XL.a. OCULAR SPECTRA. / 



X. 3. of this fedtion. The apparent fize of U 

 alfo be variable according to its fuppofed diftance. 



As thefe fpe&ra are more eafily obfervable when ( 

 our eyes are a little weakened by fatigue, it has 

 frequently happened, that people of delicate confti- 

 tutions have been much alarmed at them, fearing a 

 beginning decay of their light, and have thence 

 fallen into the hands of ignorant oculifts ; but I be- 

 lieve 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 fen(i- 

 bility, 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 mif- 

 taking the appearance of thefe mufca wlitantes in 

 their eyes. Benvenuto Celini, an Italian artilt, a 

 man of ftrong abilities, relates, that having paifed 

 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 tfee 

 tops of the houfes, as they pafled 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. Placeafquare inch of white paper on a large 

 piece of ftraw-coloured filk; look fleadily fome time 

 on the white paper, and then move the centre of 

 your eyes on the filk, and a fpectrum of the form 

 of the paper will appear on the (ilk, of a deeper 

 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 



