8 Wen HAM on Binocular Vision. 
day or candle-light. I have not yet tried this arrangement of 
prisms, but intend to do so, as I have a favourable opinion of 
the method, although the one next to be described is pro- 
bably better. 
If we consider the relative position of the two reflecting 
surfaces of the prisms h b, figs. 5 and 6, they will form the 
same angle represented by the 
top line of fig. 7. It is there- 
fore evident, that if a rectan- 
gular plate of speculum metal is 
ground and polished, so as to 
form two reflecting facets in- 
clined to each other at the re- 
quired angle, as represented by fig. 7 ; and this being placed 
at an angle of 45°, with the division of the facets intersecting 
the axis of the object-glass, we shall divide the rays, and 
reflect them horizontally, just in the same way as represented 
in figs. 5 and 6, merely by means of one single reflection. 
Any other direction than a right angle, with respect to the 
axis of the object-glass, may of course be given to the rays, 
by inclining the reflector more or less. From the simplicity 
of this contrivance, and the facility with which it may be 
constructed, I shall take an early opportunity of giving it a 
trial. The only question I have is, whether a material may 
not be found that will reflect more light than even speculum 
metal : I have heard an alloy of cast- steel and platinum well 
spoken of, but have never seen any of it. 
In considering the aberrations which the thickness of glass 
contained in the reflecting prisms must inevitably produce 
when placed immediately behind the object-glass, it occurred 
to me, that if the same prisms were placed close to the top lens 
of the eye-piece, these errors, not being magnified, would be 
less sensibly felt. 
I have before mentioned, that the final image of an object, 
when it leaves the eye-piece, is compounded of several different 
images or perspectives of the object, all blended together, 
and which are as equally capable of separation there as be- 
hind the object-glass itself, as exemplified by figs. 2 and 3, 
which bear exactly the same appearance when under view, 
with the alternate sides of either the object-glass or eye-piece 
stopped off. 
Fig. 8 represents the methods that I have contrived for ob- 
taining the effect of bringing the two eyes sufficiently close to 
each other to enable them both to see through the same eye- 
piece together, a a a are rays converging from the field lens 
of the eye-piece. After passing the eye lens b, if not inter- 
cepted, they would come to a focus at c, but they are arrested 
