The reflection of long X-rays

.—For X-rays passing through matter, theory and experiment give a refractive index μ slightly less than unity, so that if a beam of X-rays falls on the plane surface of a solid at a small glancing angle less than the critical glancing angle, it is totally reflected. Determinations of the refractive indices of short wave X-rays by the total reflection method have been carried out by a considerable number of observers. The work of Forster and of Doan is of particular importance in that determinations have been made with many refinements of technique for various substances over a range of wave-lengths.