Chap, i.] Opinions of Ariftotle^ P^oger Bacon, &c. 157 



occafioned by the reflexion of the fun's beams in differ- 

 ent circumftances. We have reafon to believe, that the 



ufe 



12. Plane mirrors or fpeculums are thofe reflecting bodies, the 

 furfaces of which are perfectly plain or even, fuch as our common 

 looking-gla'fles. Convex and concave mirrors are thofe the fur- 

 faces of which are curved. 



j 3. An incident ray is that which comes from any body to the 

 reflecling furface ; the rejttQidraj is that which is fent back or re- 

 flected. 



14. The angle of incidence is the angle which is formed by the 

 line which the incident ray defcribes in its progrefs, and a line 

 drawn perpendicularly to the reflecting furface; and the angle of 

 reflection is the angle formed by the fame perpendicular and the 



a b 



V/ 



reflected ray, thus, * c ; where a is the angle of incidence, 

 b the angle of refl p clion, and c the reflecting furface. 



15. By a medium opticians mean any thing which is tranfparent, 

 fuch as void fpace, air, water, or glafs, through which confe- 

 quently the rays of light can pafs in itrait lines. 



1 6. The refrattien of the rays of light is their being bent, or 

 attradlad out of their courfe in paffirtg obliquely from one medium 

 to another of a different denfity, and which caufes objects to ap- 

 pear broken or diftorted when part of them is feen in a different 

 Kiedium. It is from this property of light, that a ftick or an oar 

 which is partly immerfed in water Appears broken. 



17. A lens is a tranfparent body of a different density from the 

 furrounding medium, commonly of glafs, and ufed by opticians to 

 collect or difperfe the rays of light. They are ingeneral either con- 

 vex; that is, thicker in the middle than at the edges, which colled 

 and by the force of refraftion converge the rays, and confequently 

 magnify ; or concave, that is, thinner in the middle than at the 

 edges, which by the refraction difperfe the rays of light, and dimi- 

 nifh the objedls that are feen through them. 



1 8. Vifion is performed by a contrivance of th : .s kind. The 

 cryllalline humour, which is feated in the fore part of the hu- 

 man eye, immediately behind the pupil, is a perfedl convex 

 lens. As therefore every objeft is rendered vifible by beams 

 or pencils of light which proceed or diverge from eve*y radi- 

 ant point of the objeft, the cryftalline lens collects all thefe di- 

 vergent rays, and caufes them to converge on the back part of 

 the eye, where the retina or optic nerve is fpread out ; and the 

 points where each pencil of rays is made to converge on the re- 

 tina. 



