Improvements in Instruments for Calculating Photographic Exposures.
Year: 1893

page_text: Date of Application, 25th July, 1893-Accepted, 16th Sept., 1893
COMPLETE SPECIFICATION.
Improvements in Instruments for Calculating Photographic
Exposures.
I, ALFRED WATKINS, of the Imperial Flour Mills, Hereford, Master 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 partly to improvements in the instrument for calculating
photographic exposures described in my Patent No. 1388 of 1890, and has for its
object to simplify the construction and manipulation of the said instrument. For
this purpose
I omit the scale indicated by the letter i and the corresponding
pointer S and I arrange the other scales in such manner that by setting the
pointer P to a number on the scale representing the sensitiveness of the plate, and
the pointer D to a number on a scale representing the relation of the diaphragm
used to the focal length of the lens, (which scale is combined with the pointer P)
then the actinometer scale which is combined with the pointer D will be so set
against the exposure scale that the correct duration of exposure can be read off
against the division on the actinometer scale representing the value of the light
for the time being. I also dispense with the cap attached by a chain that served
as a seconds pendulum, so that the cylindrical case only serves to contain the roll
of sensitive paper.
Furthermore, I provide in front of the bromide of silver actinometer a piece of
coloured glass (blue by preference) that, by cutting off any red colour in the
darkening sensitive paper, renders the operation of matching the tint much easier
than formerly, and removes a difficulty often felt in judging the tint.
Lastly I employ in front of the sensitive paper a mask of opaque paper or
other suitable substance having an aperture through which the sensitive paper is
exposed to the light, and provided next to this aperture with a piece of paper
having the exact size and configuration of the said opening, and coloured with the
standard tint with which the tint of the sensitive paper is to be compared. I prefer
to produce the standard tint by dyeing instead of by painting as heretofore.
The above described arrangement of scales may also be applied to straight sliding
scales or slide rules for the same purpose. These may also with advantage be
provided with the projecting pointers and stops.
Figure 1 of the accompanying drawings shews a perspective view and Figure 2
a longitudinal section of the before described improved construction of the
cylindrical form of the instrument.
The metal tube a has at one end the actinometer d which is removeable for the
introduction of the sensitive paper c, the other end of the tube being closed. On
the cylindrical surface is firstly a fixed scale h representing the degree of sensitive-
ness of the plate employed, secondly a rotatable ring carrying the projecting
pointer P and having a scale representing the relation of the diaphragm used to
the focal length of the lens; thirdly, another rotatable ring g carrying the
projecting pointer D and having the actinometer scale; and lastly a fixed scale
representing the exposure in seconds.
Thus the ring j being turned till the pointer P is upon the required division
of the scale h, and ring g being then turned until pointer D is upon the required
division of the diaphragmn scale j, then the required division on the actinometer
scale of ring 9 will be opposite the proper division of the exposure scale i.
In the actinometer d is provided a mask m of black paper or other opaque
material having an opening n through which the sensitive paper c is exposed to the
light, a piece o of paper or other fabric coloured with the standard tint being fixed

page_text: on the mask next the opening; this standard tint is made of exactly the same size
and configuration as the opening n, as shewn in the detached views of the mask at
Figs. 3, 4 and 5 which shew various forms of the opening and standard tint. The
mask m is covered externally by a disc of glass p of a blue or other colour that
will cut off the red colour as before described.
At the back, the mask and strip of sensitive paper care held in position in the
cap by a disc q of cardboard or other material.
Fig. 6 shews plan of a straight slide rule, constructed with the above described
arrangement of fixed scales h and I and the sliding scales ; and g and provided with
the projecting pointers and stops P and D overlapping each other, so constructed
as to render the moveable scales mutually dependent on each other, as described
in my Specification 1388 of 1890. Fig. 7 shews a longitudinal section of the above
described construction of actinometer combined with a simple box for containing
the sensitive paper, which would be used with the above form of the slide rule.
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. In sliding scales or slide rules for calculating photographic exposures, the
use of a scale for the actinometer value of the light, moving adjacent to a scale
representing the duration of exposure, substantially as described.
2. In an actinometer for photographic purposes in which a sensitive paper or
film is used, the employment of a coloured glass in front of the sensitive paper for
cutting off any red colour in the darkened sensitive paper, substantially as and for
the purpose described.
3. In an actinometer for photographic purposes, in which sensitive paper or film
is used, the use of a mask formed of a loose disc of opaque paper or other substance
having an aperture through which the sensitive paper is exposed to light, and a
coloured standard tint displayed contiguous to such aperture, the tint and aperture
being identical in shape and size, substantially as described.
4. In an instrument for calculating photographic exposures, the combination
of scales hig and I, with pointers P and D, arranged and operating as herein
described.
Dated this 25th day of July 1893.
ABEL & IMKAY,
Agents for Applicant.