Discharge in Rarefied Gases. Ill 



Lastly, as far as the optical nature of the phenomena is 

 concerned, it cannot be doubted that, both with positive and 

 with negative light, Ave have to do with a transformation of 

 the rays of high refrangibility, whose vibrations are changed 

 into those of greater wave-length, as is the case in the pheno- 

 mena of fluorescence and phosphorescence. Previous expe- 

 riments having shown me that the illumination of the solid 

 bodies perceptibly exceeded the duration of the discharge 

 which excited it, I have regarded the appearances observed as 

 phenomena of phosphorescence, and not of fluorescence as 

 previously assumed. Further, of all the numerous substances 

 tried, not a single one was found to be transparent for these 

 rays even in the thinnest possible layers. Neither thin films 

 of glass, nor crystals of calc-spar and quartz, found by Mas- 

 cart to be so transparent for the highly refrangible "rays, 

 allowed any trace to pass. 



A glass wall, which phosphoresced brilliantly when struck 

 directly by the ends of the rays, was covered by an excessively 

 thin layer of collodion, by allowing a drop of ordinary collo- 

 dion highly diluted with ether to run rapidly over the glass 

 and volatilize. Even this film, whose thickness could only 

 have amounted to a few hundredths of a millimetre, gave on 

 the glass wall lying behind it a shadow as black as ink when 

 the electric rays fell on it, as if it had been an opaque metallic 

 substance. 



Without being able to give numerical data, it may be con- 

 cluded that the range of wave-lengths within which the vibra- 

 tions of the ether are still active as light passes beyond the 

 inferior limit assigned by Fizeau. 



On the Replacement of a Kathode. 



A kathode of any given form may be replaced, as far as all 

 relationships hitherto considered are concerned, by a system of 

 small and closely p> ached pores in an insulating surface con- 

 gruent with the kathode. By way of further explanation, I will 

 describe a tube now lying before me (fig. 2) in which a cylin- 

 drical kathode has been imitated. The tube Gr is composed of 

 a bulb K, provided with an electrode a. The tube r is melted 

 into the end of a cylinder Z of about 4 centims. diameter. A 

 paper tube P made of stiff paper rolled together but not glued 

 is pushed over r, and the other end is closed by a little glass 

 cap g. The whole surface of P is covered with numerous 

 fine needle-perforations, through which communication is made 

 from K to the electrode b at the further end of Z. If 

 now the tube is exhausted, and a connected with the ne- 

 gative and b with the positive pole of the induction-coil, the 



Phil. Mag. S, 5, Vol. 10. No. 61. Sept. 1880, 



