876 STEREOSCOPIC VISION. JUDGMENT OF SOLIDITY. 



image. If, however, the differences between the two stereoscopic figures are ex- 

 cessive, so that too widely separated retinal points are affected, or if new lines 

 are introduced into a figure that do not harmonize with the solid figure, or would 

 disturb the coalescence, the stereoscopic fusion ceases. 



The stereoscope is an instrument by means of which two similar pictures, 

 drawn in perspective, may be superposed, so that they appear single, and give 

 the impression of solidity. Wheatstone accomplished this by means of two mirrors 

 placed at an angle (Fig. 305, II), Brewster by means of two prisms (Fig. 305, I). 

 The construction and mode of action are shown by the figures. 



Even without a stereoscope some persons are able to unite two such pictures 

 by directing the visual axis of each eye to the picture opposite to it. 



Two exactly similar pictures, that is, those in which all corresponding points 

 are at an equal distance from each other (for example the identical pages of two 

 copies of a book), appear exactly on the same level under the stereoscope; 

 but just as soon as one point in one or the other is closer to, or farther from, 

 the corresponding point than the others, it appears immediately to project 

 in front of or behind the plane. In this way Dove taught how to distinguish 

 false banknotes from good ones, by their failure to yield perfectly flat images. 



Solid objects seen from a great distance, such as the remote parts of a land- 

 scape, appear flat, as in a picture, and do not stand out, because the difference 

 in the position of the eyes is too small relatively to be taken into consideration. 

 In order to obtain a stereoscopic view of such objects, v. Helmholtz constructed 

 the telestereoscope (Fig. 305, IV) , an instrument that, by means of parallel mirrors, 

 moves the points of view of both eyes to some extent farther apart. The mirrors 

 L and R throw their images respectively on the mirrors 1 and r, toward which 

 the eyes O o are directed.- According to the distance of L and R, both eyes may 

 be apparently separated several feet (to Oj Oj) . The distant landscape thus ha's 

 a distinct appearance of solidity. In order to see the distant objects more distinctly 

 and at closer range, a telescope (field-glass) can be placed before each eye. In- 

 struments of this sort, re lief -telescopes, have been constructed in great perfection 

 recently by Zeiss; instead of mirrors they contain similarly acting prisms. 



If in two stereoscopic pictures the corresponding surfaces are made black 

 in the one and white in the other (for example, if two truncated pyramids are 

 drawn, as in Fig. 305, III, and one figure is drawn exactly like L, that is with 

 white surfaces and black lines, while the other is drawn with black surfaces and 

 white lines), the body appears to shine in the stereoscope. The explanation of 

 luster is that the shining object, in a certain position, reflects bright light 

 into one eye, and not into the other; because at a given angle the reflected ray 

 cannot enter both eyes at the same time. 



An interesting experiment for illustrating stereoscopic vision is furnished 

 by Wheatstpne's pseudoscope. This consists of two right-angled prisms enclosed 

 in tubes (Fig. 306, A and B), through which the observer looks in a direction 

 parallel to their oblique surfaces. If a spherical surface is seen through this 

 instrument, the images falling in the eyes will be reversed laterally. The right 

 eye thus obtains a view usually received by the left eye, and conversely; the 

 shadow projected is also reversed. The result of this is that the ball appears 

 hollow. R. Ewald constructed the apparatus with four mirrors, and its action will 

 be readily understood from Fig. 307. 



The stereoscope can also be used to explain the "rivalry of the visual fields." 

 In other words, both eyes are almost never simultaneously and equally active in 

 binocular vision, but they rather relieve each other more or less completely, so 

 that at one time the image of one retina, at another time that of the other prevails. 

 For example, if two differently colored surfaces are placed in the stereoscope, 

 they will alternate in the common field of vision, especially if they are brightly 

 illuminated, accordingly as one or the other eye is especially active. If two sur- 

 faces are used, on which lines are so drawn that they would cross if the surfaces 

 were superposed, the lines first of one system and then of the other appear more 

 prominently. The rivalry of the visual fields is similarly shown in looking 

 through differently colored glasses at a landscape. 



