406 Prof. Emerson’s Description of his 
one-tenth of an inch until the lines upon the last card are three 
and one-half inches apart; now look at each of these in the 
stereoscope with a rather hasty glance, and it will be easy to 
determine at what distance the lines should be apart to coalesce 
perfectly and instantly. This will be the distance the centres 
of the right and left pictures should be separated for that m- 
strument. 
But further, the ordinary lenticular stereoscope does not per- 
amaximum.* Sir D. Brewster, in his work on the Stereoscope, 
pp. 162, 163, declares that—‘“ the size of the pictures is deter- 
mined,” “that nothing can be gained by using larger pictures, 
&c., &e. This is true with regard to the ordinary form of the | 
stereoscope. But this renders it none the less a very desirable 
thing to be able to see larger pictures: and _ by larger pictures 
we do not mean the same pictures magnified, for then, indeed, 
nothing would be gained, but pictures including a larger angle, 
and affording a view of more than twenty square inches as a 
m 
towards the left, and vice versa. By this means the lenses ay 
made equivalent to prisms with variable angles. An é 
method of accomplishing this is shown in the following diagt®™ 
. * The Reflecting Stereoscope of Wheatstone will exhibit larger pictures, ba * 
is not adapted to popular use, 
