84 Dr. L. Vegard on the 



by Bragg, although the particular arrangements were, some- 

 what different. The slits and the ionizati on-chamber were 

 mounted on the top of an ordinary Fuess goniometer, in such 

 a way that the incident beam of X-rays and the axis of the 

 ionization-chamber were parallel to the axes of the collimator 

 and telescope respectively. The optical arrangements, how- 

 ever, were not removed, so the instrument could at the same 

 time be used as a goniometer. 



The crystal was mounted in the usual way on the crystal 

 table with some wax. By means of a screw underneath the 

 instrument the table could be moved up and down without 

 any appreciable rotation of the crystal. 



This arrangement proved very convenient, as it was 

 possible to set the crystal-face very accurately by means of 

 the optical arrangement, and then move it upwards until its 

 central part came into level with the axis of the ionization- 

 chamber. 



The crystals used were two fine specimens which were 

 kindly lent me by Professor W. C. Brogger, of the Minera- 

 logical Laboratory. The one had the cube faces (100) and 

 the other the tetrahedron faces (111) well developed. 



The crystals were good reflectors. In order that no 

 maximum might escape notice the entire ionization curve 

 for varying angles was determined first with fairly wide 

 slits (1 mm.). Then a more accurate measurement was 

 made of the plate given by the strongest Rh line of wave- 

 length X = 0*607 10~ 8 cm., using a slit about 0*4 mm. 

 broad. 



The condition for reflexion is given by the following 

 formula of Bragg: 



nA- = 2^sin 0; 



d is the distance from one point plane to the next identical 

 plane ; 6 is the glancing angle of the incident beam, \ the 

 wave-length, and n the order-number. 



The zero position of the chamber, as determined from the 

 direct beam, is perhaps less accurate than the position corre- 

 sponding to maximum ionization from the reflected beam. 

 From the position of the first and second maxima, however, 

 we can easily calculate the true zero position and the true 

 glancing angle 6. 



Let the observed angles for the 1st, 2nd, &c. order be 

 «i, a 2 , . . ., and the true zero position u , then 



<9 1= ^p, 2 = " 2 ~"° , and 



cot 20 = 2 cosec (a 2 — a 2 ) — cot (a 2 — «!). 



