A. M. Mayer — Researches on the Rontgen Rays. 473 



substances placed on the slide of the plate-holder but at the 

 same time the transmission through the slide itself. If the 

 slide has a measurable transmission, or, screening effect, then 

 its equivalent in thickness of the substances placed on it should 

 be added to them. This criticism, however, is nought, because 

 experiments have shown that if the thickness of a portion of 

 the slide is doubled by placing on it a piece of the material of 

 which it is made, no screening of the X-rays by this piece can 

 be detected. 



Jf.. The actinic effect of the Rontgen rays varies inversely as the 

 square of the distance of the sensitive plate' from the radiant 

 source. 



The slide covering a photographic plate had on it the wire 

 netting, and the plate was exposed for 30 minutes to the X- 

 rays at a distance of 10 inches from the radiant source. Another 

 similar plate in the same plate-holder was exposed for two hours 

 at a distance of 20 inches from the radiant source. These 

 distances could be accurately measured in the apparatus used. 

 These plates were taken from the same box and developed side 

 by side in the same tray. Indeed all the conditions were care- 

 fully made the same in the two experiments except the dis- 

 tance of plates from the radiant source. On developing the 

 images on the plates they were exactly alike ; the image of the 

 netting had the same illumination on each plate, and the 

 density of the films was the same. This experiment shows 

 that the X-rays act on a sensitive plate according to the law of 

 the inverse squares. 



From this law it follows that the actinic power of the X-rays 

 is not sensibly absorbed in traversing the air ; also, that these 

 rays are not sensibly diffused by radiation from the molecules 

 of the air they traverse, the air being at ordinary barometric 

 pressures. 



These deductions, which necessarily follow from the law of 

 the inverse square, are, however, at variance with the facts 

 observed by Professor Pupin, who states in Science of April 

 10, 1896 : " There was evidently a diffuse scattering of the 



X-rays in their passage through the air." 



u These experiments prove beyond all reasonable doubt that 

 the Rontgen radiance is diffusely scattered through bodies, 

 gases not excepted." These opinions of Professor Pupin are 

 founded on experiments on the action of the X-rays on a fluo- 

 rescent screen, or, rather on a " fluoroscope," not on experi- 

 ments on their actinic effects. In the latter case I am confi- 

 dent that the X-rays act according to the law of the inverse 

 squares, and therefore are not sensibly diffused. In the former 

 case Professor Pupin finds that they are diffused in traversing 

 the air, and very sensibly diffused if I understand aright his 



