360 W. LeConte Stevens—The Stereoscope, 
ing external relations, and remembering the results of. such 
comparisons. If by any means the axial adjustment can be 
ade to differ considerably from that which usually accom- 
tance is sma , whose eyes are normal, and whose power of 
accommodation, both focal and axial, is hence large 
If an observer, who possesses but a ‘single eye, looks out upon 
a landscape, the ‘relative distance of the different objects viewed 
may be roughly estimated in terms of some standard arbitrarily 
chosen, so long as they are not precisely aligned with his eye. 
The judgment i is less accurate as the angular separation of the 
objects becomes less, and as there are fewer of them at moderate 
distances with which to compare the rest. Always, and often 
unconsciously, he employs one or more of the following ele- 
ments in judging the distance and form of each object regarded. 
ear sli subtend larger visual angles than remote 
objects of equal s 
IT. Near objects are seen more distinctly than those that are 
remote. The illusion of distance may hence be produced by 
decreasing the brightness of the object viewed, by changing the 
nature of the medium, or by increasing the contrast between 
light and shade. 
Ill. Near objects, that are almost aligned with those which 
are remote, partly cover them. Covering objects are judged 
nearer than those covere 
IV. Familiarity with the dimensions of known objects when 
near enables us to compare them when remote and thereby 
judge their relative distance. 
V. By moving from one standpoint to another and compar- 
ing the new view with what is retained in memory of the pre- 
vious one, parallax of motion thus ene to the formation 
of a judgment of both distance and for 
The mere synopsis of these So hapethigs is all that is necessary ; 
separately they are familiar enough, and to illustrate them 
would be easy. Every one of them may be employed in the 
use of each eye, either separately or in conjunction with its 
companion. For distances of more than 240™ the binocular 
observer has no advantage except that two eyes receive more 
light than one, and the combined external image hence appears 
brighter and more distinct. _ of them except the last. may 
be imitated in pictures, and some of them, notably the second, 
may be heightened by the BRS 8 effect of lenses. In study- 
