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256 Mr. J. R. Capron on the Comjiarison of some Tube 



The 5622 for the yellow line of the tube must be an error. 

 5608 seems to me, from my own observations, nearer its place; 

 and I calculate 5193 and 4825 for the other lines; so that the 

 two spectra (if I am correct) would compare thus : — 



Oxygen .... 5603 5195 4834 

 Carbon .... 5608 5193 4825 



Now these wave-length differences are so small that they 

 raise a presumption of the possibility of the spectra being iden- 

 tical ; but, on the other hand, assuming the spectra are not 

 identical, the comparison tells the other way, viz. that the differ- 

 ences are so far minute as to escape detection in instruments of 

 moderate dispersion ; and with my own instrument I found the 

 spectrum too faint to increase the dispersive power with ad- 

 vantage. Considering the extremely different character of the 

 two discharges, the identity of all the tubes, and the presence 

 of the line found between 7 and a in the spectrum, I think 

 the two spectra are independent, though I admit there is room 

 for doubt. 



I ought to mention that the tube H 2 gave the principal 

 lines of the O and H spectra combined, on a faint continuous 

 spectrum. 



I next examined two vacuum-tubes of an entirely different 

 character. The one was a tube from Geissler of stout glass, some 

 15 inches long, without electrodes, and an inch across. Within 

 this tube is a second of uranium glass with bulbs blown in it; 

 and in contact with both tubes a quantity of fluid mercury runs 

 loose. Upon shaking this tube with the hand, brilliant flashes of 

 blue white light like summer lightning flash out. These are 

 discernible (though faintly) even in daylight. 



The fine terminal wires of the coil being wrapped round each 

 end of this tube, when the current passed a bright and white 

 induced discharge, with a considerable amount of stratification, 

 was seen in. the tube. 



The other tube was that of an old mercurial siphon barometer. 

 This being placed in a stand, one terminal wire was placed in the 

 mercury in the short leg of the siphon, while the other terminal 

 was made into a little coil and placed on the upper closed ex- 

 tremity of the barometer- tube. On passing the current the entire 

 short space above the mercury was filled with a grey white light, 

 not stratified, but showing a conspicuous bright ring just above 

 the level of the mercury. 



Both these tubes, when examined with the spectroscope, 

 showed four bright rather uniform bands (the central one being 

 the brightest), which I assign to the carbon-spectrum (see 

 Plate IV. spectra 5 & 6). 



