246 ANIMAL PHYSIOLOGY. 



This might be gathered from watching the movements of 

 infants; but it has been, more distinctly demonstrated by 

 observations made on persons born blind, who have gained 

 sight after some years, by a surgical operation. Such persons 

 see every thing at first as if close at hand, and, from not 

 understanding the effects of distance, form most erroneous 

 ideas of the sizes of objects; and they handle things when 

 they look at them, so as to compare the results of vision with 

 those of common sensation. The effects of distance on the 

 eyes, which experience teaches us to translate, are of various 

 descriptions. (1) The distance of the object looked at 

 determines the degree of convergence of the eyes; (2) it 

 determines the focus; (3) it affects the intensity of light 

 and shade, and the colour, producing what is called per- 

 spective of colour; (4) it diminishes the apparent size of 

 objects, producing linear perspective; so that when from 

 custom or otherwise the size of an object is known, its 

 distance is estimated. That the convergence of the eyes is 

 of some use in enabling us to appreciate the exact distance 

 of near objects, is illustrated by the difficulty which one has 

 in threading a needle when one eye is shut. But by a little 

 practice that difficulty is overcome, which shows that the 

 use of two eyes is not essential to judging distances. 



182. The appearances of solidity and hollowness depend 

 partly on the apparent diminution of receding objects, partly 

 on the way in which the light falls, and partly on the pictures 

 presented to the two eyes being different, and bringing into 

 view a larger amount of surface than can be seen from one 

 point. The last of these three causes is supplemented 

 materially by the other two; otherwise the appearance of 

 solidity would be lost on shutting one eye. It is, however, 

 a most important element, as is shown by the effects of the 

 stereoscope, when geometrical figures are looked at. 



I have already pointed out how it is that only one figure 

 is seen when two pictures are looked at through the stereo- 

 scope. But stereoscopic effect depends on. the circumstance 

 that no solid or hollow body presents exactly the same view- 

 to both eyes. The artist provides on the stereoscopic slide 

 two views of one object, such as would be presented in nature 

 to the two eyes; and the eyes are directed by the construction 



