Improvements in the Stereoscope. 187 
coloured children’s picture-books are most admirably adapted for 
the experiment; the blues and the reds can be made to appear to 
rise and fall alternately at pleasure according to the manner they 
are looked at. If viewed by monochromatic light the effect will, 
of course, not take place. 
The next experiment tried was a kind of phoenakistoscope 
arrangement for the magic lantern, with a view to getting a 
stereoscopic effect on the screen (Fig. 8, Plate 10). One-half of an 
ordinary stereoscopic picture was placed in one lantern and the 
other half in the other; both images were then thrown simul- 
taneously on the screen superposed. They could, of course, be 
made to coincide in the centre but not at the edge. Now two 
revolving discs were arranged, one in front of the lantern and one 
ina convenient position for an observer to stand behind. The 
two were arranged so that one picture only was visible on the 
screen at once, but as the disc was rotated the images appeared 
alternately at any desired rate; the second disc, driven mecha- 
nically at an equal rate to the first, only allowed one eye ata time 
to view the screen. The result is that one eye sees, of course, one 
picture, and the other the other picture. When the dises are 
slowly revolved up to (say) four revolutions a second a jerking 
action is visible; but when the speed is increased to eight or ten 
a second the image is seen perfectly steady and in full relief. I 
mention this more as an interesting experiment than as of any 
practical utility, thougn it is quite possible to conceive an 
arrangement by which this might be adapted for a number of 
spectators. 
The next experiment had reference to using larger pictures than 
usual for the stereoscope. I found this could be done by employ- 
ing a pair of reflecting prisms, such as Professor Zéllner proposed 
for reversing one-half the spectrum in his spectroscope (Fig. 9, 
Plate 10). Using a pair of these prisms and mounting the 
pictures side by side on one piece of cardboard, jointed in the 
middle, so that the pictures could be laid as in the diagram, this 
form answered fairly. The field, however, was limited in the 
horizontal direction, which is a decided objection. 
The arrangement next tried was that of the two doubly-reflecting 
prisms, as proposed in 1873 (Fig. 7, Plate 10). 
The next arrangement tried was altogether novel. In making 
