THE EYE AS A SENSORY APPARATUS. 537 



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 becomes readibly perceptible. ^ 



Even with light-rays of the same oscillation periot^ur sen- 

 sation is not proportional to the amount of energy in th^ 

 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 also, || to a limit 

 beyond which it does not go, no matter how strong the light 

 becomes; but the increase of sensation takes place far more 

 slowly than that of the light, in accordance with the psycho- 

 physical law already mentioned. If we call the amount of 

 light given out by a single candle a, then that emitted by two 

 candles will be 2a; and so on. If the amount of sensation 

 excited by the single candle be A, then that due to two can- 

 dles will not be 2 A, and that by three will be far less than 3 A. 

 If a white surface, P, Fig 159, be illuminated by a candle at 

 c and another elsewhere, and a rod, a, be placed so as to in- 

 tercept the light from c, but not - Q 

 that from the other candle, we see 

 clearly a shadow, since our eyes 

 recognize the difference in luminos- 



5 



ity of this part of the paper, reflect- 

 ing light from one candle only, from 

 that of the rest which is illuminated 



by two: that is wo can tell the sensation due to the stimulus 

 ah-om that due to the stimulus 2 a.) If now a bright lamp be 

 brought in and placed alongside, and its light be physically 

 equal to that of 10 candles, we cease to perceive the shadows. 

 We find the sensation aroused by objective light = 12 (due 

 to the lamp and two candles) cannot be told from that due to 

 light = llaj although the difference of objective light is still 

 la as before. Most persons must have observed illustrations 

 of this. Sitting in a room with three lights not unfrequently 

 some object so intercepts the light from two as to cast on the 

 wall two shadows which partly overlap. Where the shadows 

 overlap the wall gets light only from the third candle; around 

 that, where each shadow is separate, it is illuminated by this 



