Chap, i.] theory ofjhort and imperfect Vifwn. 



which he demonftrates, that the cryftalline humour of 

 die eye is a lens, which collects the rays of light pro- 

 ceeding from external objects, and throws them on the 

 retina, or optic nerve. From this principle he was 

 led to difcover the reafon of what are called fhort and 

 imperfect fight, In the one cafe, the rays converge 

 too foon ; in the other, they do not converge foon 

 enough. Hence fhort-fighted perfons are relieved by 

 a concave glafs, which caufes the rays to diverge in 

 fome degree before they enter the eye, arid renders it 

 more difficult for them to converge fo faft as they 

 would have done after entering the cryftalline hu- 

 mour ; hence, too, he proves that a convex lens is of 

 ufe to perfons who have weak, but long fight, by 

 caufing the rays to converge fooner, and in a greater 

 quantity, than would otherwife happen. He was the 

 firft, alib, that folved a problem, which had caufed 

 much perplexity in the ancient fchools, refpecting the 

 fun's image appearing round, though the rays that 

 form it are tranfmitted into a dark room through an 

 angular aperture. He confidered, that as the rays of 

 light are conftantly proceeding, in every direction, 

 from every part of the fun's difk *, " they muft be 

 crofiing each other from the extreme part of it in 

 every point of the aperture ; fo that every Tuch point 

 will be the apex of two cones, of which the bafe of the 

 one is the fun's difk, and that of the other his image 

 on the oppofite wall." The whole image, therefore, 

 confifts of a number of images, all of which are cir-r 

 cular j the image of the fun formed of thofe images 

 muft be circular alfo ; and it will approach .the nearer 

 a perfect circle, the fmaller the aperture, and the more 

 diftant the image. 



Nearly about the fame time, Johannes Baptifta 



* The face of the fun. 



Porta, 



