Dr. Carpenter, on Stereoscopic Binoculars. Ill 
the eye, instead of resembling a hemisphere, looks like 
the small end of an egg. If, then, the aperture of such 
an Objective be reduced to 60° by a diaphragm placed 
behind its back lens, the exaggeration is diminished, 
though not removed ; the hemispherical surface now 
looking like the large end of an egg. But if the aper- 
ture be further reduced to 40° by the same means, it is at 
once seen that the hemispheres turned towards the eye are 
truly represented ; the effect of projection being quite ade- 
quate, without being in the least exaggerated. — Hence it may 
be confidently affirmed — alike on theoretical and on practical 
grounds — that when an Objective of wider angle than 40° is 
used with the Stereoscopic Binocular, the object viewed by 
it is represented in exaggerated relief, so that its apparent 
form must be more or less distorted. 
There are other substantial reasons, moreover, why Objec- 
tives of limited angle of aperture should be preferred (save in 
particular cases) for use with the Stereoscopic Binocular. As 
the special value of this instrument is to convey to the mind a 
notion of the solid forms of objects, and of the relations of their 
parts to each other, not merely on the same but on different 
planes, it is obvious that those Objectives are most suitable to 
produce this effect, which possess the greatest amount of pene- 
tration or focal depth ; that is, which show most distinctly, 
not merely what is precisely in the focal plane, but what lies 
nearer to or more remote from the Objective. Now, as 
increase of the angle of aperture is necessarily attended with 
diminution of penetrating power, an Objective of 60° or 80° 
of aperture, though exhibiting minute surface-details which 
an Objective of 40° cannot show, is much inferior in suit- 
ability .to convey a true conception of the general form of 
any object, the parts of which project considerably above the 
focal plane or recede below it.* 
The Author would further draw attention to two important 
advantages he has found the Stereoscopic Binocular to 
possess, his own experience on these points being fully 
* In accordance with these principles, the Author has caused Messrs. 
Powell and Lealand to construct for him an Objective of half-inch focus 
with an angular aperture of 40°; and he has found it to answer most 
admirably for the purpose for which it was intended — the examination of 
Opaque objects with the Stereoscopic Binocular. For not only are these 
represented in their true forms, but the relations of their different parts 
are seen with a completeness not otherwise attainable. And an Objective 
so constructed has this great advantage over one whose originally larger 
aperture has been reduced by a diaphragm — that the distance between its 
front lens and the object is so much greater, as to admit far more con- 
venifntLy of side illumination. 
