Vol.. 8, 1922 
PHYSICS: DAVIS AND TERRILL 
357 
tory explanation has been found for the profound effect of such a small 
amount of gas or the behavior of the tube. 
The investigation is being continued and a more detailed account will 
be published in the near future. 
1 Barkhausen and Kurz, Phys. Zs., Leipzig, 21, No. 1, Jan. 1920, p. 1. 
2 Whiddington, Radio Review, Nov. 1919, p. 53. 
3 GiU and Morrell, Phil. Mag., 44, No. 259, July 1922, p. 161. 
THE REFRACTION OF X-RAYS IN CALCITE 
By Bkrge^n Davis and H. M. 
Department of Physics, Columbia University, N. Y. 
Communicated, October 26, 1922 
The effect of refraction in X-ray spectra has been discussed by Sten- 
strom, ^ who made some determinations of the index of refraction from the 
relative displacement of the several orders. In most cases the effect was 
too small to admit of measurement, but for sugar and gypsum crystals he 
obtained some values for wave-lengths greater than 2.5 A. 
The present paper applies a modification of the same method to the 
reflection from calcite of the Kai line of Molybedenum, 70783 A. 
Since we measure the angle from the crystal face, and not the normal, 
the customary equation for the index of refraction becomes 
V = 
cos 9 
cos~d' 
(1) 
where 6 is the glancing angle outside the crystal and 6' the angle of the 
beam inside. We will use a subscript to indicate orders higher than the 
first. Placing = 1 — 5, Stenstrom computes the values of 5 from the 
equation 
/ sin Sm Y' _ (sin^rX 
\ m J \ / 
/ cos _ dn \ 
\ m j \ ^ / 
where m and n are any two orders. 
As the shift is small in any case, it seemed desirable to express the value 
of b directly in terms of angle, in order to more readily determine the effect 
of errors of observation which are liable to be of the same order of magni- 
tude as 5 itself. Now from (1) we have 
sin^ (9' = 1 - j,-2 cos'^ e 
