THE STEREOSCOPE. 



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be too great, so that parts of the retina too wide apart are excited thereby, or 

 when new lines are present in a picture, and do not admit of a stereoscopic effect, 

 or disturb the combination, then the stereoscopic effect ceases. 



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

 in perspective may be superposed so that they appear single. Wheatstone (1838) obtained this 

 result by means of two mirrors placed at an angle (fig 570); Brewster (1843) by two prisms (fig. 

 571). The construction and mode of action are obvious from the illustrations. 



Some pairs of two such pictures may be combined, without a stereoscope, by directing the 

 visual axis of each eye to the picture held opposite to it. 



Two completely identical pictures, i.e., in which all corresponding points have exactly the 

 same relation to each other as the same sides of two copies of a book, appear quite flat under 

 the stereoscope ; as soon, however, as in one of them one or more points alters its relation to 

 the corresponding points, this point either projects or recedes from the plane. 



Telestereoscope. When objects, placed at a great distance, are looked at, e.g., the most 

 distant part of a landscape, they appear to us to be fiat, as in a picture, and do not stand out, 

 because the slight differences of position of our eyes in the head are not to be compared with 

 the great distance. In order to obtain a stereoscopic view of such objects, v. Helmholtz 

 constructed the telestereoscope (fig. 572), an apparatus which by means of two parallel mirrors, 

 places, as it were, the point of view of both eyes wider apart. 

 Of the mirrors, L and R each projects its image of the landscape 

 upon I and r, to which both eyes, 0, 0, are directed. Accord- 

 ing to the distance between L and R the eyes, 0, 0, as it were, 

 are displaced to O,, o r The distant landscape appears like a 

 stereoscopic view. In order to see distant parts more clearly 

 and nearer, a double telescope or opera-glass may be placed in 

 front of the eyes. 



Take two corresponding stereoscopic pictures, with the surfaces 

 black in one case and light in the other. Draw two truncated 

 pyramids like fig. 569, make one figure exactly like L, i.e., 







Fig. 572. 

 Telestereoscope of v. Helmholtz. 



Fig. 573. 

 Wheatstone's Pseudoscope. 



with a white surface and black lines, and the other with white lines and a black surface, then 

 under the stereoscope such objects glance. The cause of the glancing condition is that the 

 glancing body at a certain distance reflects bright light into one eye and not into the other, 

 because a ray reflected at an angle cannot enter both eyes simultaneously {Dove). 



Wheatstone's Pseudoscope consists of two right-angled prisms (fig. 573, A and B) enclosed 

 in a tube, through which we can look in a direction parallel with the surfaces of the 

 hypothenuses. If a spherical surface be looked at with this instrument, the image formed in 

 each eye is inverted laterally. The right eye sees the view usually obtained by the left eye, 

 and conversely ; the shadow which the body in the light throws upon a light ground is reversed. 

 Hence the ball appears hollow. 



Struggle of the Fields of Vision. The stereoscope is also useful for the following purpose : 

 In vision with both eyes, both eyes are almost never active simultaneously and to the same 

 extent ; both undergo variations, so that first the impression on the one retina and then that on 

 the other is stronger. If two different surfaces be placed in a stereoscope, then, especially when 

 they are luminous, these two alternate in the general field of vision, according as one or other 

 eye is active (Panum). Take two surfaces with lines ruled on them, so that when the surfaces 

 are superposed the lines will cross each other, then either the one or the other system of lines is 

 more prominent (Panum). The same is true with coloured stereoscopic figures, so that there is 

 a contest or struggle of the coloured fields of vision. 



403. ESTIMATION OF SIZE AND DISTANCE. Size. We estimate the 

 size of an object apart from all other factors from the size of the retinal image ; 

 thus the moon is estimated to be larger than the stars. If, while looking at a 



