614 Q GUI, A 11 SPECTRA. Stcr P XL. j, 



this experiment be made by the light of a tallow 

 candle, the ipot will be yellow in (lead of red ; fur 

 tallow candles abound much' with yellow light, 

 which pafles in greater quantity and force through 

 the eyelids than blue light ; hence the difficulty of 

 diflinguifhing blue and green by this kind of candle 

 light. 1 he colour of the fpeclrtim may poffibly 

 vary in the daylight, according to the different co- 

 Iptir of the meridian or the morning or evening 

 light. 



M. Bcguelin, in the Berlin Memoires, V. IL 

 1771, obie-rves, that, when he held a book, fo that 

 the fun fhone upon his half-clofed eyelids, the 

 black letters, \vhich he had long infpedlcd, became 

 red, which mud have been thus occaiioncd- 1 hole 

 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 

 paffed through the eyelids, was perceptible. There 

 is a finiilar (lory told, 1 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, afier looking for a minute on the 

 black fpots of a die, and carelefsly clpfing 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 

 liave long continued, the light of a bright day be- 

 comes intolerable to the eye fora confiderable time, 

 owing to the excefs of fenfibility exifting in the eye, 

 af.er having been long expofed to little or no ftimuT 

 lus. This occafions us immediately to contract the 

 iris to its fmalleft aperture, which becomes again 

 gradually dilated, as the retina becomes accuflomed 

 to the greater ftimulus of the daylight. 



The 



