472 A. M. Mayer — Researches on the Bontgen Hays. 



TT¥> or, -gJg, then the departure of these fractions from -^-^ will 

 affect the constant by 4 units in the third decimal place. 



These experiments, made in the conditions we have indi- 

 cated, give for the formula of transmission through the glass 

 used, a crown glass made by Chance & Co. : 



I' = I X "92* 



Having this formula as a basis, it is comparatively easy to 

 determine the constants of another substance, by placing the 

 substance on the netting with piles of glass of graded thickness 

 and exposing them to the X-rays. We then see what thick- 

 ness of glass gives the same illumination to the image of the 

 netting as does the known thickness of the substance. Thus, 

 the netting in the photograph of a disc of herapathite *9 mm 

 thick had the same brightness as in the photograph of a pile of 

 glass -69 mm thick. By the formula glass -69 mm thick transmits 

 •5636 of the incident beam and herapathite of , 9 ,nm transmits 

 the same. From this we compute that V = I X "9382* is the 

 formula for the transmission of the X-rays through herapathite, 

 which for t = "69 gives '5636 of transmission. 

 In the same manner it was found that 



2'05 mm of aluminum transmits the same as 2-44 of glass 



•02 " platinum " " 2'9 mm " 



2-0 " green tourmaline " " 2*0 " " 



. 69 mm « herapathite " " -9 " " 



From these determinations we have computed the formulae 

 of transmission of these substances : 



Glass r = Tx*92 t 



Aluminum 1' = I x '905* 



Platinum I' = I X '00063* 



Green tourmaline I' = I X '92* 



Herapathite F= Ix^S* 



Taking the amount of transmission through aluminum of 

 T 1 - mm and of l mm as unity, we have, in the following table, the 

 relative transmission through the substances experimented on. 



1 mm ^ — ^r 



"— TO 



Aluminum 1 * 1 * 



Glass 1'016 1-180 



Green tourmaline . . 1-016 1*180 



Herapathite 1-036 1*435 



Platinum -000696 



Platinum -06 mm thick is practically impervious to the rays 

 from the Crookes tube used, transmitting only -005 of incident 

 beam. 



This method of determining the transmission of the X-rays 

 through various substances may be criticised, because in the 

 experiments I obtain not alone the transmission through the 



