

THE ORGANS OF SENSE. 133 



blind. It may further be assumed that the function of rhodopsin is 



tO sensitise thfi rnrl^ and aO_ P^fjjTfLtihflTn f" nrA -ri*oh1n i-n fhn woa.Tr 



stimuli for which they.. are. adapted. 



Adaptation. If two persons enter a room in which there is a 

 moderate degree of light, one from bright daylight, the other after 

 being for some time in a dark room, the former will experience a 

 sensation of comparative darkness, whereas the latter may be dazzled. 

 The eyes in the one case are light-adapted, in the other dark-adapted, 

 and the condition of adaptation determines the degree of sensation 

 produced by the stimulus. In the same way, if one goes out of doors 

 from a lighted room at night, at first one must grope one's way, but 

 objects gradually become more distinct as the eyes become adapted to the 

 darkness. After ten minutes the retina is twenty-five times as sensitive 

 as it was on first leaving the bright light. Further, the dark-adapted 

 eye is colour-blind, and must become light-adapted once again before 

 colours can be recognised. 



VISUAL SENSATIONS. 



Although the sensation of light can be excited by various forms 

 of stimulus applied to the eye, for example by a blow, the adequate 

 stimulus consists of the waves in the ether which emanate from 

 luminous bodies. The sensation of white light results from a com- 

 pound stimulation, for white light can be dispersed into a series of 

 rays of differing wave-length by passing it through a prism, each 

 particular wave-length giving rise to a different quality of sensation 

 known as colour. The dispersed rays constitute the spectrum of white 

 light, and only some of these act as an adequate stimulus to the retina. 

 The visible rays range from those which give rise to the sensation of 

 red, with a wave-length of 760 millionths of a millimetre, through 

 orange, yellow, green, blue, indigo to violet, with a wave-length of 

 397 millionths of a millimetre. The ultra-red and ultra-violet rays 

 do not excite any visual sensation. 



The images formed upon the retina are merely records of light, 

 shade, and possibly colour. Light and shade are due to varying 

 intensity of white light. The fact that certain objects reflect only 

 particular coloured rays depends upon their property of absorbing 

 the rays which they do not reflect. Thus grass absorbs the rays from 

 both ends of the spectrum and reflects those of the middle, while a 

 scarlet poppy absorbs all the rays of short wave-length, reflecting only 

 the longer waves of the red end of the spectrum. 



The invisibility of the ultra-violet rays, or at least of a certain 

 number of them, is due to the fact that they are "absorbed by the 



