EMISSION AND TRANSMISSION OF RONTGEN RAYS. 



137 



Penetrating Powers of tJie Radiations, 



So far attention has been directed more to the relative intensities than to the 

 penetrating powers of the different radiations. A new series of experiments was 

 carried out with a potential on the tube of about 20,000 volts. The anticathode was 

 not changed until a complete set of measurements of the intensity of the rays had 

 been made for all the different thicknesses of a metal screen. Figs. 7,8, and 9 are 

 derived from the results obtained by inserting screens of aluminium, copper, and 

 platinum in the paths of the radiations from aluminium, iron, nickel, copper, and 



platinum. The thickness of screen is plotted against Iog 10 =- , where I is the initial 



-lo 



intensity of the beam as it leaves the aluminium window, and I its intensity after 

 transmission through a screen. The slope of the tangent to the curve at any point 

 gives (when multiplied by 2'3) the value of the coefficient -of absorption (X.) at that 

 region. X is defined by the relation I = I e~* d , where d is the thickness of screen at 

 the point. If the relation is homogeneous over any region, the graph will, of course, 

 be a straight line. 



VOL. CCIX. A. T 



