Improvements in or relating to Prisms for Optical purposes.
Year: 1899

page_text: Date of Application, 6th July, 1899-Accepted, 12th Aug.,
1899
COMPLETE SPECIFICATION.
Improvements in or relating to Prisms for Optical purposes.
I, WILHELM VOLKMANN, Georgenkirschstrasse 70, Berlin, in the Empire of
Germany, Physician, 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:―
The present invention relates to a direct vision prism for optical purposes
which combines with smaller dimensions, higher powers and greater simplicity
than those used up to the present day.
The prism, as represented in the accompanying drawings, consists of a sym-
metrical pentagonal piece of glass. (The inclination of the sides of the prism
to each other depends on the co-efficiency of refraction of the glass used and
the desired degree of dispersion.)
The two symmetrical, non-adjacent surfaces a are silvered, while the two
symmetrical adjacent surfaces b remain clear, while the fifth side c (the bottom
of the prism) does not affect the use of the prism. A ray of light coming from
the left side in the direction of the arrow d, and striking against the left surface b
is refracted to the right reflecting side a, reflected thence on to the left reflecting
surface a from which it is reflected to the right free side b, where it is again
refracted.
If we examine the passing of a yellow ray e and of a blue ray f which simul-
taneously fall on the prism, after the first refraction the blue ray will be found
at the right side of the yellow, (considering the view as a plan view) then on
the left side after the first reflection, then again on the right side after the
second reflection. Owing to the double reflection therefore the second refraction
is produced with the rays in the same direction and position as at first.
As contrasted with the direct vision prisms ordinarily in use (amici) the
direct vision prism according to this invention has the advantage of greater
simplicity and wider dispersion. Inasmuch as it contains only two surfaces of
refraction it can be set at the minimum position for every bundle of coloured
rays by a very slight turning motion while such a thing is impossible with amici
prisms for all cemented surfaces. The present prism has the advantage over
Kessler prism as described in "Poggendorfs Annalen" Volume 151 as with
equal power it is a quarter of the size. This is due to the peculiar crossed
passage of the rays in the interior of the prism, rendered possible by the employ-
ment of metallic reflectors instead of the total reflection preferred by Kessler.
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. A direct vision prism made of a single piece of glass in which a reduction
of size as compared with other prisms at present in use is obtained whilst main-
taining the power two of the sides of the prism being plated and act as reflectors,

page_text: and in which the rays of light cross in the interior of the prism, substantially
as described.
2. A prism substantially as described and illustrated in the accompanying
drawings.
1
1
1000