98 EXPERIMENTS, &c. 



now have their focuses in the retina, the rays 

 of which would have crossed each other, before 

 they fell upon it, had both the axes been 

 directed to the object. 



3. Spectacles were long employed, before the 

 manner in which they assisted sight was known. 

 About the year 1601, this was proposed as the 

 subject of a question to Kepler,* by his prin- 

 cipal patron at that time, Ludovie L. B. a Die- 

 trickstein, a learned nobleman of Austria. The 

 first answer he gave was, that convex glasses 

 were of use, by occasioning objects to appear 

 larger. But his patron observed, that if ob- 

 jects were rendered by them more distinct, be- 

 cause larger, no person would be benefited by 

 concave glasses, since these diminish objects. It 

 was not till three years after, that, in conse- 

 quence of finding out in what manner vision is 

 performed, he was able to give a just solution 

 of this problem, though his attention had been 

 directed to it during the whole of that interval. 

 According to the discovery he then made, con- 

 vex glasses were said by him to assist the sight 

 of presbytic persons, by so altering the directions 

 of rays diverging from a near object, that they 

 shall afterward fall upon the eye, as if they 

 had proceeded from a more remote one 5 and 



* Paralipomena in Vitellionem, p. 200. 



