328 



KNOWLEDGE. 



September, 1913. 



lines from running into each other when developed. 



If we consider any two adjacent spaces of the 

 composite transparency in relation to a single open- 

 ing in the line screen, as shown by Figure 349, the 

 remarkable similarity in principle to Elliott's early 

 apparatus will be evident. The Ives stereograph 

 consists, in fact, of hundreds of tiny Elliott stereo- 

 scopes, all inspected simultaneously. While the 

 results obtained are unexceptionable, there are two 

 obstacles to the general 

 adoption of this system. 

 One is the necessity of 

 precise adjustment of the 

 distance of the ruled screen 

 from the plate in relation 

 to the camera extension and 

 the separation of the two 

 view points, besides the 

 corresponding care required 

 in binding up the positive, 

 the other being the fact that 

 only transparencies can be 

 made. 



Yet a third method of 

 stereoscopy , utilising neither 

 reflection nor refraction, is 

 the anaglyph system of 

 Louis Ducos Du Hauron 

 (1890) in which the two 

 pictures forming the stereo- 

 graph are printed, one 

 over the other, in red and 

 blue ink respectively. The 

 resulting jumbled combina- 

 tion is viewed through 

 spectacles containing a red 



and a blue glass, or Figure 346. 



similarly tinted pieces of 

 gelatine. Through the red 



glass, the red image is invisible and the green one 

 appears black, while through the green glass the 

 reverse is the case. As each eye, therefore, sees 

 only its particular picture, and the two are super- 

 posed, stereoscopic vision results. Prints of any 

 size may be produced and viewed in this way, and 

 the stereographs evidently occupy only half the space 

 that would be necessary if the two pictures were 

 placed side by side in the usual manner. The 

 drawbacks are a considerable loss of light from the 

 presence of the colour filters, and the fact that the 

 prints cannot be inspected without the viewing 

 arrangement, as is often desired. Though Du 

 Hauron was undoubtedly the first to employ the 

 foregoing method with printed stereographs, it 

 should be stated that the principle was foreshadowed 

 by Dove in 1843, and by Rollmann in 1853, while 

 D'Almeida, in 1857, used a very similar system, but 

 with red and green glasses for stereoscopic projection 

 by the lantern. 



Turning now to refracting stereoscopes, that of 

 Sir David Brewster, familiar as the old box-form 

 pattern of early Victorian days, comes first in order. 



Initially studied by its inventor in 1844, and com- 

 municated to the British Association in 1849, this 

 instrument was not placed on the market till 1851, 

 when Duboscq, of Paris, undertook its manufacture. 

 The principle of Brewster's stereoscope is probably 

 so well known that explanation is almost superfluous. 

 A few words, however, may be desirable. The 

 lenses or prisms A and B (Figure 350), cut from 

 the two halves of a bi-convex lens and turned with 



their thin edges inwards, so 

 refract the rays from the 

 two pictures C and D that 

 the eyes of the observer 

 see them converged 

 together, both images being 

 superposed at E. Since, 

 nevertheless, each eye sees 

 only its own proper half of 

 the stereograph, the result 

 is stereoscopic relief. A 

 more convenient form of 

 Brewster's instrument, de- 

 signed by Oliver Wendell 

 Holmes in 1861, with the 

 further improvement of a 

 sliding view-holder, added 

 by J. L. Bates in 1864, is 

 now in general use. 



The refracting stereo- 

 scope has the advantages 

 that no light is lost and 

 that the image is magnified. 

 Counterbalancing these 

 merits arc the facts that in 

 magnifying the prints the 

 grain of the paper is also 

 enlarged, detracting from 

 the fineness of the image; 

 that only pictures of a 

 small and rather awkward size can be employed ; 

 that the instrument has to be focused to suit differ- 

 ences of vision ; and, finally, that a slight degree of 

 distortion, if not of other optical aberrations, is 

 present. It may also be added that some observers 

 have difficulty in combining the two images with 

 this type of apparatus. 



Variations of Brewster's instrument have been 

 numerous. Whole lenses have, for instance, been 

 used, as in the telescopic stereoscope of De la 

 Blanchere, which resembled a pair of opera glasses 

 with the two tubes adjustable as to convergence. It 

 is not, by the way, generally known that if we look 

 through an ordinary opera-glass pointed wrong way 

 round at a stereoscopic slide, a single small image in 

 relief will be seen. Another interesting stereoscope, 

 devised by Steinhauser, had lenses contrary to those 

 of Brewster's apparatus, causing the optic axes to 

 cross, so that untransposed stereographs could be 

 utilised. The chief modern improvements on 

 Brewster's instrument, apart from the American 

 hood and sliding holder, have been the employment 

 of achromatic instead of single lenses, and provision 



