450 OCULAR SPECTRA. SECT. XL. 4. i, 



fome time delineated. This pains the eye in a fimilar manner 

 as when we come fuddenly from a dark room into bright day- 

 light, and gives the appearance of bright fcintillations. Hence 

 the ftars twinkle moft when the night is darkeft, and do not 

 twinkle through telefcopes, as obferved by Mufchenbroeck j and 

 it will afterwards be feen why this twinkling is fometimes of 

 different colours when the object is very bright, as Mr. Melvill 

 obferved in looking at Sirius. For the opinions of others on 

 this fubjedt, fee Dr. Prieftley's valuable Hiftory of Light and 

 Colours, p. 494. 



Many fads 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 approaching the fire after having handled fnow ; 

 and the inflamed heels from walking in fnow. Hence thofe 

 who 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 periftied by hunger has deflroyed 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 is excited, which 

 terminates in gangrene or fever. 



IV. OF DIRECT OCULAR SPECTRA. 



A quantity of Jlimulus fomewhat greater than natural excites tht 

 retina into fpafmodic acllon y which ceafes in afeiv feconds. 



A CERTAIN duration and energy of the ftimulus of light 

 and colours excites the perfect aftion of the retina in vifion ; 

 for very quick motions are imperceptible to us, as well as very 

 flow ones, as the whirling of a top, or the fhadowon a fun-dail. 

 So perfect: darknefs does not affect the eye at all j and excefs 

 of light produces pain, not vifion. 



i. When a fire-coal is whirled round in the dark, a lucid cir- 

 cle remains a confiderable time in the eye , and that with fo 

 much vivacity of light, that it is miftaken for a continuance of 

 the irritation of the object. In the fame manner, when a fiery 

 meteory (hoots acrofs the night, it appears to leave a long lucid 

 train behind it, part of which, and perhaps fometimes the whole, 

 is owing to the continuance of the aftion of the retina after 

 having been thus vividly excited. This is beautifully illuftrated 

 by the following experiment : fix a paper fail, three or four 

 inches in diameter, and made like that of a fmoke jack, in a tube 



of 



