Prof. Helmholtz on the Telestereoscope. 23 



surprisingly prominent relief. When the large mirrors only are 

 turned, the small ones being left at the angle of 45°, an exag- 

 gerated relief is obtained. If the dimensions in the direction of 

 the depth of the field of view to those on the surface are to 

 retain their right relations, the large mirrors must always remain 

 parallel to the small ones. The aspects of near objects, particu- 

 larly of the human figure, are strikingly beautiful in the tele- 

 stereoscope. The impression differs from the reduction produced 

 by concave glasses, in the circumstance that it is not reduced 

 pictures that the observer imagines he sees, but actually reduced 

 bodies. 



Magnifying power may easily be connected with the tele- 

 stereoscope : it is only necessary to place a double opera-glass 

 between the eyes of the observer and the small reflectors ; it is 

 still preferable for the field of view, to separate the eye-glass from 

 the object-glass of the instrument, and so fix them in the tele- 

 stereoscope that the light at each side first strikes the large mir- 

 ror, then the object-glass, then the small reflector, and finally the 

 eye-glass ; so that in this arrangement the optic axis of the tele- 

 scope itself is broken at a right angle. The greater the mag- 

 nifying power, the greater of course must be the perfection of 

 the plane reflectors, but then it is not necessary to choose them 

 larger than the object-glass of the telescope. 



These views, at the same time telescopic and stereoscopic, also 

 exceed to an extraordinary degree the common image of the tele- 

 scope in vividness. In the simple telescopic images, difference 

 of distance disappears totally : the objects look exactly as if they 

 were painted on a plane surface. By the ordinary combination 

 of the two Gahleo's telescopes, the appearance of relief for nearer 

 objects is in some degree obtained ; and hence it is that a double 

 opera-glass gives a much livelier impression of relief than a 

 single one. But in the usual construction of the instrument 

 the relief is false : the objects appear as if they were squeezed 

 together in the direction of depth. In the case of human faces, 

 on which, for the most part, opera-glasses are directed, this is 

 very striking. Wlicn they are regarded from the front, they 

 appear much flatter than they really are, and when looked at in 

 profile, they ajjpear too narrow and sharp. In both cases the 

 expression of the countenance is essentially altered. 



When a double opera-glass is turned round and the observer 

 looks through the object-glass, the deep dimensions of objects 

 are magnified out of proportion. While, therefore, through a 

 simple telescope all objects appear as paintings, through a double 

 t)pera-glass, complete objects are seen us Ims-rdii'fs, while by re- 

 versing the opera-glass, objects appear in high relief. 



From the known laws of the tehacope and stereoscopic vision. 



