110 Dr. Carpenter, on Stereoscopic Binoculars. 
the light to pass through the remaining pair, we should find 
that the two pictures we should receive of the object would 
vary sensibly, according as they are formed by the bands 
a a ' , b b' , or c c'. For, supposing the pictures taken through 
the bands b b ' to be sufficiently dissimilar in their perspective 
projections to give, when combined in the Microscope, a 
sufficient but unexaggerated Stereoscopic relief, those taken 
through the bands a a' on either side of the centre would be 
no more dissimilar than two portraits taken at a very small 
angle between the Cameras, and their combination would 
very inadequately bring out the effect of relief ; whilst, on 
the other hand, the two pictures taken through the extreme 
lateral slips c c' would differ as widely as portraits taken at 
too great an angle of divergence between the Cameras, and 
their combination would exaggerate the actual relief of the 
object. Now, in each of the bands b b' , a spot v v' may be 
found by mathematical computation, which may be desig- 
nated the visual centre of the whole Semi-lens ; that is, the 
spot which, if all the rest of the Semi-lens were stopped off, 
would form a picture most nearly corresponding to that given 
by the whole of it. This having been determined, it is easy 
to ascertain what should be the angle of aperture ( op q , 
fig. 5) of the entire Lens, in order that the angles v p v' 
between the “ visual centres ” of its two halves should be 15°. 
The investigation of this question having been kindly under- 
taken for the Author by his friend Prof. Hirst, the conclu- 
sion at which he has arrived is, that the angle of aperture of 
the entire Lens should be about 
36*6°. This, which he gives as 
an approximate result only (the 
requisite data for a complete Ma- 
thematical solution of the ques- 
tion not having yet been ob- 
tained), harmonises most remark- 
ably with the result of experi- 
mental observations made upon 
objects of known shape, with 
Objectives of different angular 
apertures. 
When spherical objects, such 
as the globular forms of Polycys- 
tina or the pollen-grains of the 
Malvaceae, are placed under a 
Stereoscopic Binocular, provided 
with an Objective of one half or four-tenths of an inch focus, 
having an angular aperture of 80° or 90°, the effect of 
projection is so greatly exaggerated, that the side next 
