266 SUPPLEMENT TO THE 



annul! of equal breadth, those which are farthest removed 

 from the centre must inevitably, from their greater size, 

 reflect or refract the greatest quantity of light, (see 1, figure 

 1, and the rings, 2, 3, 4, 5) ; the outside rays, therefore, 

 are by far the most important, and their condition must 

 be particularly attended to in judging of the merit or 

 demerit of any optical instrument. The inside rays, if 

 taken very near the centre, exhibit no sensible aberration, 

 however bad the object-glass or metal may be, as, for 

 example the little disc in the centre of figure 1 , marked 1 . 



Fig. 1. 



2dly. Every perfect object-glass and metal draws every 

 pencil of light forming the image to a perfect point, see F, 

 figure 2 ; consequently the rays must be in the same state, 

 and there will be the same degree of brightness and in- 

 tensity of light, both within and without the focus, at any 

 two given points, as C and C, equidistant from the focus 

 either way.* 



3dly. Imperfect metals and object-glasses have their 

 outside rays either shorter or longer than those which 

 form the principal focus ; in which case it is evident that if 



* The reader must not suppose that I mean to assert that there is not 

 exactly the same quantity of light at any point taken from the object-glass or 

 metal, &c. to an infinite distance from it, under every possible circumstance, 

 both of aberration and of freedom from it. What is asserted relates only to 

 the distribution of the rays, or to their state of condensation or diffusion. 



