ON LIGHT AND COLOURS. 



137 



PROPOSITION V. 



The disposition impressed upon the rays, whether to be 

 easily deflected or easily inflected by a second bending body, 

 is strongest nearest the first bending body, and decreases as 

 the distance between the two bodies increases. 



Fig. 11. Let A B = a be the distance between the 

 first bending body and a given point, more or less arbi- 



Eg.11. 



trarily assumed ; P the second body ; A P = x ; P M = y, 

 the force exerted by the second body at P ; C = the chart ; 

 P M = y is in some inverse proportion to A P, but not as 



or , because it is not infinite at A, but of an assign- 



--~i- i- 3C 



able value there; therefore y = -. ; and the curve 



(a + ar)-' 



which is the locus of P has an asymptote at B, when 

 x = a. The fringes being received on the chart at C, 

 it might be supposed that the difference in their breadth, 

 by which I measure the force, or y, is owing to P ap- 

 proaching the chart C, in proportion as it recedes from A, 

 and thus making the divergence less in the same proportion ; 

 but the experiments are wholly at variance with this sup- 

 position. 



Exp. 1. The following table is the result of one such 

 experiment. The first column contains the distances hori- 

 zontally of P from A, being the sines of the angle made by 

 the rays with the vertical edges ; the second column contains 

 the real distance of the second from the first edge, the secant 

 of that angle ; the third column gives the breadths of the 

 fringes at the distances given in the preceding columns ; the 



