1913] FINE SLITS IN THIN OPAQUE SCREENS 183 With the same notation as was employed in the treatment of (82) we have __ cfy _ „ f w cos 0 (cos 6 - cos a) cW _ ' "• cos3 0 (cos 6 - cos a) doc 'Jo (cos 6 — cos a)2 + «'2 . 0 (cos 6 — cos *)- + a'2 The first of these integrals is that already considered in (83). It yields STT. In the second integral we replace cos3 9 by {(cos 6 — cos a) + cos a}3, and we find, much as before, that when x' = 0 [w cos* 8 (cos 0 — cosa)dd . ., . ,nn. — - ^ - - — >~ = TT (i. + cos2 a) .............. (96) Jo (cos (9 — cos a)2 + * - \* t \ t Thus altogether for the leading term we get This is the complete solution for a fluid regarded as incompressible. We have now to pursue the approximation, using a more accurate value of D than that (log r} hitherto employed. In calculating the next term, we have the same values of D and r~1dD/dr as for (88) ; and in place of that equation we now have + d8[% sin4 0-% sin2 0 + % sin2 0 cos 0 cos a] log (± 2 (cos 0 - cos a)}, (98) The integral may be transformed as before, and it becomes 4 f ff d<j> log (2 sin <£) [| (sin4 20 cos4 a + 6 sin2 20 cos2 2</> sin2 a cos2 a .'o + cos4 20 sin4 a) - f (sin2 20 cos2 a + cos2 20 sin2 a) + f cos a cos 20 |sin2 a cos a + sin2 20 (cos3 a - 3 sin2 a cos a)}]. (99) The evaluation could be effected by expressing the square bracket in terms of powers of sin2 0, but it may be much facilitated by use of two lemmas. If/(sin 20, cos2 20) denote an integral function of sin 20, cos2 20, [ "" d0 log (2 sin 0)/(sin 20, cos2 20) = f "" d<j> log (2 cos 0)/(sin 20, cos2 20) Jo •' o " d0 log (2 sin 20)/(sin 20, cos2 20) = £ | cZ0 log (2 sin 0)/(sin 0, cos2 0), o •' o '..................(100) in which the doubled angles are got rid of.ond we have to consider