232 psYcnoiJxiY. 



attention far more than the absolute quality or quantity of 

 a given sensation is its ratio to whatever other sensations 

 we may have at the same time. When everything is dark 

 a somewhat less dark sensation makes iis see an object 

 white. Helmholtz calculates that the white marble painted 

 in a picture representing an architectural view by moon- 

 light is, when seen by daylight, from ten to twenty thousand 

 times brighter than the real moonlit marble would be.* 



Such a difference as this could never have been sensibly 

 learned ; it had to be inferred from a series of indirect con- 

 siderations. There are facts which make us believe that 

 our sensibility is altering all the time, so that the same 

 object cannot easily give us the same sensation over again. 

 The eye's sensibility to light is at its maximum when the 

 eye is first exposed, and blunts itself with surprising rapid- 

 ity. A long night's sleep will make it see things twice as 

 brightly on wakening, as simple rest by closure will make 

 it see them later in the day.f "VYe feel things differently 

 according as we are sleepy or awake, hungry or full, fresh 

 or tired ; differently at night and in the morning, differently 

 in summer and in winter, and above all things differently in 

 childhood, manhood, and old age. Yet we never doubt that 

 our feelings reveal the same world, with the same sensible 

 qualities and the same sensible things occupying it. The 

 difference of the sensibility is shown best by the difference 

 of our emotion about the things from one age to another, or 

 when we are in different organic moods. What was bright 

 and exciting becomes weary, flat, and unprofitable. The 

 bird's song is tedious, the breeze is mournful, the sky is 

 sad. 



To these indirect presumptions that our sensations, fol- 

 lowing the mutations of our capacity for feeling, are always 

 undergoing an essential change, must be added another 

 presumption, based on what must happen in the brain. 

 Every sensation corresponds to some cerebral action. For 

 an identical sensation to recur it would have to occur tho 

 second time in an unmodified brain. But as this, strictly 



* Populare Wissenschaftlicbe Vortriige, Drittes Heft (1876). p. 72. 

 \ Fick, in L. Hermann's Handb. d. Physiol., Bd. in. Th. i. p. 225. 



