VISION PURPLE. 513 



within their reach; and they can also distinguish colors, or 

 at least some colors, as green. Moreover, the vision purple 

 is only found in the outer segments of the rods; there is 

 none in the cones, and yet these alone exist in the yellow 

 spot of the human eye, which is the seat of most acute 

 vision; and animals, such as snakes, which have only cones 

 in the retina, possess no vision purple and nevertheless see 

 yery well. 



It may be that other bodies exist in the retina which are 

 also chemically changed by light, but the changes of which 

 are not accompanied by alterations in color which we can 

 ^ee; and in the absence of the vision purple seeing might be 

 carried on by means of these, which may be less quickly 

 destroyed by light and so still persist in the bleached 

 retinas of the frogs above mentioned. For the present, 

 however, the question of the part, if any, played in vision 

 by such bodies must be left an open one. 



The Intensity of Visual Sensations. Light considered 

 as a form of energy may vary in quantity; physiologically, 

 also, we distinguish quantitative differences in light as 

 degrees of brightness, but the connection between the in- 

 tensity of the sensation excited and the quantity of energy 

 represented by the stimulating light is not a direct one. 

 In the first place some rays excite our visual apparatus 

 more powerfully than others: a given amount of energy in 

 the form of yellow light, for example, causes more powerful 

 yisual sensations than the same quantity of energy in the 

 form of violet light; and ultra-violet rays only become 

 visible, and then very faintly, when all others are suppressed; 

 but if they be passed through some fluorescent substance 

 (see Physics), such as an acid solution of quinine sulphate, 

 which, without altering the amount of energy, turns it into 

 ethereal oscillations of a longer period, then the light be- 

 comes readibly perceptible. 



Even with light-rays of the same oscillation period our 

 sensation is not proportional to the amount of energy in 

 the light; to the amount of heat, for example, to which it 

 would give rise if all transformed into it. If objective 

 light increase gradually in amount our sensation increases 



