514 THE HUMAN BODY. 



also, up to a limit beyond which it docs 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 accord- 

 ance with the psycho-physical law mentioned on page 473. 

 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 candles will not be 2.4, and that, 

 by three will be far less than 3 A. If a white surface, P, 

 Fig. 140, be illuminated by a candle at c and another else- 

 j where, and a rod, o, be placed 



so as to intercept the light 

 from c, we see clearly a shadow, 

 since our eyes recognize the 

 difference in luminosity 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 

 we tell the sensation due to the stimulus a from that due 

 to the stimulus 2a. 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 shadow s. 

 That is the sensation aroused by objective light = I2a 

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

 due to light = 11; although the difference of objective 

 light is still la as before. Most persons must have ob- 

 served 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 and one other candle; 

 and the wall in the neighborhood of the shadows by all 

 three. Objectively, therefore, the difference between the 

 deep shadow and half shadow is that between the light 

 of one candle and that of two. The difference between 

 the half shadows and the wall around is that between the 

 light of two and three candles. But as a matter of sensa- 

 tion the difference between the half shadow and the full 



