Sect. XL. 3. f. OCULAR SPECTRA. 440 



of the form of the black fpot : for that part of the retina, on which 

 the black fpot was delineated, being now more fenfible to light 

 than the other parts of it, which were expofed to the white pa- 

 per, is capable of perceiving the red rays which penetrate the 

 eyelids. If this experiment be made by the light of a tallow 

 candle, the fpot will be yellow inftead of red ; for tallow can- 

 dles abound much with yellow light, which paries in greater 

 quantity and force through the eyelids than blue light ; hence 

 the difficulty of diftinguifhing blue and green by this kind of 

 candle light. The colour of the fpectrum may poffibly vary in 

 the daylighf, according to the different colour of the meridian 

 or the morning or evening light. 



M. Beguelin, in the Berlin Memoires, V. II. 1771, obferves, 

 that when he held a book fo that the fun (hone upon his haif- 

 clofed eyelids, the black letters, which he had long infpedted, 

 became red, which mull have been thus occafioned. Thofe 

 parts of the retina which had received for fome time the black 

 letters, were fo much more fenfible than thofe parts which had 

 been oppofed to the white paper, that to the former the red 

 light, which palled through the eyelids, was perceptible. There 

 is a fimilar ftory told, I think, in M. de Voltaire's Hiftorical 

 Works, of a Duke of Tufcany, who was playing at dice with 

 the general of a foreign army, and, believing he faw bloody 

 fpots upon the dice, portended dreadful events, and retired in 

 confufion. The obferver, after looking for a minute on the 

 black fpots of a die, and carelefsly clofing his eyes, on a bright 

 day, would fee the image of a die with red fpots upon it, as 

 above explained. 



5. On emerging from a dark cavern, where we have long 

 continued, the light of a bright day becomes intolerable to the 

 eye for a confiderable time, owing to the excefs of fenfibility ex- 

 ifting in the eye, after having been long expofed to little or no 

 ftimulus. This occafions us immediately to contract the iris to 

 its fmalleft aperture, which becomes again gradually dilated, as 

 the retina becomes accuftomed to the greater ftimulus of the 

 daylight. 



The twinkling of a bright ftar, or of a diftant candle in the 

 night, is perhaps owing to the fame caufe. While we continue 

 to look upon thefe luminous objects, their central parts gradu- 

 ally appear paler, owing to the decreafmg fenfibility of the part 

 of the retina expofed to their light ; whilft, at the fame time, 

 by the unfteadinefs of the eye, the edges of them are perpetually 

 falling on parts of the retina that were juft before expo Ted to. 

 the darkneis of the night, and therefore tenfold more fenfible to 

 light than the part on which the ftar cr candle had b^en for 



Vol. I. K k k fom* 



