186 Scientific Proceedings, Royal Dublin Society. 
Two other forms of stereoscopes were introduced since the in- 
vention of the Brewster, but I have not been able to find any 
description of them, 
One was introduced by M. Claudet many years ago, but more, 
I believe, as a curious experiment than as a practical instrument. 
The other was called, | believe, “Swan’s Crystal Cube,” in which 
the observer, looking into a cube of glass, saw the object in relief. 
This was effected by attaching two stereoscopic pictures respec- 
tively to the back and one side of the cube, which was formed 
by two right angled prisms laid together. In this way one pic- 
ture (the back) was seen by transmission with one eye, and the 
other picture was totally reflected by the base of one of the prisms 
and seen by the other eye. This was, of course, more a scientific 
toy than a practical form of the instrument. 
The first experiments I made were for the purpose of investi- 
gating the cause of the apparently wonderful stereoscopic effect 
which we get in looking at some of those coloured photographs 
of flowers ordinarily sold in the shops, when viewed with both 
eyes through a single magnifying lens, such as is supplied with 
the graphoscope. That it is a true stereoscopic effect is evident 
from the fact that the moment you shut one eye the effect is lost ; 
but how a stereoscopic effect could be obtained from a single 
picture was more than I could at first understand. One single 
experiment, however, resolved it. Instead of one single large 
lens I took two, each equal in focus to the lens of the grapho- 
scope. On looking at the coloured photograph through these, 
keeping the eyes in the centres of each lens, no effect of relief was 
visible at all. On drawing the lenses closer together, so that the 
eyes looked through the outer edges of the lens, the red flowers 
stood up. On drawing the lenses asunder, so that the eyes looked 
through the inner edges of the lens, the reverse effect took place, 
and the red flowers appeared to sink below the paper. 
The explanation is, therefore, quite clear. The edges of the 
lens act as prisms as well as lenses, and fringe everything with 
red on one side and blue on the other; in other words, the out- 
line of the red flower is a little extended on one side to the right 
eye, and on the other side to the left eye. Thus the two pictures 
we see are really dissimilar, and dissimilar just in such a manner 
as to cause the appearance of relief. I afterwards found that 
