IX. Bakerian Lecture.— X-rays and crystal structure

The method of investigating crystalline structure by the use of X-rays has already been explained in papers read before this Society. It will be convenient nevertheless to re-state its principle very briefly in order to introduce some further considerations which I propose to lay before you. The statement of the principle may be made in the following way. Let a train of waves of length λ be passing through a medium in which are particles having the power of scattering the radiation. Suppose, further, that the scattering power is not distributed evenly through the medium, but that directions can be found along each of which there is a periodic variation of the scattering power of the material contained in strata perpendicular to the given direction, strata being, of course, taken of equal thickness for comparison. Let the distance of recurrence or spacing be called d. Let θ be the angle between the rays and the strata. Then there will be a “reflection" of the radiation by the medium ofnλ =2d sinθ, wheren is any integer.