496 



correction ^) in connection with the thickness of the preparation. 

 This correction is not needed, when the absorption of the charac- 

 teristic rays in the preparation can be neglected, wliich was the 

 case for many of the materials, which were till now examined by 

 the method of Debijk and Schkkrer. This does not hold however in 

 onr case (Oi-radiation), in which the radiation penetrated oidy to 

 a very small depth, practically not at all in the preparation. The 

 corrected values of sin* */, ^) are found in the fourth column of the 



table. 



During the computations for the drawing up of a formula for 

 sin^ V, ^ it appeared that the majority of the values in the fourth 

 column, in dividing them by 0,01734, give a quotient, which differs 

 only little from a whole number. These quotients are found in the 

 tifth column. The values of these quotients, smoothed to half a 

 unity, are inserted in the sixth column. 



Miller ') has measured the crystal-form on crystals obtained by 

 electrolysis. It appeared to belong to the tetragonal system with a 

 proportion of axes a -. a : r = 1:1: 0,3857. So it lay at hand, to 

 tiy to find agreement of the fourth column with a formula of 

 the form : 



sin' {&^A (h,' + h,') + Bh,' = A (h,' -i- /,,' 4^ vA/O. 



iu which A = — and B = Aq = . By n and c is meant now in this 



the edges of the elementary parallelepiped or of the elementary 

 cell. It lies also very much at hand to choose for A the above- 

 given value 0,01734, and so to seek furlher agreement of the numbers 

 in the sixth column with an expression /<i' -j- h^^ -\- qht\ in which 

 evidently q was then to differ little from a whole number. 



By choosing (/ = 6, a pioper agreement appears to be obtained. 

 In order to see this, it is advisable to compose a table of the values 



n Gomp. also P. Debije, Phys. Z.S. 18 (1917) p. 5. For the value of this 

 correction we found (to diminish the value of sin'^ Vo Ö with) : 



-— -sew 6* -^ + cos ^ + / \ cos6 ^ \ 



4Z2 i d y d* ^ d ^ \ 



Here r means the radius of the circular cross section of the bar, R that of the 

 camera, d the distance of the centre of the camera to the opening of the screen, 

 by which the rays enter. Second and higher powers of ri'R are neglected. [Added 

 during translation : The deduction of this formula has in the mean time been 

 inserted in A. J. Bijl. Thesis for the Doctorate, Utrecht 1918 p. 22]. 



») W. H Miller, Ann. d. Phys. u. Ch. 58 (1843), p. 660. See also H. von 

 FouLLON. Jahrb. d. Kais.-Kön. Geo). Reichsansl. VVien ^54 (1884), p. 367. • 



