232 PSYCHOLOGY. 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 us 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 Avill make it see them later in the day.f We 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 caj)acity 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 the second time in an unmodified brain. But as ' this, strictly
- Populare Wlssenschaftliclie Vortrage, Drittes Heft (1876), p. 72
f Fick, in L. Hermann's Handb. d. Physiol., Bd. iii. Th i. p. 225.
