INTENSITY OF VISUAL SENSATIONS. 515 



shadow seems much greater than that between the half 

 shadow and the rest of the wall; in other words the differ- 

 ence, , between a and 2a, is a more efficient stimulus than 

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

 total stimulus increases the same absolute difference is less 

 lelt or may 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 Wa and that 

 by the letters 2a, we may say we perceive with a' certain 

 distinctness a luminous difference equal to one fifth of 

 the whole. If we now take the book into the daylight the 

 total light reflected from both the letters and the imprinted 

 part of the page increases, but in the same proportion. Say 

 the one now is 5Qa and the other 10; although the 

 absolute difference between the two is now 40# instead of 

 Sa we do not see the letters any more plainly than before. 

 The smallest difference in luminous intensity which we 

 can perceive is about T ^-<y of the whole, for all the range of 

 lights we use in carrying on our ordinary occupations. 

 For strong lights the smallest perceptible fraction is con- 

 siderably 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; which in a stronger 

 light it would not do. Bright objects in general obscurity 

 thus appear unnaturally bright when compared with things 

 about them, and indeed often look self-luminous. A cat's 

 eyes, for example, are said to "shine in the dark;" and 

 painters to produce moonlight effects always make the 

 bright parts of a picture relatively brighter, when compared 



