ocular SpefUra of Light and Colours . 319 
that we do not fee them on all obje&s every hour of our lives. 
But as the nerves of very weak people lofe their fenfibility, in 
the fame manner as their mufcles lofe their a&ivity, by a fmall 
time of exertion, it frequently happens, that fick people in the 
extreme debility of fevers are perpetually employed in picking 
fomething from the bed-cloaths, occafioned by their miftaking 
the appearance of thefe mu fete volitantes in their eyes. Benve- 
nuto Celini, an Italian artift, a man of ftrong abilities, re- 
lates, that having paffed the whole night on a diftant mountain 
with fome companions and a conjurer, and performed many 
ceremonies to raife the devil, on their return in the morning to 
Rome, and looking up when the fun began to rife, they faw 
numerous devils run on the tops of the houfes, as they patted 
along ; fo much were the fpe&ra of their weakened eyes mag- 
nified by fear, and made fubfervient to the purpofes of fraud 
or fuperftition. (Life of Ben. Celini.) 
3. Place a fquare inch of white paper on a large piece of 
ftraw- coloured filk ; look fteadily fome time on the white 
paper, and then move the center of your eyes on the filk, and 
a fpe£trum of the form of the paper will appear on the filk, 
of a deeper yellow than the other part of it ; for the central 
part of the retina, having been fome time expofed to the ftimu- 
lus of a greater quantity of white light, is become lefs fenfible 
to a fmaller quantity of it, and therefore fees only the yellow 
rays in that part of the ftraw- coloured filk. 
Fa£ls fimilar to thefe are obfervable in other parts of our 
fyftem : thus, if one hand be made warm, and the other ex- 
pofed to the cold, and then both of them immerfed in fubtepid 
water, the water is perceived warm to one hand, and cold to 
the X)ther ; and we are not able to hear weak founds for fome 
time after we have been expofed to loud ones ; and we feel a 
chillinefs 
