Dr. Carpenter, on Stereoscopic Binoculars. 109 
the vertical direction. This is especially important when 
horizontal sections are being made with tine scissors ; since 
the course of the section can thus be so regulated to pass 
through the plane desired, with an exactness totally un- 
attainable by the use of any Monocular Magnifier. 
On the Angle of Aperture best suited for Stereoscopic Vision. 
— The Stereoscopic Binocular is put to its most advantageous 
use when applied either to opaque objects of whose solid 
forms we are desirous of gaining an exact appreciation, or to 
transparent objects which have such a thickness as to make 
the accurate distinction between their nearer and their more 
remote planes a matter of importance. That its best and 
truest effects can only be attained by Objectives not exceeding 
40° of angular aperture, may be shown both theoretically and 
practically. Taking the average distance between the pupils 
of the two Eyes as the base of a triangle, and any point of 
an Object placed at the ordinary reading-distance as its apex, 
the vertical angle enclosed between its two sides will be from 
12° to 15° ; which, in other words, is the angle of divergence 
between the rays proceeding from any point of an Object at 
the ordinary reading-distance to the two Eyes respectively. 
This angle, therefore, represents that at which the two pic- 
tures of an object should be taken in the Photographic 
Camera, in order to produce the effect of ordinary Binocular 
vision without exaggeration ; and it is the one which is 
adopted by Portrait-photographers, who have found by expe- 
rience that a smaller angle makes the image formed by the 
combination of the pictures appear too flat , whilst a larger 
angle exaggerates its projection. Now, in applying this 
principle to the Microscope, we have to treat the two lateral 
halves (l, r, fig. 4) of the Objective as the two separate 
lenses of a double Portrait Camera, and to consider at what 
angle each half should be entered by 
the rays passing through it to form 
its picture. To any one acquainted 
wuth the principles of Optics, it must 
be obvious that the picture formed 
by each half of the Objective must 
be (so to speak) an average or gene- 
ral resultant of the dissimilar pic- 
tures formed by its different parts. 
Thus, if we could divide the lateral 
halves or Semi-lenses l, r, of the 
Objective by vertical lines into the 
three bands a b c and a b' c ', and could stop off the two 
corresponding bands on either side, so as only to allow 
Pig. 4. 
