A. M. Mayer — Researches on the Bontgen Bays. 471 



substances to the X-rays. This wire netting he places on the 

 slide covering the sensitive plate, and on the netting he places 

 substances of various thickness. The netting completely 

 screens the X-rays, and its image on the negative is the brightest 

 possible to obtain in the given conditions. If a plate of a sub- 

 stance should also entirely screen the X-rays, then the image of 

 the netting is invisible in the photograph of this substance, 

 and as plates of substances allow more and more of the X-rays 

 to traverse them the images of the netting in the photographs 

 of these substances are more and more bright. If the image 

 of the netting in the photographs of two substances should be 

 equally bright, then these two plates transmit equal actinic 

 action to the sensitive plate. Thus this ingenious device serves 

 as a very delicate photometer in determining the fact just men- 

 tioned. 



It occurred to me that the wire netting could also give me 

 data with which to determine the constants of the formulae of 

 transmission of the X-rays through various substances. The 

 method devised is as follows : On a wire netting with 8 meshes 

 to the inch placed on the slide of the plate-holder, are cemented 

 piles of glass discs, (each glass about T \ mm thick); these piles 

 gradually increase in thickness. These piles of glass were 

 exposed to the action of the X-rays, so that the photographic 

 plate was 25 cms distant from the radiant source. In the appara- 

 tus used this distance could be accurately measured. After an 

 exposure of two hours it was found, on developing the plate, 

 that the netting was just visible in the photograph of the pile 

 5*35 mms thick, and that it was not visible in the photograph of the 

 pile 5*5 mm thick. In this last case, though the X-rays had pene- 

 trated to the sensitive film, yet the difference in the screening 

 effect of the netting and of the glass was not visible, because 

 the eye cannot distinguish between the illumination of jux- 

 taposed surfaces which differ in illumination by about y-jL-. 

 Hence, through the last pile of 5*5 mm about y^ of the actinic 

 intensity of the incident beam had passed. 



Now % the formula of transmission of rays through a substance 

 which does not reflect these rays is V = la 1 . Where V = the 

 intensity of the transmitted beam ; I = the intensity of the 

 incident beam ; a = a constant depending on the substance, 

 and t is an exponent of a, and t is the thickness of the sub- 

 stance. We have taken T V mpi as the unit of t. 



From the conditions of the experiment F = \a x = T ^ and 

 as t is known, a is readily computed. The accuracy of the 

 determination of a depends on the value of the least percep- 

 tible difference in illumination that the eye can distinguish in 

 two juxtaposed surfaces. We have adopted -j-j^ as the most 

 probable value.* But suppose that the fraction is not y^- but 



* Photometric Experiments, 0. N. Rood, this Journal, July 18*70. 



