September. 1913. 



KNOWLEDGE. 



331 



for altering the separation between them to suit the 

 distance between the eyes of any observer. Such 

 refinements, however, are still only met with in the 

 higher-priced lenticular stereoscopes. 



In the opinion of many, the reflecting principle 

 offers the fewest disadvantages, now that its earlier 

 constructional drawbacks have been overcome. It 

 is not without interest to trace the evolution of the 

 reflecting stereoscope, from the cumbrousness and 

 complexity which at first distinguished it to its 

 latest developments and refinements. The earliest 

 stereoscopic apparatus making use of reflection was 

 invented by Sir Charles Wheatstone in 1838 — next in 

 order, therefore, to Elliott's apparatus. As shown by 

 Figure 351, it had two plane mirrors, A and B, inclined 

 together at an angle of 90°, silvered side outward, 

 the two pictures C and D composing the stereograph 

 being placed one at each side, and the observer 

 viewing their images in the mirrors. As both images 

 were reflected into an identical position at E, and 

 thereby made to coalesce, while, at the same time, 

 each eye saw only its own particular picture, stereo- 

 scopic vision resulted. The chief drawbacks of this 

 arrangement were that the two pictures had to be 

 inserted separately ; that, if any adjustment were 

 needed, both pictures had to be advanced towards or 

 withdrawn from the mirrors simultaneously, which 

 involved the use of a clumsy double-threaded screw ; 

 that the reflected image was necessarily reversed as 

 regards right and left ; and that the efficient lighting 

 of both pictures, from two contrary directions, 

 presented serious difficulties. This instrument was 

 therefore soon superseded. It may be remarked 

 that, besides claiming credit for the inception of the 

 first reflecting stereoscope, strong ground exists for 

 believing that Wheatstone was aware of the principle 

 of — if he had not actually constructed — the 

 lenticular form prior even to Brewster. 



There have been many variants on Wheatstone's 

 instrument, since it is obvious that the two prints 

 and mirrors may be arranged in numerous different 

 ways. One of these, known as the Reflectascope, is 

 illustrated by Figure 352. As will be noticed, two 

 plane mirrors are placed at A and B, joining at a 

 very obtuse angle. The stereoscopic slide, the back 

 only of which is seen in the figure, is inserted in a 

 holder at C, and the observer, looking over the slide, 

 sees in the mirrors a single picture in relief. The 

 chief drawback here is that the join between the 

 mirrors shows, though almost imperceptibly, as a 

 thin line across the centre of the combined image. 

 If the two mirrors are inclined inwards away from 

 the slide, instead of outwards, the arrangement 

 otherwise remaining the same, non-transposed 

 stereoscopic slides may be used. In both cases the 

 image is reversed as regards right and left. 



It is an evident advantage that optical aids should 

 be dispensed with so far as possible, in order that the 

 eyes view the stereograph direct. Except in Elliott's 

 instrument, the drawbacks of which have been 

 pointed out, this desideratum is incapable of practical 

 fulfilment. There are, however, several systems in 



which one eye at least regards its own picture direct. 

 Various suggestions to that effect were made, at 

 different times, by H. W. Dove, Brewster, and 

 Rollmann ; but, as a modern instance of efficient 

 performance combined with convenience, Theodore 

 Brown's tiny pocket stereoscope (1895), doubtless 

 the smallest made, may be mentioned. In this, as 

 shown by Figure 353, two small mirrors, A and B, 

 are enclosed in a case, nearly but not quite parallel 

 with each other. Looking through an aperture in the 

 case, the right eye views its proper picture by double 

 reflection, apparently superposed on the left-hand 

 picture, the latter being seen direct by the left eye. 

 The same idea is obviously applicable to a double 

 reflection prism having sides of suitable angle. 



Mirrors, to give optically perfect results, should 

 be surface-silvered, but are then unfortunately liable 

 to tarnish, and are very susceptible to injury by 

 moisture, scratches, or abrasion. Reflecting prisms 

 serve all the purposes of mirrors and are free from 

 these disadvantages, besides being more easily 

 cleaned. Many prismatic stereoscopes have been 

 constructed, among which may be recorded that of 

 Girard-Teulon (1861), in which four total reflection 

 prisms were used, and several forms of apparatus 

 invented by Sir Howard Grubb (1878), in which the 

 two stereographic prints were mounted one above 

 the other, instead of side by side. In another 

 instrument by Grubb the two pictures of an 

 ordinary stereoscopic transparency were thrown in 

 superposition on to a concave mirror by means of a 

 combination of prisms and lenses, the observer, at a 

 suitable distance, seeing an aerial image in relief. It 

 may be mentioned that Duboscq and also Jequezel, 

 both in 1857, made stereoscopes having prisms as 

 well as lenses, so that both reflection and refraction 

 were employed. These instruments were, however, 

 practically modifications of Brewster's apparatus. 



Whether mirrors or prisms are used, it is fairly 

 evident that a double reflection must involve more 

 loss of light, and must require a more exact adjust- 

 ment than a single one. There is therefore a 

 special advantage in the prism stereoscope invented 

 by Professor Leon Pigeon, of Dijon (1910), who is 

 also known as the contriver of the compact and 

 useful book-form mirror stereoscope named the 

 " Dixio " (1905). In Professor Pigeon's prism 

 stereoscope (see Figure 354) the right-hand picture, 



A, is viewed direct, while the left-hand one, B, is 

 seen by a single reflection in the prism, C, apparently 

 superposed on A. It is necessary that the left-hand 

 print should be laterally reversed, a condition easily 

 fulfilled, among other obvious ways, by making the 

 prints in carbon, one by single and the other by 

 double transfer ; or, if film negatives are used, by 

 printing the left-hand picture from the back. As 

 shown by Figure 355, the apparatus consists of an 

 upright panel, A, supported by a narrow cross-piece, 



B, the prism being mounted in a hood, C, which 

 screens stray light from the left eye. The prints, 

 which may be of quite large size if desired, are laid 

 flat on a table and the instrument is placed over the 



