Sect. XL. 2. 3. OCULAR SPECTRA. 447 



fpectrum is fuppofed to be contiguous, but to other caufes men- 

 tioned in No. X. 3. of this fe<Sion. The apparent fee of it 

 will alfo be variable according to its fuppofed diftance. 



As thefe fpeclra are more eafily obfervable when our eyes are 

 a little weakened by fatigue, it has frequently happened, that peo- 

 ple of delicate conftitutions have been much alarmed at them, 

 fearing a beginning decay of their fight, and have thence fallen 

 into the hands of ignorant oculifts ; but I believe they never are 

 a prelude to any other difeafe of the eye, and that it is from 

 habit alone, and our want of attention to them, that we do not 

 fee them on all objects 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 activity, by a fmall time of 

 exertion, it frequently happens, that fick people in the extreme 

 debility of fevers are perpetually employed in picking fome- 

 thing from the bed-clothes, occafioned by their miftaking the 

 appearance of thefe mufca volitantes in their eyes. Benvenuto 

 Celini, an Italian artift, a man of ftrong abilities, relates, that 

 having palled the whole night on a diilant 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 dev- 

 ils run on the tops of the houfes, as they pafTed along ; fo much 

 were the fpectra of their weakened eyes magnified 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 

 flraw-coloured filk ; look fteadily fometime on the white paper, 

 and then move the centre of your eyes on the filk, and a fpec- 

 trum of the form of the paper will appear on the filk, of a deep- 

 er yellow than the other part of it : for the central part of the 

 retina, having been fome time expofed to the ftimulus of a greater 

 quantity of white light, is become lefs fenfible to a fmaller quan- 

 tity of it, and therefore fees only the yellow rays in that part of 

 the flraw-coloured filk. 



Facts fimilar to thefe are obfervable in other parts of our 

 fyftem : thus, if one hand be made warm, and the other expofed 

 to the cold, and then both of them immerfed in fubtepid water, 

 the water is perceived warm to one hand, and cold to the other •, 

 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 on com- 

 ing into an atmofphere of temperate warmth, after having been 

 fome time confined in a very warm room : and hence the itom- 

 ach, and other organs of digeliion, of thofe who have been ha- 

 bituated to the greater ftimulus of fpirituous liquor, are not ex- 

 cited 



