2 
Wenham on Binocular Vision, 
by the relative intensity of the light, for that portion of the 
object farthest from the lens will receive the greatest share. 
This objection may probably be removed by a different sys- 
tem of illumination, but of this I shall treat hereafter. If the 
single lens is provided with a proper stop, what is known as 
the angle of aperture will be so exceedingly small that a series 
of uniform opaque particles lying behind each other in an 
object will be only seen by the direct light that they intercept, 
and the underlying ones will be invisible. These are the 
reasons why most microscopic objects, which we know must 
have a visible thickness, appear so perfectly thin, that we 
might almost imagine that they were painted on the glass 
slide upon which they are mounted. This illusion may be 
attributed to the natural effects of monocular vision, and in 
this case the only remedy is to view the object from different 
points at the same time with each eye, under equal magnify- 
ing power. I shall now enter into various methods of effect- 
ing this. One of the most simple and obvious is to employ 
two lenses, one to each eye, only differing from an ordinary 
pair of spectacles in the foci being shorter and the optic axes 
converging till the points where the foci intersect become 
coincident. 
Binocular vision may also be obtained through a single 
lens, if the diameter is sufficiently large to allow both eyes to 
see through it at the same time as in the common reading- 
glass. In these instances we cannot well use glasses of shorter 
focus than four or five inches ; and in cases where higher 
magnifying powers are required it becomes necessary to adopt 
some method which shall produce the effect of bringing the 
two eyes proportionately closer together, to suit the diminished 
diameter and shorter focus of the lens. This may be accom- 
plished by means of a system of four plane reflectors, inclined 
at an angle of 45 degrees, and fixed behind the lens in a 
line at right angles to its axis, or else by four rectangular 
prisms in the same position; both these can be made 
to adjust to suit the 
diameters of various 
lenses and difference of 
distance between the 
eyes. 
The arrangement that 
I have tried for lenses 
of short focus is repre- 
sented by fig. 1 : a a is 
a plano-convex lens, be- 
hind which is placed the usual stop bb. c c are two rhomboidal 
