786 
DR. OLIVER LODGE ON ABERRATION PROBLEMS. 
Effect of Motion on Diffraction Grating. 
56. To avoid any confusion about motion relatively to source, and the alteration of 
wave-length thus caused, it will be best to abandon our usual convenient plan of letting 
the ether move, and attend explicitly to the motion of the grating with its telescope 
and observer ; all else being stationary. 
Pig. 14. 
Details of Doppler effect -with moving grating, AB, and telescope OP. 
Consider a plane wave, A^Bq, advancing through a stationary medium with 
ordinary velocity, V, towards a stationary grating. Let A^A = AC = Xq be an ordi¬ 
nary wave-length, while AB = s is the width of one complete element of the grating; 
then BC is a wave-front, and AO is the ray, inclined to the normal to grating at 
angle CBA = 0^. 
Now let the grating advance with velocity v to meet the Avave a distance 
BB' = AA' = CC' in one period; the disturbance Bq Avill only have to go as for as 
B', and the disturbance A only as far as C' ; so draAving a tangent B'D to the sphere 
of radius AC', Ave get the AA^ave-front appropriate to moving grating ; AD is the ray, 
inclined to the normal to grating at angle DB'A.' = r/). 
