12 Cady — On the Energy of the Cathode Rays. 



This bolometer was apparently free from the faults of its 

 predecessors: therefore only the usual errors of observation 

 need be considered, especially that arising from the evolution 

 of gas. This error can hardly have amounted to more than 

 3 per cent with weak cathode-current ; hence it follows from 

 the last table that the true value of the ratio i V/Q lies be- 

 tween 0-80 and 0-86. 



Since the bolometer foil was only 0'001 mm thick, the ques- 

 tion arises whether an appreciable part of the rays can have 

 penetrated through to the other side. In this case it might 

 easily happen that the particles on passing through would lose 

 a part of their energy while retaining their charge. Mc- 

 Clelland* and Wienf have shown that cathode rays after trav- 

 ersing an aluminium window carry a negative charge; but 

 Lenard did not succeed by means of a thermo-element in 

 detecting any heating effect.;}: Still if we assume according to 

 Lenard that for solids the quotient of absorptive-power divided 

 by density = 3200,§ we find from his formula that not more 

 than 1 per cent of the rays can have passed through the bolo- 

 metric foil used. This can have had practically no effect upon 

 the ratio under discussion. 



Conclusion. 



It only remains to compare the limiting value of the ratio as 

 experimentally determined, with the equation (4<z) 



i V i—r 



r > 



Q 1 — rr 



where r denotes the reflected part of the rays, while upon 

 reflection the rays lose their energy in the ratio r' \\. 



Starke| has found that platinum reflects about 40 per cent of 

 the rays. Assuming, therefore, that for the bolometer, r =0*4, 

 we find that when i V/Q = 0-83, r' : 0*7. 



This result is in contradiction with the observations of 

 Merritt already referred to. The latter are unfortunately 

 not above criticism, as the author himself admits. Merritt 

 compared the defiectibility of direct and reflected rays by 

 observing the phosphorescence on the walls of the tube. It is 

 hardly to be supposed that the spot from the reflected ray can 

 have been even approximately as distinct as that from the 



* J. A. McClelland, Proe. Roy. Soc., lxi, p. 227. 1897. 



fW. Wien, Verh. Phys. Ges. Berlin, xvi, p. 165, 1897. 



% Ph. Lenard, Wied. Ann., li, p. 239, 1894. This experiment was, however, 

 performed in air at ordinary pressure, which rapidly absorbs the energy of the 

 cathode rays. 



§ Ph. Lenard, Wied. Ann., lvi, p. 274, 1895. 



|| H. Starke, Wied., lvi, p. 58, 1899. 



