1042 



THE SENSES 



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taller than B, and C smaller than either A or B. In the same 

 figure the lines D and E are of the same length, but E seems con- 

 siderably longer than D. 



Illusions of movement are among the most interesting optical 

 illusions. If two similar objects are momentarily shown to the 

 eye in rapid succession and at points in space not separated by too 

 great a distance, the illusion is produced that the first object has 

 moved to the position of the second. Such illusions are the basis 

 of the so-called ' moving pictures ' shown by the cinematograph. 

 A series of instantaneous photographs of a movement are taken, 

 recording the successive positions assumed by the moving body. 

 When these are thrown on the retina in the same order and in rapid 

 succession, an illusion of the original movement is produced. 



The apparent size and form of an object is intimately related to 

 the size, form, and sharpness of its image on the retina. We are, 



therefore, able to dis- 



A B C criminate with great 



precision the un- 

 stimulated from the 

 excited portions of 

 that membrane, es- 

 pecially in the fovea 

 centralis, and also 

 the degree of excita- 

 tion of neighbouring 

 excited parts. But 



instead of localizing the image on the retina as we localize on the 

 skin the pressure of an object in contact with it, we project the 

 retinal image into space, and see everything outside the eye. 



In vision, in fact, we have no conception of the existence of either 

 retina or retinal image; and even the shadows of objects within the 

 eye for instance, an opacity or a foreign body in any of the refractive 

 media are referred to points outside it. Generally opacities in the 

 vitreous humour are movable, in the lens not. 



Purkinje's Figures. As was first pointed out by Purkinje, the 

 shadows of the bloodvessels in the retina itself, and even of the 

 corpuscles circulating in them, although neglected in ordinary 

 vision, may be recognized under suitable conditions, a conclusive 

 proof that the sensitive layer must lie behind the vessels. 



If a beam of sunlight is concentrated on the sclerotic as far as possible 

 from the margin of the cornea, and the eye directed to a dark ground, 

 the network of retinal bloodvessels will stand out on it. Another 

 method is to look at a dark ground while a lighted candle, held at one 

 side of the eye at a distance from the visual line, is moved slightly to 

 and fro. In the first method, a point of the sclerotic behind the lens 

 is illuminated, and rays passing from it across the interior of the eyeball 

 in every direction cast shadows of the vessels of the retina on its sensi- 



Fig. 441. Illusions of Space-Perception. 



