6x* OCULAR SPECTRA. SECT. XL. $, 



is become lefs fenfible to a fmaller quantity of it, 

 and therefore fees only the yellow rays in that pars 

 of the ftraw-coioured lilk. 



Facts fimilar to thefe are obfervable in ottar parts 

 of our fyftem : thus* if one hand be made warm* 

 arid the other expofed to the cold, and then both 

 of them intrncffed 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 forne 

 time after we have been expofed to loud ones ; 

 and we feel a chillinefs on coming into an atmof- 

 phere of temperate warmth, after having been fome 

 time confined in a very warm room : and he,,ce the 

 ftotnaeh, and other organs of digeilion, of thofe who 

 have been habituated to the greater ftimulus of fpi- 

 fituous liquor, are not excited into their due action 

 by the left ftimulus of common food alone ; of 

 which the immediate conference is indigeftion and 

 hypochondriacifm- 



III. OF SPECTRA FROIvi EXCESS OF SENSIBILITY, 



The retina is more eafity. excited into a&'wn by greater 

 irritation after having been lately fitbjefted to lefs. 



f, IF the eyes are clofed, and covered perfectly 

 With a hat, for a minute or two, in a bright day j 

 on femoving the' hat, a 1 red or crirufon light Js feen 

 through the eyelids, la this experiment the retina, 

 after being fome time kept in the dark, becomes fo 

 fenfible to a fmall quantity of light., as to perceive 

 ciiiiinctly the greater quantity of red rays than of 

 others \vhich pafs through the eyelids* A limilar 

 toioufed light is feen to pafs through the edges of 

 the fingers, when the open hand is oppofed to the 

 fiame of a candle, 



2. If 



