Improvements in Instruments for Calculating Photographic Exposures.
Year: 1898

page_text: Date of Application, 13th Dec., 1898.
Complete Specification Left, 12th Sept., 1899-Accepted, 4th Nov.. 1899
PROVISIONAL SPECIFICATION.
Improvements in Instruments for Calculating Photographic
Exposures.
I, ALFRED WATKINS, of the Imperial Mills, in the City of Hereford, Miller,
do hereby declare the nature of this invention to be as follows:-
My invention relates to the watch form of exposure meter or actinometer and
is similar in shape and size to that described in my Patent 25000 of 1894.
In the place however of a strip of sensitive paper, I adopt in my present inven-
tion flat circular discs of sensitive paper with a hole in the centre, circular by
preference.
The instrument is preferably made in two forms. In one form the body which
is a shallow box, contains the sensitive discs, the top disc being separated from
those beneath by an opaque pad or plate faced by preference with a sheet of
india rubber. A lid is fitted to the body like the lid of a watch, it is capable of
revolving and contains a glass face also capable of revolving. The lid also-con-
tains an opaque mask which revolves with it, and in which are arranged excen-
trically one or more (two by preference) apertures each having alongside a
painted or dyed tint as described in my Patent 14,364 of 1893. If two separate
tints and apertures are used they take the place of the tints on the two sides of
the reversible mask described in my Patent 25000 of 1894, but in this case both tints
are on the same side of the mask and in different parts of it, by preference on
opposite sides of the centre. I prefer to arrange the apertures at different distances
from the centre of the mask so that cach commands a different path on the
sensitive paper when the lid is revolved.
Where
In order that only one aperture and tint should be in sight at one time, an
opaque mask is cemented to the front glass, bearing an aperture somewhat larger
than the tint and aperture to be exposed. This front mask may also bear the
design which forms the front face of the instrument, and revolves in order to
bring in sight whichever tint is selected, covering up the other or others.
only one tint is provided in the instrument the front mask is not required.
I use by preference, in place of the coloured glass specified in my Patent 14364
of 1893 a coloured film of gelatine, celluloid or other transparent substance or
varnish interposed between the tint mask and the front glass in order to absorb
red colour in the darkened sensitive paper. I prefer a blue colour for this
purpose.
The back of the instrument is occupied by a revolvable lid which bears a
circular moveable scale similar by preference to that described in my Patent 25000
of 1894. The body of the instrument bears a milled rim which projects a little
between the two lids. The two lids are joined together by a bridge, on which
by preference is placed the handle of the instrument, so arranged that although
the front lid can be lifted off to change the paper, both lids revolve together on
the body of the instrument when it is desired to expose a fresh surface of sensi-
tive paper.

page_text: In order that the front lid may be moved just a sufficient distance to expose
a fresh surface of sensitive paper and no more, a series of notches or indents are
made at regular intervals round the circumphrence of the body, and a click or
small ball presses into these by means of a spring. This click arrangement is
not essential to the working of the instrument and may be omitted.
The second form of the instrument is an actinometer only and is not provided
with a moveable calculating scale.
The body of the box, front lid, arrangement of tint mask, front mask, sensi-
tive paper, and coloured film or glass are the same as in the instrument just
described but there is no revolvable back lid. The handle is by preference
fastened to the front lid, which revolves to expose fresh surfaces of sensitive
paper. At the back of the box is engraved a scale or diaphragms, in which by
preference is called 1, 2, 4, 8, and so on. This information enables
the user of the actinometer to easily calculate the exposure to give the plate by
means of a simple plan which I have devised.
F
F F
11 16
F
22
With every speed of plate there is a certain size of diaphragm which if used
makes the correct exposure equal to the time the actinometer takes to darken
to the standard tint. This diaphragm is usually inconveniently small, but with
every other larger diaphragm the camera exposure is a definite fraction of the
actinometer time. This fraction I express by dividing the diaphragm number
by the plate speed.
F
22
Thus a photographer using Ilford ordinary plates which are speed 35, with
diaphragm, which is No. 8, would always have to give ths or approximately of
the actinometer time whether the light is bright or dull, and the subject out of
doors or indoors, if the actinometer is exposed to the light falling on the subject.
This plan is easy to remember and requires only a simple mental calculation.
I propose to stamp on the back of this second form of my invention a formula
which embodies this plan × A = E. D being the diaphragm, P the plate speed,
D
P
A the actinometer number, and E the exposure.
Dated this 12th day of December 1898.
Improvements in Instruments for Calculating Photographic
Exposures.
I, ALFRED WATKINS, of the Imperial Mills, in the City of Hereford, Miller,
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:―
My invention relates to the watch form of exposure meter or actinometer and
is similar in shape and size to that described in the Specification to my Patent
No. 25000 of 1894. In the place however of a strip of sensitive paper, I adopt
according to my present invention flat circular discs of sensitive paper with a
hole in the centre, by preference of circular form. The instrument is preferably
made in two modifications. In one form, the body which is a shallow box con-
tains the sensitive discs, somewhat similar to the construction described in the

