S*CT. XL. 3. OCULAR SPECTRA. 61$ 



The twinkling of a bright ftar, or of a diftant 

 candle in the night, is perhaps owing to the lame 

 caufe. While we continue to look upon thcfe lu- 

 minous objects, their central parts gradually appear 

 paler, owing to the decreafing fenfibili.y of ihe part 

 of ihe retina expofcd to their light ; whilft, at ihe 

 fame time, by the unfteadinefs of the eye, the edges 

 of them are perj etually falling on parts of the re- 

 tina that were jufl before cxpofed to the darknefs of 

 the uighr, and therefore tenfold more fenfible to light 

 than the fart on which the i at* or candle had I cen for 

 fome time delineated. 1 his pains the eye in a fimilar 

 manner as when we come fuddcnly from a dark room 

 into bright day-light, and gives the appearance of 

 bright fcintillations. Hence the flars twinkle moft 

 when ihe night is darkeft, and do not twinkle through 

 telefcopes, as cbferved by Muficherbroeck ; and it 

 will afterwards be feen why this twinkling is fome- 

 times of different colours when the object is very 

 bright, as Mr. Melvill obferved in looking at Sirius. 

 For the opinions of others on this fubjecl, fee Dr. 

 Prieftley's valuable Hiftory of Light and Colours, 

 p. 494. 



Many facls obfervable in the animal fyftem are 

 fimilar to thefe ; as the hot glow occafioned by the 

 ufual warmth of the air, or our clothes, on coming 

 out of a cold bath ; the pain of the fingers on ap- 

 proaching the fire after having handled fnow; and 

 the inflamed heels from walking in fnow. Hence 

 thofe v\ho have been expofed to much cold have 

 died on being brought to a fire, or their limbs have 

 become fo much inflamed as to mortify. Hence 

 much food or wine given fuddenly to thofe who have 

 almoft perifhed by hunger has deftroyed them ; for 

 all the organs of the famifhed body are now become 

 fo much more irritable to the ftimulus of food and 

 wine, which they have long been deprived of, that 



inflammation 



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