250 Mr. J. R. Capron on the Comparison of some Tube 



are also compared with other lines in the violet light 4654 and 

 4601 ; and the Professor then concludes that it may be in general 

 assumed that the feeble bands of the aurora- spectrum belong to the 

 spectrum of the negative pole, possibly changed more or less by 

 additions from the banded or the line air-spectrum. 



4thly. That the only probable explanation of the bright line is 

 that it owes its origin to fluorescence or phosphorescence, the Pro- 

 fessor remarking on this point that " an electric discharge may 

 easily be imagined which, though in itself of feeble light, may be 

 rich in ultra-violet light, and therefore in a condition to cause a 

 sufficiently strong fluorescence/' remarking also that oxygen 

 and some of its compounds are fluorescent, 



5thly. That there is no need, in order to account for the spec- 

 trum of the aurora, to have recourse to the " very great variability 

 of gas-spectra according to the varying circumstances of Q pressure 

 and temperature" (Dr. Vogel's theory). Professor Angstrom 

 does not admit such variability, and does not admit that the way 

 a gas may be brought to glow or burn can alter the nature of the 

 spectrum. 



In order to test the Professor's conclusions in an experimental 

 way, I have recently examined some tube and other spectra not 

 only for line-positions, but also for general resemblance to an 

 aurora-spectrum. These experiments, few at first, led to others ; 

 and the results, though not perhaps developing any striking or 

 new features, may help to clear the way on some points, and to 

 suggest further experiments bearing on an interesting subject, 

 which up to the present time has been almost entirely in the hands 

 of foreign spectroscopists. It did not seem desirable to use power- 

 ful currents : a J-inch-spark coil worked by a quart bichromate- 

 cell was found sufficient to illuminate the tubes steadily. The 

 spectroscope used was one made for me by Mr. Browning speci- 

 ally for auroral purposes ; of the direct-vision form, with five large 

 prisms, the collimator and telescope having clear 1-inch lenses. 



The field of view extends at one glance from near C to near G, 

 the extremities of the spectrum being obtained by traversing with 

 the telescope ; with a fine slit the D line is widely separated. 

 In the eyepiece is inserted a reduced scale (photographed on 

 microscopic glass and mounted in balsam) of bright lines or 

 spaces upon an opaque ground. The faintest spectrum brought 

 in part upon this scale shines through the spaces and renders 

 the divisions visible, showing the spectrum-lines above and 

 touching them. The drawings were made on enlarged photo- 

 copies of the microscopic scale itself, so as to avoid any chance 

 of error as between the actual scale and the copy used for the 

 drawings. I found this plan the most effective for rapid and 

 correct sketching; and it gives close results as compared with 



