Notes on some Points in the Theory of Light. 215 



cidence ; while for all less values of the refractive index the in- 

 tensity will be least at the perpendicular incidence. 



Let i and i' be the angles of incidence and refraction, and 

 put" 



, _ sin i cos i 



M = r,, a = r, J 



sm ^ cos i 



then if / be the intensity of the reflected light, when common 

 light is incident, FresnePs expression 



= JL I sin2 (* " tan 2 (t - Q J 

 ? ( sm^ + + tan 2 (t + f) )' 



in which the intensity of the incident light is taken for unity, 

 may be put under the form 



1 V /I ^ 



(---Ml + i?--^ 



J _y j u ^y yjf y 



\ 2 ' 



which has a minimum value when 



+ l = M+ 8 



^ + u ~ + M 1 ; 



the value of I being in that case 

 I vi 



and the corresponding angle of incidence being given by the 

 formula 



H , !/.. 1\ 



sin z = x , where = J Jt 4 ^ . 



, t V?-i V ^/ 



Since ju + - cannot be less than 2, it is easy to see that, when 



