Wenham on Binocular Vision. 
9 
by the inclined surfaces d d, of two solid glass prisms. From 
the refraction of the under incident surface of the prisms the 
focus of the eye- 
piece becomes 
elongated, and 
falls within the 
substance of the 
glass at e. The 
rays then diverge, 
and, after being 
reflected by the 
second inclined 
surface y, emerge from the upper side of the prism, when 
their course is rendered still more divergent, as shown by the 
figure. The reflecting angle that I have given to the prisms 
was 47^°. I also find it is requisite to grind away the contact 
edges of the prisms as represented, as it prevents the extreme 
margins of the reflecting surfaces from coming into operation, 
which can seldom be made very perfect. 
The definition with these prisms is good, but they are 
liable to objection, on account of the extremely small portion 
of the field of view that they take in, and which arises from 
the distance that the eyes are of necessity placed beyond tlie 
focus of the eye-piece, where the rays being divergent, the 
pupil of the eye is incapable of taking them all in ; also there 
is great nicety required in the length of the prisms, which 
must differ for nearly every different observer. 
I have constructed an adjusting binocular eye-piece, not 
differing in principle from the last. The first reflection is 
performed by means of a triangular steel prism, with the two 
inclined facets very highly polished ; this is represented by 
the dotted outline fig. 8. The rays, after having been 
reflected at right angles, are taken up by two rectangular 
glass prisms, shown by the dotted lines ^tff. 
The loss of light in this is much greater than in the former 
instance, and the field of view more contracted ; for the rays 
from the eye-piece, after being reflected from the surface of 
the steel prism, fall to their natural focal distance, instead of 
being elongated, as in the solid prism, consequently the eye is 
still further removed from the focus. I had cliosen hard steel 
for the reflector, on account of the property this material pos- 
sesses, of allowing the figure of a small flat surface to be 
retained, or even perfected, during the operation of polishing. 
I have also tried a combination of prisms over the field-glass, 
using two eye lenses, but with no good result. 
The best effect that I have yet produced in the way of 
