glit. 145 



describes in its progress, and a line drawn per- 

 pendicularly to the reflecting surface; and the 

 angle of reflection is the angle formed by the 

 same perpendicular and the reflected ray. Thus, 

 in fig. 47, AB is the reflecting surface, CG is 

 a line drawn perpendicularly to that surface, e is 

 a ray of light incident at G, and reflected tof; 

 and the angle CGe of incidence is evidently equal 

 to the angle CGjfof reflection. 



15. By a medium, opticians mean any thing 

 which is transparent, such as void space, air, 

 water, or glass> through which consequently the 

 rays of light either may or do pass in straight 

 lines. 



16. The refraction of the rays of light is their 

 being bent, or attracted out of their course in 

 passing obliquely from one medium to another 

 of a different density, and which causes objects 

 to appear broken or distorted when part of them 

 is seen in a different medium. It is from this 

 property of light that a stick, or an oar, which is 

 partly immersed in water, appears broken. 



17. A lens is a transparent body of a different 

 density from the surrounding medium, com- 

 monly of glass, and used by opticians to collect 

 or disperse the rays of light. Lenses are in gene- 

 ral either convex, that is, thicker in the middle 

 than at the edges, which collect and, by the force 

 of refraction,, converge the rays, and consequent- 

 ly magnify; or concave, that is, thinner in the 

 middle than at the edges, which by the refrac- 



VOL. i. H 



