4 
Wenham on Binocular Vision, 
the right or left hand portion of the front lens, it will be seen 
that, during each alternate change, certain parts of the object 
will alter in their rela- 
tive position. To il- 
lustrate this, figs. 2 
and 3 are enlarged 
drawings of a portion 
of the egg of the com- 
mon bed-bug {Cimex 
lecticularis), the oper- 
culum which covers the 
orifice having been 
forced off at the time 
the young was hatched. The figures exactly represent the 
two positions that the inclined orifice will occupy when the 
right and left hand portions of the object-glass are stopped 
off. It was illuminated as an opaque object, and drawn 
under a two- thirds object-glass of about 28° of aperture. 
If this experiment is repeated, by holding the card over 
the eye-piece, and stopping off alternately the right and left 
half of the ultimate emergent pencil, exactly the same changes 
and appearances will be observed in the object under view. 
The two different images thus produced are just such as are 
required for obtaining stereoscopic vision. It is therefore 
evident, that if, instead of bringing them confusedly together 
into one eye, we can separate them, so as to bring figs. 2 and 
3 into the left and right eye, in the combined effect of the two 
projections we shall obtain all that is necessary to enable us 
to form a correct judgment of the solidity and distances of the 
various parts of the object. 
I shall now explain some plans for effecting this. The 
most obvious method is to have two microscopes placed side 
by side, and converging towards the object, each tube to be 
furnished with a similar objective and eye- piece. For very 
low powers this would, no doubt, be the most perfect form of 
binocular microscope, but it is liable to objection, firstly, on 
account of its expense, and, secondly, from the difficulty, if 
not impossibility, of using the higher powers. I do not think 
that it would be practicable to use anything beyond the half- 
inch object-glass ; but I believe where vision is assisted by 
the use of both eyes together, it would be of advantage to 
employ objectives of smaller angular aperture, in this case, the 
focus would then fall at a greater distance from the front lens. 
T should also mention that a microscope of this description 
would require the two tubes to be placed at a different angle 
of convergence for every pair of object-glasses employed, of 
either longer or shorter focus. 
