112 Dr. Carpenter, on Stereoscopic Binoculars. 
confirmed by that of others. In the first place, the pene- 
trating power or focal depth of the Binocular is greatly 
superior to that of the Monocular Microscope ; so that an 
object whose surface presents considerable inequalities, is 
very much more distinctly seen with the former than w r ith 
the latter. The difference may in part be attributed to the 
practical reduction in the Angle of aperture of the Objective, 
which is produced by the division of the cone of rays trans- 
mitted through it into two halves ; so that the picture received 
through each half of an Objective of 60° is formed by rays 
diverging at an angle of only 30°. But that this Optical 
explanation does not go far to account for the fact, is easily 
proved by the simple experiment of looking at the object in 
the first instance through each eye separately (the prism 
being in place), and then with both eyes together ; the dis- 
tinctness of the parts which lie above and beneath the focal 
plane being found to be much greater when the two pictures 
are combined, than it is in either of them separately. 
In the absence of any Optical explanation of the greater 
range of focal depth thus showed to be possessed by the 
Stereoscopic Binocular, the Author is inclined to attribute it 
to an allowance for the relative distances of the parts which 
seems to be unconsciously made by the Mind of the observer, 
when the solid image is shaped out in it by the combination 
of the two pictures. This seems the more likely from the 
second fact to be now mentioned, namely, that when the Bino- 
cular is employed upon objects suited to its powers, the pro- 
longed use of it is attended with very much less fatigue than is 
that of the Monocular Microscope. This, again, may be 
in some degree attributed to the division of the work 
between the two eyes ; but the Author is satisfied that unless 
there is a feeling of discomfort in the eye itself, the sense of 
fatigue is rather mental than visual, and that it proceeds from 
the constructive effort which the Observer has to make, w T ho 
aims at realising the solid form of the object he is examining, 
by an interpretation based on the flat picture of it presented 
by his vision, aided only by the use of the Focal Adjustment, 
which enables him to determine what are its near and what 
its remote parts, and to form an estimate of their difference 
of distance. Now, a great part of this constructive effort is 
saved by the use of the Binocular, which at once brings before 
the Mind’s eye the solid image of the object, and thus gives 
to the Observer a conception of its form usually more com- 
plete and accurate than he could derive from any amount of 
study of a Monocular picture.* 
* It has happened to the Author to be frequently called on to explain 
