1913] FINE SLITS IN THIN OPAQUE SCREENS 181 the denominator. In (30) we are to introduce under the integral sign the additional factor M>2sin2#. As regards the second term of (87) we have dDj _ [+by(y-r,)dyl dD dy dy y ~] _6 V(&2 - 2/2) r ~fo ' where in - -7- we are to replace r by + (y — tj). We then assume as before y — b cos 0, rj = 1} cos a, and the same definite integrals hn suffice ; but the calculations are more complicated. We have seen already that the leading term in (87) is TT. For the next term we have n . ikr I dD k* k* f , ikr £ = 7 + logT, ?^ = 4- and thus % cos20 + |cosacos#)log±2(cos0-cosa). ...(88) o The latter integral may be transformed into /V 2 cZ</){l-fcos2(2<j!)~cO + .£cosa cos (2<j> - a) Jo 4- 1 - f cos2 (20 + a) + £ cos a cos (2^E> + a)} log (2 sin 0), and this by means of the definite integrals h is found to be -~ (1 + 2 sin2 a). o To this order of approximation the complete value is -^ = ,r + iwM>%-sina.a + logiM?&) ............. (89) CLX For the next two terms I find -f --^- [(1 -I- 4 cos2 a) (1 - 47 - 4 log J ikb) oL2 -|- 3 sin4 a 4- ^ cos4 a + 6 sin2 a cos2 a] 7rM>° T + oriT-fi (^ + f ^s2 a + 1 cos4 a) (7 + log ^B - f ) Zl . IS . U [_ + Q-^-YK cos8 a - 5^-5 cos4 a sin2 « - -T cos2 a sin4 a - ^r-^ sin8 a ... .(90) cr . lo o . o o o^ . o J When cos a = 0, or ±1, the calculation is simpler. Thus, when cos a = 0,t departure from (86) is entailed when kb is no longer very small.