VISUAL SENSATIONS 641 



element, the cones being absent or very few in number. Von Kries 

 has suggested that in all probability the retina is endowed with two 

 kinds of vision. 



(a) Vision by means of rods, which are colour-blind, so that on 

 stimulation by any rays of the retina a sensation of white or grey 

 is produced. The rods are chiefly excited by the more refrangible 

 rays of the spectrum, being totally unaffected by the red rays. They 

 show great power of adaptation. This form of rod vision may be 

 connected with the visual purple. In the dark-adapted eye this pig- 

 ment is found pervading the whole of the outer limbs of the rods ; it 

 rapidly fades on exposure of the eye to light, so that it must be absent 

 in the light-adapted eye. On examining its absorption spectrum we 

 find that its absorptive power is greatest for the rays in the green part 

 of the spectrum, and that it allows the red rays to pass almost without 

 absorption, i.e. it absorbs just those rays which experiment shows us 

 have the greatest effect in producing a sensation of light in the dark- 

 adapted eye. 



(b) The cones, on this view, would represent a more highly dif- 

 ferentiated apparatus of vision. They alone are present in that part 

 of the retina which we use exclusively for distinguishing the finer details 

 of surrounding objects ; they are sensitive to all colours, and when 

 stimulated by all the rays of the spectrum simultaneously give rise to a 

 sensation of white light. Their sensitiveness to illumination is, however, 

 inferior to that of the rod apparatus. According to this theory, there- 

 fore, whereas in a dim light we determine the position of surrounding 

 objects and differences in their luminosity by means of the rods, the 

 greater part of our visual impressions, including all that we obtain 

 by daylight and our knowledge of the finer visual qualities of things, 

 are brought to us by the intermediation of the cones. 



COLOUR VISION 



If a ray of white light be passed through a prism it is widened out 

 into a bright coloured band or spectrum, the red rays, which are 

 least refrangible, being at one end, and the blue rays at the other. 

 It is usual to divide the colours of the spectrum into seven red, 

 orange, green, yellow, blue, indigo, violet ; but the division is an 

 arbitrary one, and the colours shade into one another so gradually 

 that no two observers would agree exactly on the limits between 

 them. The difference of wave-length necessary to give a distinct 

 difference in colour varies according to the part of the spectrum 

 which is under observation. In the middle of the spectrum, 

 it is between 0'7 fj-fj. and 2'0 /UL/U.. At the red end a difference of 

 4'7 fj.fi is required to evoke a new quality of sensation, and at the 

 extreme end, both red and violet, there is a section of the spectrum 



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