?o MOTIONS OF THE RETINA. SECT. HI, 5. 



of light, but that the animal adivity of the retina 

 conftitutes vifion. 



i. Much has been conjectured by philosophers 

 about the momentum of the rays of light ; to fubjecl: 

 this to experiment a very light horizontal balance 

 was conftrucled 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 phiiofophy, in my prefence, on one 

 wing of this delicate balance, and it receded from 

 the light ; thrown on the other wing, it approached 

 towards the light, and this repeatedly ; fo that no 

 fenlible impulie 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 muft be much too weak in its dilute 

 ftate to make any mechanical impreflion on fo tena- 

 cious 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 im- 

 pulfe of light;- which, according to the obfervati- 

 ons related by Mr. Melvil in the Edinburgh Literar 

 ry Effays, only communicates heat, and fhould there- 

 fore only communicate momentum, where it is ob- 

 ftrucled, reflected, or refracted. -From whence alfo 

 niay be collected the final caufe of this degree of 

 tranfparency of the retina, viz. left by the focus of 

 flronger lights, heat and pain mould have been pro- 

 duced in the retina, inftead of that ftimulus 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 fainter, till at 

 length it entirely vanifhes, though the eye is kept 

 uniformly and tieadily upon it. Now if the change 

 $f motipn of the retina was a mechanical imprcffi. 



