the view obtained, though the spheres on the outskirts of the picture still
appear slightly elliptical. After all precautions have been taken, if disagree-
able effects still persist in the drawing it is customary to introduce certain
so-called corrections such as making the perspective of the spheres in Plate
II., fig. 1, all perfect circles. These are really not corrections but actual
transgressions of the rules of perspective which alter the view so that it will
not be exactly correct at any point, yet it may not be noticeably disagreeable
from any position likely to be taken by the observer.

FINALLY, mention should be made of the most perfect form of perspective
representation, the binocular. When a single perspective drawing is viewed
with both eyes the image on the retina of each eye is practically the same.
When, however, an object in space is viewed with both eyes two slightly
different images are received, one by the right and one by the left eye. It
is the fusion of these different views which gives to the observer the impres-
sion of relief or solidity.

In binocular perspective two slightly different views are made of the
same scene from two slightly different points of view some 2 3/4 in. apart, to
correspond with the views seen by the two eyes. This is most easily done
with a camera having two lenses, known as a stereoscopic camera. The two
views are then presented to the observer in such a way that the right eye
sees only the view taken by the right-hand lens and the left eye only the view
taken by the left-hand lens. An instrument designed for showing these
double pictures is called a stereoscope, and gives a result that is startling in
its realistic reproduction of distance and relief.

AQUATINT, a variety of etching (q.v.) in which effects are obtained by the
action of acid through a porous ground of sand or some powdered resinous
substance (Lat. aqua, water, and tincta, dyed). The plate is first covered
with a ground over which the resinous powder or sand is evenly dusted by

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