468 A. M. Mayer — Researches on the Rbntgen Rays. 



the drop is then brought in contact with a cover glass. This is 

 the manner in which I placed on the glass discs and blotting 

 paper the plates of herapathite used in my experiments. 



Rontgen has shown that generally the lower the density and 

 always the thinner the substance the less it screens from a pho- 

 tographic plate the action of the X-rays. " Herapathite is, 

 therefore, eminently fitted as the substance on which to make 

 experiments which are to decide whether the X-rays can or 

 cannot be polarized by having traversed crystals which, like 

 herapathite and tourmaline, transmit only one polarized beam, 

 the 'Other being absorbed by the crystal, for the density of 

 herapathite is 1*557 and plates of only # 012 mm thick are sufficient 

 to answer the question. When crystals of herapathite -012 mm 

 thick have their optic axes crossed at 90° these crossed portions 

 viewed against incident light appear black, so powerful is the 

 polarizing property of this substance. If the X-rays be polar- 

 izable these black portions should act like thick lead and com- 

 pletely screen the sensitive film from the action of the X-rays. 

 The fact is that the crossed herapathites do not screen the X- 

 rays any more than the herapathites do when superposed plates 

 have their axes parallel. In the latter case the crystals freely 

 transmit light with a faint olive-green tint. 



The thickness of crystal plates used in the experiments 

 varied from -01 mm to •025 mm , as found by focussing with a 

 micrometer-screw a powerful objective on the top surface of 

 the crystals and on the glass on which they rested. 



Six discs of glass, -IS 111111 thick and *25 mm in diameter, were 

 covered with herapathites in the manner described. They 

 crossed one another at various angles ; where they crossed at 

 90° the crossed portions were black. On a piece of yellow 

 blotting paper, f- mm thick, were also placed several layers of 

 herapathites, so deep that they reflected a green metallic luster 

 like the elytra of cantharides. These discs and the blotting- 

 paper were fastened to the slide covering the photographic 

 plate. This slide was impervious to two hours' exposure to the 

 actinic action of the sun's light. On the slide were also three 

 discs of thin glass, so overlapping that the X-rays had to pass 

 through 1, 2, 3 thicknesses of the glass before reaching the sen- 

 sitive plate. These served as standards with which to compare 

 the screening effects of the herapathites. 



The slide so prepared and covering a sensitive plate was 

 exposed to the radiations of a Crookes tube in three experi- 

 ments, for \ , 1, and 2-J- hours. On developing the plates, not 

 the slightest trace of the presence of the herapathites was visi- 

 ble. The photographs of the glass discs had not the slightest 

 mottling on their surfaces ; appearing to the unaided eye and 

 when examined through a magnifying glass with uniform illu- 



