538 THE HUMAN BODY. 



and one other candle; and the wall in the neighborhood of 

 the shadows by all three. Objectively, therefore, the differ- 

 ence between the deep shadow and half shadow is that 

 between the light of one candle and that of two. The differ- 

 ence between the half shadows and the wall around is that 

 between the light of two and three candles. But as .a matter 

 of sensation the difference between the half shadow and the 

 full s^low seems much greater than that between the 

 half shadow and the rest of the wall; in other words the 

 difference, a, between a and 2a, is a more efficient stimulus 

 than the same difference, a, between 2a and 3. When the 

 total stim^bs increases the same absolute difference is less 

 felt or mlP be entirely unperceived. An example of this 

 which every one will recognize is afforded by the invisibility 

 of the stars in daytime. 



On the other hand, as the total stimulus increases or de- 

 creases the same fractional difference of the whole is per- 

 ceived with the same ease; i.e., excites the same amount of 

 sensation. In reading a book by lamplight we perceive 

 clearly the difference between the amount of light reflected 

 from the black letters and the white page. If we call the 

 total lamplight reflected by the blank parts 10 and that by 

 the letters 2#, we may say we perceive with a certain distinct- 

 ness a luminous difference equal to one fifth of the whole. 

 If we now take the book into the daylight the total light re- 

 flected from the letters and the imprinted part of the page 

 increases, but in the same proportion. Say the one now is 

 50a and the other 10#; although the absolute difference be- 

 tween the two is now 40& instead of 8a we do not see the 

 letters any more plainly than before. The smallest difference 

 in luminous intensity which we can perceive is about T ^ of 

 the whole, for all the range of lights we use in carrying on 

 our ordinary occupations. For strong lights the smallest per- 

 ceptible fraction is considerably greater; finally we reach a 

 limit where no increase in brightness is felt. For weak 

 illumination the sensation is more nearly proportioned to the 

 total differences of the objective light. Thus in a dark room 

 an object reflecting all the little light that reaches it appears 

 almost twice as bright as one reflecting only half; in a 

 stronger light it would so appear. Bright objects in general 

 obscurity thus appear unnaturally bright when compared 

 with things about them, and indeed often look self-luminous. 



