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of the instrument, so that nearly similar parts of the pictures 

 fall on identical points of the retina. In looking at a solid 

 object, the portions of its sides brought into view are seen 

 to a greater extent by the eye on the same side than by the 

 other; but in a hollow object, they produce the broader image 

 in the eye on the opposite side ; and thus it happens that if 

 the stereoscopic views of a solid body be clipped separate, 

 raid each be placed in the instrument in the position which 

 was intended for the other figure, it is made to appear as 

 a hollow. This effect can be obtained in perfection with 

 geometrical figures without shading; but is aided by the 

 reversal of the shading when an irregular figure is looked at. 

 By means of another instrument, the pseudoscope, the rays 

 coming from actual objects are directed in such a manner 

 that the image which should be presented to one eye is made 

 to fall on the other, and by this means raised objects seem as 

 if hollow, and vice versd. But the important part which 

 experience plays in giving the idea of solidity and hollow- 

 ness, is shown by the circumstance that neither with stereo- 

 scope nor pseudoscope can the reversal of appearance, or 

 conversion of relief, as it is termed, be obtained when the 

 objects looked at are of a complex description, and so appeal 

 to our associations that they cannot be conceived otherwise 

 than as they really are.. 



183. It has already been pointed out that impressions on 

 the retina have a certain tendency to diffusion; they have 

 likewise a tendency to endure after removal of the stimulus, 

 particularly if this have been applied with much intensity, 

 or for a length of time. After gazing at the sun, or any 

 bright light, a spectrum or coloured figure remains before the 

 eye for some time; and any object, whether brilliant or not, 

 if moved rapidly, can be shown to leave impressions on the 

 retina which endure after cessation of the stimulus. When 

 a wheel of a carriage is in motion, the spokes become indistin- 

 guishable one from another, and a dull tinge of their colour, 

 brightest near the nave, is diffused round about; because the 

 spokes each affect the retina in every part of their revolution, 

 the impression made by one at any part being immediately 

 succeeded by another, and the spokes are placed most closely 

 together near the centre, This imperfection of vision would 



