M. E. Goldstein on Spectra of Gases. 339 



given out by the same discharge. If we cover a vacuum-tube 

 with lampblack and make only a small scratch in the lamp* 

 black through which the light can pass, this point is drawn 

 out into a line by means of the rotating mirror. This line 

 can be looked at by means of a small prism, the edges of 

 which are placed parallel to its length. We then see, for in- 

 stance, if the tube is filled with hydrogen, bands and lines 

 throughout the extent of the line of light. If the tube is 

 filled with moist air, the bands of nitrogen are seen every- 

 where together with the lines of hydrogen. 



Experiments with the Ley den jar also show that Wullner's 

 hypothesis does not agree with facts. 



The discharge of a Leyden jar consists, like that of an induc- 

 tion-coil, of a spark, and of an aureola which can be blown away 

 by means of a current of air, or separated from the spark by 

 means of a rotating mirror. If a Leyden jar is discharged 

 through a tube filled with rarefied .nitrogen, only the band- 

 spectrum is seen if the pressure is conveniently chosen. The 

 rotating mirror shows, however, that the discharge is composed 

 of a continuous part and a disruptive spark. 



If w r e increase the resistance, the continuous part of the dis- 

 charge is weakened and finally becomes invisible ; yet only the 

 band-spectrum is seen. It may be said that the continuous 

 discharge is so much weakened by the broadening effect of the 

 rotating mirror that it cannot be seen any more in the mirror, 

 yet it might be strong enough in the tube itself to give a band- 

 spectrum. This objection is easily answered. We can find an 

 arrangement which allows us to look at the image of the spark 

 in the rotating mirror through a prism, and we shall see that it 

 is just that spark which gives the band-spectrum. 



If the resistance is pretty great, the line-spectrum begins to 

 appear together with the band-spectrum. It is still possible by 

 regulating the resistance to reduce the discharge as seen in the 

 rotating mirror to a single image ; and this image shows, when 

 looked at by means of the spectroscope, the lines and bands of 

 air at the same time. 



The same results are arrived at with hydrogen. The dis- 

 charges through the vapours of sodium and mercury are widened 

 by means of the rotating mirror, and allow, therefore, a conti- 

 nuous discharge to pass. It is well known that these substances 

 give a line-spectrum. 



If we introduce into the circuit of a RuhinkorfPs coil two 

 tubes both filled with air, but of different diameters, we can, by 

 conveniently regulating the pressure, produce the band-spectrum 

 in one of the tubes and the line-spectrum in the other. It is 

 easily proved by means of the rotating mirror that the dis- 

 charge has exactly the same character in both tubes. 



