SECT. XL. i. 4. OCULAR SPECTRA. 445 



fudden flafli 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 fpeftra of the 

 ambient objects continue to prefent themfelves in rotation, or 

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

 ftill in motion. 



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

 eye are not owing to the mechanical impulfe of light imprefled 

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

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

 many bodies ; for in all thefe cafes the fpetlra mud either re- 

 main uniformly, or gradually diminifh ; 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 

 flam of light or colours in the prefied eye as in the third, nor the 

 rotation or libration of the fpetlra in the fourth, could exift. 



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

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

 our limbs and veflels ; (ince it confifts of fibres, like thofe, inter- 

 mixed with its medullary fubftance. To evince this ftrufture, 

 the retina of an ox's eye was fufpended 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 diflencled till it 

 breaks , which (hews that it confifts of fibres j and its fibrous 

 conftruftion became dill more diftindt 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 veflel. 

 Nor does the degree of tranfparency of the retina invalidate the 

 evidence of its fibrous ftru&ure, fince Lceuwenhoek has (hewn 

 that the cryftalline humour itfelf confifts of fibres. ( Arcana 

 Naturae, 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 thofe 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 action by irritations, both 

 aft nearly in the fame quantity of time, are alike ftrengthened 

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



when 



