The Width of X-ray Spectrum Lines

Width of $X-\mathrm{ray}$ Spectrum Lines.—(1) Four causes are discussed: ($a$) the width of the slit employed, ($b$) angular imperfections of the crystal, ($c$) finite resolving power of the crystal grating, and ($d$) lack of homogeneity of the x-rays. The effect of the first two causes is independent of the angle of reflection and therefore the same for all orders and all lines; the effect of the third is shown to vary as $\frac{1}{sin \theta cos \theta }$ and therefore to decrease as $\theta$ increases up to 45°; while the effect of the last is equal to $\left(\frac{d\lambda }{\lambda }\right) tan \theta$ and therefore increases with $\theta$. (2) Measurements of two tungsten lines, $\lambda$ 1.242 and 1.279 Å, in the first four orders from the cleavage faces of calcite and rock-salt, are given. The fact that in both cases the width is least for the second order and greatest for the fourth order shows that both the third and fourth causes contribute measurably to the observed width, as well as the first two.