tint, the figures upon the exposure scale-
may be read as minutes instead of seconds.

page_text: PHOTOGRAPHING EXCEPTIONAL SUBJECTS.
For panoramic views or open landscape with no dark objects in the foreground,
instead of setting the actinometer time against the actual plate speed number, set
it two speeds higher than this, and calculate as if the plate speed were this
higher figure. Thus, with plate speed F 45, set actinometer time to F 64; or 5
with plate speed F 78, set it to F 111.
For sea and sky, set the actinometer time six speeds higher than the actual plate
speed used, and calculate as if plate speed were this higher figure. Thus, with
plate speed F 45, set it to F 128, or with plate speed F 90, set it to F 256. If
preferred, the exposure for sea and sky may be found by calculating as for an 1
ordinary exposure, and dividing by from 8 to 10.
Firstly, the state o
ragins:
Secondly, the speed of plate: and
Thirdly, the variation of exposure due to subject; while the time scale
represents the actinometer time and also the exposure in seconds or
minutes.
The scales are also so divided and arranged that, when they are set correctly for
one diaphragm, they are simultaneously set correctly for every diaphragm from the
largest to the smallest.
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. The arrangement of an exposure meter in which the exposure required is
calculated with only two scales, one being the scale of diaphragm numbers and the
other the scale of time, substantially as described.
2. In an exposure meter, a scale of diaphragm numbers representing not merely
the scale of diaphragms but the speed of plate, also a scale of time to represent
not merely the actinometer time but the tine of exposure required for each
diaphragm.
3. In an exposure meter, the arrangement of actinometer with revolvable
sensitive discs, substantially as described.
4. The forms of exposure meter substantially as herein described, namely, the
slide rule form with scales as described, one of them sliding, and the box form with
two contiguous annular discs carrying scales, one on the lid or glass and the other
on the body of the box, and an actinometer combined therewith.
5. An exposure meter in which the exposures required for any set of conditions
are calculated without the intervention or use of stops, pointers, or similar devices
as hitherto used, substantially as described.
Dated this 5th day of March 1894.
WM. P. THOMPSON & Co.,
Of Liverpool, Manchester, Nottingham, and London, Patent Agents.