Wenham on Binocular Vision. 
3 
prisms of glass, with the reflecting ends inclined at an angle 
of 45 degrees. All the four surfaces of both prisms should 
be well polished, and their combined length when placed 
together should be such that the distance between the centres 
of the external diagonal reflecting planes should be the same 
as that between the ejes. I prefer the two solid prisms to a 
combination of four rectangular ones, as there is less loss of 
light, and error arising from external reflection. This com- 
bination makes a remarkably fine hand magnifier, giving such 
a depth and substance to objects as cannot be obtained with a 
single eye ; the field of view is also large, as we are able to 
see the object obliquely through the lens. For lenses of low 
power, as from one to two inches focus, the prisms would 
require to be separated to some extent, or we should not ob- 
tain a sufficient angle for stereoscopic vision ; in fact, we 
must consider this merely as a method of bringing the eyes 
closer together, that we may be enabled to see through a lens 
of small diameter with both of them at the same time, in a 
similar way as with the ordinary reading-glass before re- 
ferred to. 
On looking through the prisms, fig. 1, without the magni- 
fier, a singular illusion is produced, for the vision with the 
two eyes is brought so nearly to a state of parallelism that 
they are in effect blended into one, and we so far lose the 
power of appreciating distance, that we appear able to grasp 
objects several feet away from us, as the deceptions arising 
from monocular vision are increased by seeing with the two 
eyes from the same position as with one. 
In obtaining binocular vision with the compound achromatic 
microscope, in its complete acting state, there are far greater 
practical difficulties to contend against, and which it is highly 
important to overcome, in order to correct some of the false 
appearances, arising from what is considered the very perfec- 
tion of the instrument. All the object-glasses from the one 
inch upwards are possessed of considerable angular aperture, 
consequently images of the object are obtained from a differ- 
ent point of view, with the two opposite extremes of the 
margin of the cone of rays : and the resulting effect is, that 
there are a number of dissimilar perspectives of the object, all 
blended together upon the single retina at once. For this 
reason, if the object has any considerable bulk, we shall have 
a more accurate notion of its form by reducing the aperture of 
the object-glass. 
Select any object lying in an inclined position, and place it 
in the centre of the field of view of the microscope, then, 
with a card held close to the object-glass, stop off alternately 
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