REFRANGIBlirrr of LIGHT. 67 



If 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. 



But if the purple fringe had appeared round the object, 

 when the eye is conformed to too fmall a diftance for feeing it 

 diftinctly, 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 

 neceffary, 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. Poflibly fuch difficulties might be 

 obviated by diluting the humours with fome mild fluid of 

 known optical properties. 



The aberration from unequal refrangibility not being cor- 

 rected in the eye, is one caufe why virion through a good tele- 

 fcope is more perfect, independent of magnifying power, than 

 naked vifion when moft perfect ; a fact which rauft appear fo 

 extraordinary, that it can fcarcely be expected to be credited, 

 except by thofe who have convinced themfelves of it by expe- 

 rience; 



In order to explain this, it mull: be obferved, that the ulti- 

 mate effect required to be produced by a telefcope or microfcope, 



I 2 is 



