228 W. LeConte Stevens—New form of Reversible Stereoscope. 
prisms for the semi-lenses, as in fig 2. Slide the stereograph 
nearly to the end of the stereoscope. He GN Sis perspective 
is at once reversed; whether this is enoug overpower the 
other elements of perspective must depend ae the nature of 
the picture. If the stereograph be that of the moon, the 
reversion is complete; if of terrestrial scenery, some objects may 
be apparently transposed in position while others are not. 
same picture may be examined successively with each iilusive 
effect several times in as many minutes. As soon as reversion 
is attained, the stereograph may be drawn up as close as may 
e convenient. 
If both semi-lenses and Hae are ered, the instrument 
becomes a direct-vision stereoscope; in some ‘respects similar 
to that deaisibed by Professor William B. ee | in 1855. To 
secure natural perspective, press the screens flat, pull the stereo- 
graph up as close as possible, and gaze as if ‘through it at a 
remote object, with the muscles of the eyes relaxed. ‘The two 
pictures, imperfectly focalized, are dimly seen apparently to 
overlap. The stereograph is then pushed out to the end of the 
stereoscope, and the pictures are binocularly combined by optic 
divergence. The stereograph may now be pulled up as near as 
Sivanient 
o secure reversion of perspective by direct vision, pos the 
brass hinges and lift the wooden screen, as in . Push this 
out (0’ 6’) as near as porible to the stereograph_ at re or of 
the instrument, then pull it up, keeping the gaze fixed upon 
the projection Cd ) at the top. This grows dim as it approaches 
its previous position. Without changing the direction of the 
visual lines, except slightly to lower them, transfer the atten- 
tion to the stereograph beyond. The combined picture is seen 
in reverse perspective, apparently much smaller and nearer 
than when the prisms were employed. 
Those who have tried this instrument ts far have usually 
The use of aes screws for the semi-lenses of the stereo- 
scope is, of course, not a novelty. They were thus applied by 
Petes. |e 
