185 On the Catoptrical and Dioptrical 



magnified. And thus the objects will appear as clear arid 

 well defined, as it' viewed through a good telescope. 



15. " Secondly, If the eye of the observer be placed ex- 

 actly in the focus, or very near it, he will only behold a 

 very confused appearance, without distinguishing the objects 

 at all. 



16. S( Thirdly, If the observer stand further from the 

 mirror than the focus, so that his eye be distant from the 

 focus about as far as he holds a book when he reads with 

 ease, then, I say, that whatever be the state of his sight, 

 long or short, he will see the objects much magnified, with 

 very great clearness and distinctness, and as well as if he 

 viewed them with the best telescope. In this case, how- 

 ever, the objects will be inverted. — I have established 

 these facts by many experiments, some of which I shall 

 now describe. 



17. " Expcr. 1st. I took a concave glass mirror, which 

 was silvered on the convex side, its diameter being some- 

 what more than 10 inches (above 10 and 7-10ths English 

 inches), and its focal distance about 22 feet. Standing be- 

 fore this mirror, in the manner I have described, I saw 

 clearly and distinctly all the distant objects opposite to the 

 mirror. I must observe that this glass was coarsely po- 

 lished, and of a bad material j which very considerably di- 

 minished the effect. This mirror was also too small. It is 

 proper to add here, that mirrors of metal, well polished and 

 accurately wrought, are better for this purpose, on many 

 accounts, than those of glass. 



hS. u Exper. <2d. 1 took a glass lens, both whose 

 sides were of equal convexity, each side being a portion of 

 a sphere, whose diameter was about 44 feet. It was not 

 silvered, and was six inches and a half in diameter. Its 

 focus, made by reflection of the rays, which after passing 

 through the first surface were turned back by the second, 

 was distant about five feet and a half. 



19. " Having placed this glass in the situation described 

 in the preceding experiment, in order to view the objects 

 by reflection from the second surface, as if it had been sil- 

 vered, I saw them with much clearness and distinctness. 



20. " It is easy to conceive that the number of rays re- 

 flected by the lens in this situation must have been very 

 small, in comparison to what it would have reflected had it 

 been silvered ; and consequently in this last case, it would 

 have produced a much greater effect. Hence it appears 

 farther, that a concave metal, well polished, of a regular 

 figuie, and of the. same focal distance, would produce a very 



good 