page_text: Specification to Patent No. 144578, the top disc being separated from those beneath
by an opaque pad or plate faced by preference with a sheet of india rubber with
a central projecting stud. A lid is fitted to the body like the lid of a watch, it
is capable of revolving and contains a glass face which is also capable of revolving
in the lid. The lid also contains an opaque mask which revolves with it, and in
which are arranged excentrically one or more (two by preference) apertures each
having alongside a painted or dyed tint as described in the Specification to my
Patent No. 14364 of 1893. If two separate tints and apertures are used they take
the place of the tints on the two sides of the reversible mask described in the
Specification to my Patent No. 25000 of 1894 but in the present case both tints
are on the same side of the mask and in different parts of it, by preference on
opposite sides of the centre. I prefer to arrange the apertures at different
distances from the centre of the mask so that each commands a different path on
the sensitive paper when the lid is revolved. In order that only one aperture
and tint should be in sight at one time, an opaque mask is cemented to the front
glass bearing an aperture somewhat larger than the tint and aperture to be
exposed. This front mask may also bear the design which forms the front face
of the instrument, and revolves in order to bring in sight whichever tint is
selected, covering up the other or others. Where only one tint is provided in
the instrument the front mask is not required.
The back of the instrument is occupied by a revolvable lid which bears a
circular moveable scale similar by preference to that described in the Specification
to my Patent No. 25000 of 1894. The body of the instrument bears a milled
rim which projects a little between the two lids. The two lids are joined together
by a bridge on which by preference is placed the handle of the instrument, so
arranged that although the front lid can be lifted off to change the paper, both
lids revolve together on the body of the instrument when it is desired to expose
a fresh surface of sensitive paper.
In order that the front lid may be moved just a sufficient distance to expose
a fresh surface of sensitive paper and no more, a series of notches or indents are
made at regular intervals round the circumference of the body, and a click or
small ball presses into these by means of a spring. This click arrangement is
not essential to the working of the instrument and may be omitted.
The second form of the instrument is an actinometer only and is not provided
with a moveable calculating scale.
The body of the box, front lid, arrangement of tint mask, front mask, sensi-
tive paper, and coloured film or glass are the same as in the instrument just
described and there is no revolvable back lid. The handle is by preference
fastened to the front lid, which revolves to expose fresh surfaces of sensitive
paper. At the back of the box is engraved a scale of diaphragms, in which by
preference is called 1,. 2, 4, 8, and so on. This information enables
TI 16
the user of the actinometer to easily calculate the exposure to give the plate by
means of a simple plan which I have devised.
F
8
F
F
F
22
With every speed of plate there is a certain size of diaphragm which if used
makes the correct exposure equal to the time the actinometer takes to darken
to the standard tint. This diaphragm is usually inconveniently small but with
every other larger diaphragm the camera exposure is a definite fraction of the
actinometer time. This fraction I express by dividing the diaphragm number
by the plate speed. Thus a photographer using Ilford ordinary plates which
are speed 35, with diaphragm, which is 8, would always have to give ths or
F
22
approximately of the actinometer time whether the light is bright or dull;
and the subject out of doors or indoors if the actinometer is exposed to the light
falling on the subject. This plan is easy to remember and requires only a simple
mental calculation.
I propose to stamp on the back of this second form of my invention a formula

