284 



THE APPLICATIONS OF PHYSICAL FORCES. [BOOK in. 



exact photographic reproductions, carefully taken from well chosen 

 positions, and with favourable conditions of light. 



It is important that the two images be equally illuminated. This 

 is secured by holding the stereoscope so that the light falls equally 

 on both pictures through the opening arranged for the purpose. If 



the photographs are on glass, the 

 apparatus may be placed opposite 

 the daylight or lamplight. In 

 this case the back of the stereo- 

 scope is provided with a piece of 

 ground glass, which evenly distri- 

 butes the light, and intercepts the 

 vision of exterior objects. 



The stereoscope not only gives 

 the impression of relief, it also 

 produces the effect of converging- 

 lenses or magnifying glasses, for 

 it magnifies objects, and conse- 

 quently facilitates the accurate 

 study of details. 



To increase these effects the prisms are replaced by combinations 

 of lenses, as represented in section on Fig. 213. This form was 

 arranged by Helmholtz. Besides the alteration of the eye-pieces, it 

 is distinguished by a special mechanism, by which the distance of the 



FIG. 211. Refracting stereoscope : section. 



FIG. 212. Refracting stereoscope: external view. 



two eye-pieces can be regulated, and the distance of the eyes or the 

 lenses from the stereoscopic pictures can be increased or diminished 

 at will. This arrangement is useful, because stereoscopic images are 

 not always so placed that the distance of the corresponding points 



