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W. LeConte Stevens—New form of Reversible Stereoscope. 227 
semi-lens, and the optic angle is positive and small, or the vis- 
ual lines may be sensibly parallel. If the observer’s interocu- 
by 2. 
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achromatism in the semi-lenses. By now turning the screws so 
as to press the glasses closer together, any degree of optic di- 
vergence may be attained that the observer is willing to endure, 
while the picture is still seen in natural perspective, distances 
being apparently slightly magnified, and diameters also in the 
same ratio. 
To secure reversed perspective, lift up the wooden screen, 
fold the brass hinges outward in front, and substitute the 
