optics. 265 



will be larger than that of C D. And hence it is 

 that objects appear to diminish in proportion to 

 their distance. The angle made at the eye, by rays 

 from the utmost extremities of a body, is called 

 the visual angle. 



Parallel rays falling upon a convex mirror are 

 rendered divergent; which will be obvious, by con- 

 sidering that each ray is reflected so as to make 

 the angle of incidence and reflection equal. Let 

 A B (Plate 26. fig. 5.) be a convex mirror, being 

 part of a sphere whose centre is C ; then the line 

 C D, being a radius of the sphere, is perpendicular 

 to the surface of the mirror at C. Let F C and 

 F B be two parallel rays falling upon the convex 

 mirror: the ray E C will be reflected in the di- 

 rection C G, making the angle D C G equal to 

 E C D; and the ray F B will be reflected in the 

 direction B H, making the angle I B H equal to 

 FBI. But G C and H B, if continued, would 

 meet in K, and consequently form an angle, and 

 have become divergent. In the same manner it 

 may be shown, that divergent rays falling upon a 

 mirror are rendered still more divergent, and 

 convergent rays are rendered parallel, or less 

 convergent. 



When parallel rays, as dfa, Cm b f e I c, (Plate 

 13. fig. 2.), fall upon a concave mirror, A B, they 

 will be reflected back from that mirror, and meet 

 in a point m, at half the distance of the surface of 

 the mirror from C, the centre of its concavity. 

 Thus, let C be the centre of concavity of the 

 mirror A b B, and let the parallel rays dfa, C m b, 

 and e I c, fall upon it at the points a b and c. 

 Draw the lines C i a, C m b, and Che from the 

 centre C to these points: these lines will be per- 

 pendicular to the surface of the mirror, because 



