318 T11E ANIMAL KINGDOM. 



the sense of vision. By the sense of sight we recognise 

 all forms, colours, and gradations of light and shade, and 

 also (assisted by the expe- 

 rience gained by the sense 

 of touch) the qualities of 

 surface, such as roughness, 

 smoothness, &c. ; for in- 

 stance, we know by touch 

 that a piece of glass is smooth, 

 that a piece of velvet has 



-I-/V. * , i j ( ,1 FIG. 84. FROST VIE"W OP EYE 



a different kind of smooth- 

 ness, &c., and the mode in which these surfaces reflect the 

 light, teaches us to recognise them so that we could predict 

 from touch what appearance an object would have, or by 

 the look how it would feel. Our knowledge of the solidity 

 of bodies is known to the sense of sight by (unconsciously) 

 looking at them with each eye alternately, so as to see to a 

 certain extent round them, or somewhat on either side 

 alternately. By this fact the principle of the stereoscope is 

 explained, the pictures looked at through it, consist of two 

 views of the same object, taken at points of sight a short 

 distance apart (corresponding to the distance between the 

 eyes) and they give us, when steadily looked at, the same 

 result that real objects do when looked at with each eye, 

 alternately. It is a curious fact, but one which has been 

 clearly demonstrated by optical experiments, that the 

 picture represented on the "retina" is inverted, but from 

 habit we refer things to their natural position, and how 

 naturally and easily this is done, may be ascertained by 

 holding the head down close to the floor, and looking at 

 objects through one's legs, in this position they are all 

 inverted ; but we naturally make ourselves satisfied that 

 they are still as they were before, so that they really do 

 not appear to be inverted. The impression formed by 

 objects upon the retina is not immediately destroyed upon 

 their removal, but remains for an instant permanent ; this 

 gives rise to a great many curious phenomena, among which 

 the most familiar is the circle of light produced by the 

 rapid evolution of any luminous body, as the red-hot end of 



