446 W. LeConte Stevens—The Stereoscope, 
The experiment just described does not imply any unusual 
conditions in the stereoscope except that the higher value, 8°, 
given: to the optic angle is greater than usual. ssuming 
12° the mean already found for the stereographic fore- 
ground interval, the corresponding angle of convergence after 
allowing for deviation of rays is a little less than 2° the inter- 
section of axes is hence still far from the point to ‘which the 
focal adjustment is pee im This fact explains the difficulty 
experieuced by so persons in obtaining distinct vision 
through the sas ctae especially those who have passed 
beyond middle age and lost in great measure the power of focal 
accommodation 
Most of the stereographs in common use are pictures in 
which physical perspective is strong. When these are properly 
mounted and viewed in the stereoscope the chief advantage 
gained by use of this instrument seems to be that it necessi- 
tates variation in the relation between the optic axes, in order 
that perfect binocular combination of the different parts of the 
superposed retinal images be secured in the subjective Cyclo- 
pean,* or combined binocular, eye. If the ere is perfect ShDe 
to test this at no distant da: . It should be so acts 
to Professor LeConte’s theory of binocular perspective.t 
What has been generally given and accepted as the mathe- 
matical theory of the stereoscope applies strictly, but only to 
the relations involved in taking the photographs with cameras 
ligt placed, so that the axes of the lenses converge 
upon some point of the object pictured. When the negatives 
are once fixed “ase proofs from them so mounted that corre- 
sponding points from the pair are focalized upon corresponding 
retinal points for the observer who binocularly combines them, 
with or without the stereoscope, the relation between the dif. 
ferent parts of the fields of view combined undergoes no senst- 
ble variation, real or apparent, except between the limits fixed 
by difference between the stereographic intervals in the back- 
ground and foreground respectively. If the eyes are comfort- 
able, after binocular combination is attained, it makes little 
* This Journal, III, vol. i p. - et seq. 
+ This Journal, In, vol. ii, p. 3 
