In these expressions we are to replace p by ks/f, or rather, since the diffraction pattern is symmetrical, by **kr**/f, where r is the distance of any point in the focal plane from the centre of the system.

Its connexion with a is expressed by a =c4'/dr; so that TZ sin 05 e'(at - **kr**) 47b 2 where the factor e int is restored.

Retaining only the real part of (16), we find, as the result of a local application of force equal to DTZ cos nt (17), the disturbance expressed by TZ sin 4, cos(nt - **kr**) ?

According to (18), the effect of the force acting at dS parallel to OZ, and of amount equal to 2b2kD dS cos nt, will be a disturbance - dS sin cos (nt - **kr**) (20), regard being had to (12).

At B there is no displacement, but at K there is displacement towards B represented by **KR**, i.e.

A2 y i-**Kr**) x2+B i,, ~~T) y+C2 y ~TT) 11 =0.

**Kr**., 1889), W.

If we also assume the ratio of the current to the heat-flow to be the same in both postulates, we have a = TB, whence 0 2 = **kr**/T.