358 W. LeConte Stevens—The Stereoscope, 
Art. XLVIL reais Stereoscope, and Vision by Optic Divergence ; 
y W. LeContEe STEVEN 
DurinG the last twelve years, Professor Joseph LeConte has 
published in this Journal a series of articles on Binocular Vision, 
naked eyes beyond the plane of the pictures, even when the 
distance between the identical points was chee than the dis- 
tance between the centers of his pupils.” He adds, “It would 
be curious to inquire, at what distance and of what size, accord- 
ing to the laws of vision, the stereoscopic image ought to seem 
in this case.’ 
While conversing with this gentleman,+ about three years 
ago, it was discovered that I possessed the same power; and 
since that time no stereograph has been found on which identical 
tested all the experiments on this subject that he has described ; 
and my results have been either identical with his, or as closely 
approximate as could be reasonably expected. To avoid repe- 
tition of what has been already sufficiently established I shall 
assume that the reader is familiar with the contents of Profes- 
sor LeConte’s papers.{ It will be found convenient to study 
optic ivaruenies especially in connection with the stereoscope. 
n normal binocular vision the two eyes may be regarded as 
uman cameras occupying slightly different positions, from 
which are obtained simultaneous views of the point upon whic 
the visual axes are converged. The apparent distance of this 
point is mainly determ ‘ied gt the intersection of these axes, if 
the optic angle is large enough to be readily appreciable. In 
‘ reading ordinary print with comfort the optic angle is rarely 
less than 
The method of preparing photographs for the es is 
too familiar to describe. It is usually assumed that, when thes 
are viewed through the instrument, the lenticular prisms are so 
adjusted that rays are deviated into the observer's eyes from 
corresponding points of the stereograph, as if coming from 
single objects in front; so that he may easily imagine his own 
* IT, ix, 162-163, March, 1875. + Mr. James Wood Davidson, of New York. 
This Journal, IT, vol. xlvii, pp. 68 and 153; III, vol.1, p. 33; vol. ii, pp. 1, 
315, and 417; vol. ix, p. 159. 
