Letter from F J M [Frederick John Marrian] Stratton, Solar Physics Observatory, Cambridge University, to Sir Joseph [Larmor] Stratton has Larmor's letter, with the interesting point, which is not easy to solve experimentally. If he understands it, a monochrmoatic source of light diffacted by a grating will give a series of images of different orders. The second order image may be of the simple grating with an overlay of the first order image. The only hope of disentangling them would be if the two image components were of slightly different wavelengths, so they might be separated by a prism. The most likely people ot be able to do this would be T.R. [Thomas Ralph] Merton or J.A.[John Anthony] Carroll and he wonders if Larmor would like the point to be followed up. 24 July 1939