144 
Proceedings of Royal Society of Edinburgh. [sess. 
Attempt to Polarise the Eontgen Eays. 
Different views have been expressed about the possibility of 
polarising the x-rays by means of tourmalines ; and although the 
following experiments seem to indicate a negative result, I take 
the liberty of placing them on record. 
Apparatus employed . — The source of electricity was the main, 
and the measurements across the terminals, with Lord Kelvin’s 
cell tester and ampere gauge, were 10 volts and 10 amperes. The 
spark of the coil was 6 inches, and a mercury interrupter was used. 
An ordinary Crooke’s focus tube, enclosed in cardboard to exclude 
all light, was excited by the above, and the vacuum carefully 
arranged to give the maximum fluorescence by gently heating 
the bulb with a spirit-lamp. Screens of barium-platino-cyanide, 
potassium-platino-cyanide, and lithium-rubidium-platino-cyanide 
were tried. The two tourmalines were got as nearly alike as 
possible; the measurements of each were : — length, 47 mm. ; 
breadth, 12 mm. ; thickness, 2 mm. 
First Observation . — On placing one tourmaline between the 
source of the x-rays and the screen, and directly in contact with 
the latter, a distinct shadow was seen, due to absorption of the 
rays. On placing the second tourmaline parallel with the first, a 
difference in density of the shadow was immediately observed. 
When the tourmalines were gradually turned at right angles to each 
other, a dark square area could be seen where the two crossed. A 
source of error was, however, suggested in this experiment. One 
of the tourmalines could not be in as close contact with the screen 
as the other, and, on account of the manner in which the x-rays 
pass from a point on the platinum plate in such a Crooke’s tube, 
differences were observed in the shadows of the four arms of the 
cross formed by the tourmalines. (1) For example, if the horizon- 
tal tourmaline were next to the screen, and the vertical one behind 
it, the two arms above and below the square dark central area were 
less sharply defined than the two arms on each side of it, and 
consequently the shadows appeared to be different. (2) Although 
on the square portion corresponding to where the tourmalines 
