Year: 1888

page_text: I, Sir DAVID LIONEL SALOMONS Baronet, of Broomhill, Tunbridge Wells, in
the County of Kent, do hereby declare the nature of this invention to be as
follows:-
In determining the standard or the comparative rapidity of a lens it is first
necessary to find the ratio of the aperture to the focal length. This having been
done the operator can calculate or refer to a table which gives the desired result.
Different makers howover employ different standards which necessitates the use of
several tables or the multiplication by some constant or co-efficient.
My slide rule not only performs the first calculation but also shows these relations
expressed as standards without the necessity of referring to a separate table or tables.
The rule contains three or more scales one being a sliding scale. All the scales are
logarithmic. One scale is marked at focal lengths, the sliding scale next to it is
marked as a series of apertures. The third scale shows the ratios. The figure
corresponding to any given aperture is brought opposite that one representing any
given focal length and the index then shows the ratio between the two, and at the same
time the standard of any maker or makers. The scale also has devices for measuring
diameter of stops and the time of exposure may then be found by any known
method.
In order to make the instrument more compact one of these scales may be upon the
bottom of the recess receiving the slide.
L
L 1000
CARPMAEL & Co,
Agents.

page_text: 1, Sir DAVID LIONEL SALOMONS Baronet, of Broomhill, Tunbridge Wells, in the
County of Kent, 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 determining the comparative rapidity of a lens with a standard lens it is first
necessary to find the ratio between the aperture and the focal length. This having
been done the operator can calculate or refer to a table which gives the desired result
expressed according to some artificial notation. Different makers however employ
different standards which necessitates the use of several tables or the multiplication by
some constant or co-efficient. Two standards only are used in practice viz. called
the "Uniform" and "Decimal" standards.
:
My slide rule not only performs the first calculation but also shows these relations
expressed in the two usual standards (or others if required) without. the necessity of
referring to a separate table or tables. The rule contains three or more scales one
being a sliding scale. All the scales are logarithmic. One scale is marked as focal
lengths, the sliding scalo next to it is marked as a series of apertures. The third
scale shows the ratios and against them are the standards. The figure corresponding
to any given aperture is brought opposite that one representing any given focal length
and the index then shows the ratio between the two, and at the same time the standard
of any maker or makers. The scale also has devices for measuring diameter of stops
and the time of exposure may then be found by any known method.
may be
In order to make the instrument more compact one of these scales
upon the bottom of the recess receiving the slide, or at the back of the slide, or
on both.
DESCRIPTION OF THE DRAWINGS..
Figure 1 is a plan of a slide rule constructed according to my invention. Figure 2
is the same view with the slide A figure 1 removed to show the graduations at the
bottom of the groove in which it slides. In this figure the graduations on the face of
the rule are omitted for the sake of clearness. Figure 3 is an edge view.
The logarithmic scale gives a range of 4" to 40" focal length. The sliding bar A
gives the diameter of the stops or apertures from " to 5". Below A is the ratio
and below this again is placed the Dallmeyer standard or any makers
scale R
=
-A
f"
standard or standards.
Beneath the slide A, on bottom of channel or groove is engraved the standard scale
of the Photograph Society known as "Uniform" standard, the end edge of slide A
serving as index.

page_text: For convenience of measuring the diameter of the stops or apertures the edges
of the slide-rule are provided with an inch and centimetre or other scales as occasion
inay require. One end on each side of slide rule has a knife edge of metal to insure
accurate measuring of circular openings.
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 slide rule for photographic purposes having three logarithmic scales (one of
them a sliding scale) graduated respectively for focal lengths, apertures and the ratio
between the two and with or without other scales substantially as described.
2. A slide rale for photographic purposes having one of the scales upon the bottom
of the recess receiving the slide substantially as described.
3. A slide rule for photographic purposes substantially as described and shown in
the drawings.
4. A slide rule substantially as described for comparing one standard with another,
and with ratios.
5. A slide rule substantially as described with which all the usual calculations
required by a photographer in reference to comparing lenses, lens settings, equivalent
values for setting lenses and their stops, can be accomplished without trouble, either
at a glance or by setting the slide suitably and observing the result.
Dated this Seventh day of November 1888.