SECT. III. 3. i. THE RETINA. ir 



firft eroded, and the hair like fibres remained floating in the vef- 

 fel. Nor does the degree of tranfparency of the retina invali- 

 date this evidence of its fibrous ftru6lure, fince Lseuwenhoek 

 has fhewn, that the cryftalline humour itielf confilis of fibres. 

 Arc. Nat. V. I. 70. 



Hence it appears, that as the mufcles confill of larger fibres 

 intermixed with a fmaller quantity of nervous medulla, the or- 

 gan of vifion confifts of a greater quantity of nervous medulla 

 intermixed with fmaller fibres. It is probable that the locomo- 

 tive mufcles of microfcopic animals may have greater tenuity 

 than thefe of the retina , and there is reafon to conclude from 

 analogy, that the other immediate organs of fenfe, as the portio 

 mollis of the auditory nerve, and the rete mucofum of the Ikin, 

 poflefles a fimilarity of flrufture with the retina, and a fimilar 

 power of being excited into animal motion. 



III. The fubfequent articles (hew, that neither mechanical 

 impreffions, nor chemical combinations of light, but that the 

 animal activity of the retina conflitutes vifion. 



1. Much has been conjectured by philofophers about the 

 momentum of the rays of light ; to fubject this to experiment 

 a very light horizontal balance was conftructed by Mr. Michel, 

 with about an inch fquare of thin leaf copper fufpended at each 

 end of it, as defcribed' in Dr. Prieflley's Hiftory of Light and 

 Colours. The focus of a very large convex mirror was thrown 

 by Dr. Powel, in his lectures on experimental philofophy, in 

 my prefence^ on one wing of this delicate balance, and it reced- 

 ed from the light : thrown on the other wing, it approached 

 towards the light, and this repeatedly : fo that no fenfible im- 

 pulfe could be obferved, but what might be well afcribed to the. 

 afcent of heated air. 



Whence it is reafonable to conclude that the light of the day 

 muft be much too weak in its dilute (late to make any mechan- 

 ical impreflion on fo tenacious a fubftance as the retina of the 

 eye. Add to this, that as the retina is nearly tranfparent, it 

 could therefore make lefs refinance to the mechanical impulfe 

 of light ; which, according to the obfervations related by Mr. 

 Melvil, in the Edinburgh Literary EiTays, only communicates 

 heat, and mould therefore only communicate momentum, 

 where it is obflructed, reflected, or refracted. From whence 

 alfo may be collected the final caufe of this degree of tranfpar- 

 ency of the retina, viz. left by the focus of ftronger lights, heat 

 and pain mould have been produced in the retina, inftead of 

 that ftimuius which excites it into animal motion. 



2. On looking long on an area of fcarlet filk of about an inch 

 in diameter laid on white paper, as in Plate I. the fcarlet colour 



