﻿516 Prof. R. W. Wood on Fluorescence and 



pressure or a powerful magnetic field. Experiments in this 

 direction will be made next winter. 



In both of these cases, in each of which we have excitation 

 bylines of nearly the same wavelength, the wider-spaced 

 series is produced when the stimulation is by the longer wave- 

 length. It remains to be determined whether we take hold 

 of different absorption-bands and excite entirely different 

 series, or whether we stimulate the same vibrator in each 

 case, the spacing of the resulting lines depending upon how 

 nearly we approach its natural period in our exciting- 

 vibrations. 



In addition to the regularly spaced lines in the violet, we 

 have a complex assortment of lines in the yellow-green region, 

 the intervening portion being totally devoid of lines. One of 

 these lines, \ = 5300, has a broad diffused wing, and it is 

 perhaps worthy of remark that in the spectrum excited by 

 the two zinc radiations we have a hazy doublet at this point, 

 in the spectrum excited by zinc 4811,, a single line, and in 

 the spectrum excited by cadmium 480, two faint lines. Some 

 of the other lines have wings, as will be seen from the chart, 

 and at wave-length 516 we find a broad hazy band. All of 

 these peculiarities complicate things ; and I have drawn 

 attention to them merely to show that we must not expect to 

 explain matters by too simple a mechanism. 



A word or two about the bismuth arc may not be out of 

 place. Various plans were tried, such as immersion of the 

 electrodes in water, burning in the carbon arc, &c. The best 

 arrangement was found to be a shallow iron dish about 4 cms. 

 in diameter (pounded from a piece of thin sheet iron), filled 

 nearly full of molten bismuth, and kept hot over a small 

 burner. The dish of metal formed the positive electrode, the 

 negative being a bar of iron which could be raised or lowered 

 by a rack and pinion. The arc required constant attention, 

 fresh metal being put into the dish every ten or fifteen minutes, 

 and as exposures of eight hours were necessary, it will be 

 seen that an enormous amount of very fatiguing work was 

 necessary in all cases where open-air ares were used. 



Copper Excitation. 



I have been unable thus far to obtain photographs of the 

 fluorescence excited by the separated copper radiations. The 

 lines are close together, and the arc climbs about o\ er the 

 electrodes. I hope next year to improve matters in this 

 respect. On Plate XII. a, is seen the fluorescence excited by 



