ACCORDING TO THE UNDULATORY THEORY. 497 
or 
or 
It is now evident that , or the number of the rays which have tra- 
versed after the second reflection, must be equal to m — 1, or amount 
to one less than the number of the reflecting surfaces. Bearing this in 
mind we have 
g = (m1) —(m—2) pt p™ 
oe 
Although we dare not here suppose m to be, properly, indefinitely 
great, it must however be so great that we may consider p” in com- 
parison with (m— 2) p as evanescent, and m — 1, as well as m — 2, 
equal tom. By this we have 
Ss = pda 
i Pp 
If we now put this value of S in the formula (9), and instead of p 
its value, and moreover represent 2 7 = for shortness’ sake by g, we 
have 
A! (cost + / —1 . sin?) 
_a(l —r)" mr? (cosg + VW —1.sing) 
1—(1 —7r)?(cosg + V=1.sing)" 
