608 OCULAR SPECTRA. SECT. XL. i, 



light is excited in the eye by a ftroke on it. (New- 

 ton's Opt. Q.^16.) 



4. When any one turns round rapidly on one 

 foot, till he becomes dizzy, and falls upon the 

 ground, the fpe&ra of the ambient objects continue 

 to prefent themfelves in rotation, or appear to li- 

 brate, and he feems to behold them for fome time 

 ftill in motion. 



From all thefe experiments it appears, that the 

 fpe&ra in the eye are not owing to the mechanical 

 impulfe of light impreffed 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 fpectra muft 

 either remain uniformly, or gradually diminifh; and 

 neither their alternate prefence and evanefcence as 

 in the firft experiment, nor the perpetual changes of 

 their colours as in the fecond, nor the flafh of light 

 or colours in the prefTed eye as in the third, nor 

 the rotation or libration of the fpeclra as in the 

 fourth, could exift. 



It is noi abfurd to conceive, that the retina may 

 be ftimulated into motion, as well as the red and 

 white mufcles which ferm our limbs and veffels ; 

 iince it conlifts of fibres, like thofe, intermixed with 

 its medullary fubftance. To evince this ftru&ure, 

 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 become fmooth like fimple 

 mucus, when it is diftended till it breaks ; which 

 fhews that it confifts of fibres ; and that its fibrous 

 conduction became Hill more diftindt to the fight, 

 by adding fome cauftic alkali to the water, as the 

 adhering mucus was firft eroded, and the hair-like 

 fibres remained floating in the veflel. Nor docs the 

 degree of tranfparency of the retina invalidate the 

 evidence of its fibrous ftrudure, fince Leeuwenhoek 



has 



