PHOTOGRAPHY AND SOME OF ITS APPLICATIONS. 629 
for example — wliicli are exhibited at the Crystal Cube Company's 
establishment at Charing Cross^ are singularly true and life-like. 
In some of them_, you can almost fancy the possibility of 
plucking at the nose (if such an operation were desirable) of 
the inhabitant of the crystal cube. The effect of solidity in 
the case of this application is almost equal that of the stereo^r 
scope, and is the result of optical phenomena in some measure 
the same as those of Wheatstone^s instrument. The diagram 
beneath will help to explain the principle of Mr. Swanks 
device. To produce the effect, two photographs {Ji and li!) 
taken at the proper angles, and 
two rectangular prisms {i j h 
and i I A;), whose angles are 
somewhat different, are required. 
The prisms are placed upon 
each other, so as to produce 
a nearly square block of glass, 
and the photographs are arranged, 
one [li) behind this block, and 
the other (/f) at the side. If now 
the two eyes (a? and y) be placed 
in front of the crystalline cube 
(a little less to the side than is 
indicated in the diagram), it will 
happen that each eye will per- 
ceive one particular image, and 
one only, and thus the stereoscopic effect will be produced. 
This must follow from the laws of reflexion and refraction. 
The rays of light proceeding from the photograph 7f, will 
pass laterally inwards till they meet with the polished 
surface of the lower prism at the point z ; from this they 
will be reflected at an angle equal to that of their in- 
cidence, and not meeting the surface (7 j) of the upper 
prism vertically, they will be refracted on entering the air, and 
will pass on to the eye y. The pencils of light from the pho- 
tograph lij will be transmitted without either refraction or 
reflexion, and will enter the eye a’, and thus perfectly binocular 
vision will be produced. It is necessary to mention that the 
photographs are generally executed on some transparent 
medium, such as talc, which is then attached to the prism. 
From the expensive character of the apparatus employed, and 
the difficulty of giving the photographs transparent colour to 
exactly the same extent in the two, the crystal cube miniatures 
have not yet been sold at a price calculated to make them as 
popular as the carte, but we believe Mr. Swan is now 
engaged in experiments, with a view to discover a method of 
rendering the whole process simpler and cheaper. 
