162 fycbo Brahe> &c. [Book IIL 



advance no nearer the truth than the obfervation, that 

 when the incident ray does not make an angle of more 

 than thirty degrees with the perpendicular, the refract- 

 ed ray proceeds in an angle which is about two-thirds 

 of it. Many difputes arofe about the time of Kepler 

 (i6t>o) upon this fubject, but it appears that little was 

 effected by them in the caufe of truth. 



Kepler was more fuccefsful in purfuing the difco- 

 veries of Maurolycus and B. Porta. He demonftrated 

 that images of external objects were formed upon the 

 optic nerve by the foci of rays coming from every 

 part of the object; he alfo obferved, that thefe images 

 are inverted ; but this circumilance, he fays, is recti- 

 fied by the mind, which, when an imprefllon is made 

 on the lov/er part of the retina, confiders it as made 

 by rays proceeding from the higher parts of the object. 

 Habit is fuppofed to reconcile us to this deception, 

 and to teach us to direct our hands to thofe parts of 

 objects from which the rays proceed. Tycho Brahe, 

 obferving the apparent diminution of the moon's difk 

 in folar eclipfes, imagined that there was a real dimi- 

 nution of the difk by the force of the fun's rays j but 

 Kepler faid, that the difk of the moon does not ap- 

 pear lefs in confequence of being unenlightened, but 

 rather that it appears at ether times larger than it 

 really is, in confequence of its being enlightened. For 

 pencils of rays from fnch diftant objects generally come 

 to their foci before they reach the retina, and confe- 

 quently diverge and fpread when they reach it. For 

 this reafon, he adds, different perfons may imagine the 

 difk to be of different magnitudes, according to the; 

 relative goodnefs of their fight. 



In the fixteenth century alfo many improvements 



were made in perfpective j the ingenious- device, in 



particular, of the reformation of diftorted images by 



2 concave 



