Sect, XL. i. 4, OCULAR SPECTRA. 445 



fudden fiafh of light is excited in the eye by a ftroke on it. 

 (Newton's Opt, Q^i6.) 



4. When any one turns round rapidly on one foot, till he 

 becomes dizzy, and falls upon the ground, the fpeclra of the 

 ambient objects continue to prefent themielves in rotation, or 

 appear to librate, and he feems to behold them for fome time 

 ft ill in motion. 



From all thefe experiments it appears, that the fpeclra in the 

 eye are not owing to the mechanical impulfe of light imprefTed 

 on the retina, nor to its chemical combination with that organ, 

 nor to the abforption and emiilion of light, as is obferved in 

 many bodies ; for in all thefe cafes the fpeotra muft either re- 

 main uniformly, or gradually diminim ; and neither their alter- 

 nate prefence and evanefcence as in the firft experiment, nor 

 the perpetual changes of their colours as in the fecond, nor the 

 flaih of light or colours in the prelTed eye as in the third, nor the 

 rotation or libration of the fpeclra as in the fourth, could exiifc. 



It is not abfurd to conceive, that the retina may be (limulated 

 into motion, as well as the red and white mufcles which form 

 our limbs and veffcls ; fince it confifts of fibres, like thofe,. inter- 

 mixed with its medullary fubftance. To evince this ftruclure, 

 the retina of an ox's eye was lufpended in a glafs of warm 

 water, and forcibly torn in a few places ; the edges of thefe 

 parts appeared jagged and hairy, and did not contract, and be- 

 come fmooth like fimple mucus, when it is diilended till it 

 breaks ; which {hews that it confifts of fibres : and its fibrous 

 construction became {till more diftimft. to the fight, by adding 

 fome cauftic alkali to the water, as the adhering mucus was firit 

 eroded, and the hair-like fibres remained floating in the vcflel. 

 Nor does the degree of tranfparency of the retina invalidate the 

 evidence of its fibrous Structure, fince Leeuwenhoek has (hewn 

 that the cryftalline humour itfelf confifts of fibres. (Arcana 

 Natune, Vol. I. p. 70.) 



Hence it appears, that as the mufcles have larger fibres inter- 

 mixed with a fmaller quantity of nervous medulla, the organ of 

 vifion has a greater quantity of nervous medulla intermixed with 

 fmaller fibres -, and it is probable that the locomotive mufcles, 

 as well as the vafcular ones, of microfcopic animals have much 

 greater tenuity than thefe of the retina. 



And befides the fimilar laws, which will be (hewn in this 

 paper to govern alike the actions of the retina and of the muf- 

 cles, there are many other analogies which exift between them. 

 They are both originally excited into a&ion by irritations, both 

 a£t, nearly in the fame quantity of time, are alike itrengrhened 

 or fatigued by exertion, are alike painful if excited into action 



when 



