REFRANGIBILITY of LIGHT. 67 
Ir-the coloured penumbras, inftead of being red when the 
.eye is conformed to a greater diftance ‘than’ that of the object 
obferved, and blue when conformed to a lefs diftance, had been 
purple in the former cafe, and green in the latter, it would be 
reafonable to:conclude, that the vitreous, humour was.a difper- 
five medium of the. fame kind with effential oils, and fuch as 
owe’ this property to metalline particles with. which they are im- 
pregnated.. . 
Bur if the purple fringe had uppeared round ‘the object, 
Hilton the eye is conformed to too fmall.a diftance for feeing it 
diftin@lly, and with a green fringe under the contrary circum- 
ftances, this would indicate a difperfive power in the vitreous 
humour, fimilar to that of the muriatic acid. 
In fome animals, and particularly in_ birds of prey, the 
images of objects on the retina are required to be more perfect 
than in the human eye. It would be an object of fome mo- 
ment in comparative phyfiology, to'determine whether there be 
any partial or total correction of aberration from the difference 
of refrangibility in the eyes of thefe animals, which, if found 
neceflary, will without doubt be the cafe. In fome experiments . 
which I once attempted with the vitreous humour, I found ir- 
regularities arife in the refraction, from giving it a figure diffe- 
rent from its natural one. Poffibly fuch difficulties might be 
obviated, by diluting the humours with fome mild pad of 
known optical properties. 
Tue aberration from unequal refrangibility not being. cor- 
rected in the eye, is one caufe why vifion through a good tele- 
{cope is more perfect, independent of magnifying power, than 
naked vifion when moft perfect ; a fa&t which muft appear fo 
extraordinary, that it can fearcely be expected to be credited, 
except by thofe who have convinced themfelves of it by expe- 
rience: 
In order to explain this, it muft be obferved, that the ulti- 
mate effect required to be produced by a telefcope or microfcope, 
IT 2 is 
