168 Dr. Tycho E:son Auren on 



is a leaden screen (N) at a distance of 20 cm. from the focus. 

 In the screen there are also made two round openings of a 

 radius of 2 cm., through which are admitted the ray-pencils 

 that are to pass through the absorption vessels. In front of 

 the openings are placed aluminium plates (P), which in the 

 experiments have been used as filters for the rays. These 

 niters were of four thicknesses — i. e. 1*25, 2*5, 5*0, and 

 10*0 mm. — and are marked, in the following, by I., II., 

 III., IY. The mean wave-length of the four different com- 

 positions of radiation when using these niters is indicated 

 in Table I. 



Concerning the 7-rays Keetman * has shown that, in order 

 to obtain reliable values of the absorption coefficients, two 

 conditions must be fulfilled. First, the absorbing layer 

 should be exposed as little as possible to oblique radiation ; 

 secondly, the rays coming out of the absorbing layer in 

 such a direction should, as mncli as possible, be excluded 

 from the ionization chamber. There can be no doubt but that 

 experiments for the purpose of determining the absorption 

 coefficient in the case of hard X-rays give more or less 

 inaccurate results if the said conditions are not approximately 

 fulfilled. As in the experimental arrangements above 

 mentioned the ray- pencil entering the ionization chamber 

 seen from the focus and the absorbing layer has not had 

 greater opening angles than S°*5 and 12° respectively, and 

 as the diameter of the absorbing layer has not been more than 

 3 cm., the conditions mentioned may safely be considered as 

 very nearly fulfilled. To be sure that no error might arise 

 from the possible admission of scattered rays from the 

 absorbing layer into the ionization chamber, special expe- 

 riments were undertaken, in which the absorption vessel 

 (E 2 and E 2 ) when being irradiated was displaced so that the 

 distance to the ionization chamber was diminished from 12 

 to 1*5 cm. without the direction of the incident rays being 

 changed in relation to the vessel. The experiments have 

 shown that when the distance is greater than 3 or 4 cm. the 

 position of the absorption vessel has scarcely any measurable 

 influence on the results. In regard to this fact, 0. Klein f 

 has undertaken a mathematical explanation, to which I here 

 beg to refer. The same problem has also recently been 

 discussed by Glocher J, and his results agree very well with 

 those found by Klein. 



The relation between the intensity of both the ray -pencils 



** Keetmann, Ann. der Ph. lii. p. 720 (1917). 

 + O. Klein, infra, p. 207. 

 % Glocher, Phys. Z. xix. p. 251 (1918). 



