Chap. i>] Defearfes, Scheiner, csV. . 163 



concave or convex fpeculums was invented, but it is 

 uncertain by whom. 



The true law of refraction was difcovered by Snel- 

 lius, the mathematical profeflbr :it Leyden j but not 

 living to complete it, the difcovery was publimed and 

 explained by Profeflbr Hortenfius. Some difcoveries 

 of lefier imporrance were made at this time, among 

 others by Defcartes, who very clearly explained the 

 nature and caufe of the figure of the rainbow, though 

 he was able to give no account of the colours j he 

 however confidered the fmall portion of water, at which 

 the ray ifiues, as having the effect of a prifm, which 

 was known to have the property of exhibiting the 

 light, tranfmitted through it, coloured. 



In 1625, the curious difcovery of Scheiner was pub- 

 lifhed at Rome, which afcertains the fact, that viiion 

 depends upon the images of external objects upon the 

 retina. For taking the eye of an animal, and cutting 

 away the coats of the back part, and prefenting diffe- 

 rent objects before it, he.difp)ayed their images diftinct- 

 ly painted on the naked retina or optic nerve. The 

 fame philofopher demonftrated by experiment, that 

 the pupil of the eye is enlarged in order to view re- 

 mote objects, and contracted when we view thofe which 

 are near. He fhewed, that the rays proceeding from 

 any object, and palling through a fmall hole in a pafte- 

 board, crofs one another before they enter the eye$ for 

 if the edge of a knife is held on the fide next the eye, 

 and is moved along till it in part covers the hole, it 

 will firft conceal from the eye that part of the object 

 which is fituated on the oppofite fide of the hole. 



Towards the middle of the feventeenth century the 



velocity of light was difcovered by fome members of 



the Royal Academy of Sciences at Paris, particularly 



Cafmi and Roemer, by obferving the eclipfes of Ju- 



M 2 piter's 



