Sect. III. 3. 1. THE RETINA. n 



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 ftructure, fince Leeuwenhoek 

 has fhewn, that the cryft aline humour itfelf confiits of fibres. 

 Arc. Nat. V. I. 70. 



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

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

 gan of vilion confiftsof a greater quantity of nervous medulla 

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

 tive mufcles of microlcopic 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 fkin, 

 poflefs a fimilarity of ftructure with the retina, and a fimilay 

 power of being excited into animal motion. 



III. The fubfequent articles mew, that neither mechanical 

 imprefiions, nor chemical combinations of light, but that the 

 animal activity of the retina conftitutes 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 cbnftructed by Mr. Michel, 

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

 end of it, as defcribed in Dr. Prieftley'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 irn- 

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

 afcent of heated air. 



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

 mult be much too weak in its dilute ftate to make any mechan- 

 ical impreilion 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 refrilance to the mechanical impulfe 

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

 Melvil in the Edinburgh Literary EfTays, only communicates 

 heat, and fhould therefore only communicate momentum, 

 where it is ohftructed, 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 fhould have been produced in the retina, inftead of 

 that itimulus 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 



becomes 