page_text: which embodies this plan
× A = E. D being the diaphragm, P the plate speed
A the actinometer number, and E the exposure.
On the accompanying drawing are shewn actinometers constructed according.
to my above described invention.
Figs. 1 to 8 shew the first described construction with revolvable back con-
taining the scales for determining the time of exposure.
Fig. 1 shews a front view of the instrument with the front glass and mask in
the position for shewing the one tint; Fig. 2 shews the same view with the
glass and its mask turned half round so as to show the second tint. Fig. 3 shews
a side view, Fig. 4 shews an enlarged vertical cross section, Fig. 5 shews the
inside view of the front lid with its fixed mask, Fig. 6 shews a cross section of
the front lid detached from the back, Fig. 7 shews a front view of the instrument
with the lid removed exposing the sensitised disc and Fig. 8 shews the revolvable
glass face with its notched opaque mask.
The body of the instrument consists of the back a with revolvable scales
similar to the arrangement described in the Specification to my Patent No. 25000
of 1894 and therefore not requiring further description. To this back is attached
the front lid g by a bridge piece k and between them is revolvably attached the
middle part b, having a projecting rim with notches or indents 61, into which
takes a spring catch e in the bridge piece k so as to hold it in the different posi-
tions into which it is turned; into this middle part fits more or less tightly a
circular pad d preferably faced with a caoutchouc disc e having a central project-
ing stud e', and on to this is laid the sensitised disc f through the centre hole f
of which the stud el passes, as shewn, so that when the middle part 6 of the casing
is revolved, the pad and sensitised disc revolve with it. On to the part bis
fitted the lid g which however is held stationary by the bridge piece k on which
is formed the handle k¹. In the lid is contained firstly a revolvable glass' h
(preferably of a blue colour) on the inner face of which is cemented the opaque
mask having a segmental notch at i¹. Secondly the lid contains a tightly fitting
disc on opposite sides of the centre of which, are formed openings, prefer-
ably at different distances from the centre as shewn; at the sides of these open-
ings are provided tints 34 of different depths, as described in the Specification
to Patent No. 14364.
Between the back a of the casing and the pad d may be provided a store of
sensitised discs such as f, for future use, the central holes f₁ of which facilitate
their removal.
In using the instrument, assuming that an exposure is required corresponding
to the tint, the glass h with its mask i is turned round so that the notch.
uncovers the opening ; and tint, while it covers up opening and tint *.
The middle part 6 of the casing is then turned one division as determined by the
catch c so as to bring a fresh part of the sensitised disc f in front of opening j',
and the usual observation relatively to the tint 3 is then made.
On the other hand if an exposure corresponding to tint is to be made, the
glass his turned half round so as to expose and cover up . By making
the openings and at different distances from the centre it will be seen on
reference to Fig. 7 that the complete circumference of the disc f will be avail-
able for each of the tints. If necessary the disc j may be made with more than
two openings and tints at varying distances from the centre. The bridge piece k
has a projecting tongue 2, which fits into a notch at on the cover g so as to
hold this securely in position relatively to the back part a, while allowing of its
removal.
Figs. 9, 10, 11 and 12 shew respectively a front view, a side view a part enlarged
vertical section and a back view of the second described construction. As before
stated this only differs from the first construction in that the middle part is
dispensed with so that in this case the back part m contains the pad d with caout-
chouc disc e on which is placed the sensitised dise f, the back m being rotated
relatively to the front part g for bringing fresh parts of the paper before the
openings of the device ; in the lid, this being constructed the same as before.

page_text: A similar click to that described with reference to Figs. 1 to 9 can also be
applied to the last described construction in which case the handle containing
the click would be fitted on a bridge fixed to g and projecting over the back m
which would be provided with notches for the click to take into.
In place of the revolving scales of the first described construction I mark on
D
66
the back m the values of F and D and the formula
× A = E" before
T
referred to.
Having now particularly described and ascertained the nature of this inven-
tion and in what manner the same is to be performed, I declare that what I
claim is:
1. In a watch shaped actinometer with sensitised disc, the use of two or more
different tints with openings in combination with a revolvable mask so arranged
that all but the one tint and opening to be displayed are covered from sight
thereby, substantially as described.
2. In a watch shaped actinometer with sensitised disc, the combination of a
front and back lid connected together by a bridge piece and a middle part con-
taining the sensitised disc revolvable between the front and back lids, substan-
tially as described.
3. In combination with a watch-shaped actinometer with sensitised disc the
use of a spring click arranged in the bridge piece for determining the distances
through which the sensitised paper is turned by the revolving body, substantially
as described.
4. In combination with a watch shaped actinometer the use of sensitised discs
with central holes, for the purpose herein described.
Dated this 12th day of September 1899.
ABEL & IMRAY,
Agents for the Applicant.