444 W. LeConte Stevens—The Stereoscope, 
should be referred to the sensation * muscular strain rather 
than to the intersection of visual line 
The effect of varying the tension of the rectus muscles of the 
eyes in modifying the estimate of relative distance has been ap- 
plied in W heatstone’s pseudoscope®* and in his reflecting stereo- 
scope, though no reference in this connection has been distinctly 
made to anything beyond variation of convergence e fol- 
lowing experiment is not difficult. Upon a large sheet of paper 
a series of vertical parallel lines are drawn, 50™ apart; the 
last line of this series forms the first of a second series 60™" 
the second series, the convergence is still farther diminished, 
and it passes into divergence when the third is successfully 
combined. The apparent distance of the first series I estimate 
at 2™5, of the second about 8™ and of the third about 3™°5. 
By intersection of axes, the first should be 6", the second 
h 
none who succeeded in attaining saa thus without ob- 
serving an apparent recession of the external image. 
o ascertain whether it oat of axes is unconsciously prac- 
ticed in the use of the stereoscope, I examined 166 stereographs 
taken at random and found the foreground interval to vary be- 
tween 60" and 95™™, the mean being 7279. The eared 
interocular distance for adults is a little less than 64; 
combine without the stereoscope, therefore, divergence is uy 
always necessary. To ascertain the mean deviating power o 
the lenticular prisms used in the best instruments, 30 pairs were 
obtained through the courtesy of Mr. H. T. Anthony, of New 
York. With but slight variation, the focal saa was found to 
* Phil. Transactions, 1852, ety or, Phil. Magazine, 1852, pp. 506-523. 
