586 Prof. R. W. Wood on the 
about over the aperture in the drum-retort of the sodium 
tube. The image of the carbons gave some trouble as well, 
exciting the complete fluorescence spectrum. Both troubles 
were obviated by focussing the arc on a small round hole in 
a screen by means of a large double-convex lens, and then 
throwing an image of the aperture into the sodium retort by 
means of a second lens. In this way it was possible to keep 
the white light away from the sodium vapour, and correct 
for the shifting of the arc by moving the lamp from side to 
side. As exposures of seven or eight hours are often 
necessary, it will be readily seen that the care of the arc is 
no light labour. A magnetic field was tried to keep the arc 
in a fixed position, but nothing appeared to be gained by this 
expedient. 
The spectra excited by the radiations from the cadmium 
and zinc arcs in exhausted quartz tubes were photographed 
with a large plane grating in combination with the Cooke 
lens, and as the wave-lengths of the resonance spectra excited 
in these two cases have been determined with consider- 
able accuracy, we will discuss them first. In each case 
the iron-spectrum was photographed in contact with the 
resonance spectrum. 
Cadmium Excitation. 
The wave-lengths of the lines in the resonance spectra 
excited by the monochromatic radiations from a cadmium- 
vapour lamp o have been determined to within one or two 
tenths of an Angstrom unit. In the previous paper I showed 
that stimulation of the vapour with the radiation of wave- 
length 4800 caused it to emit a series of bright lines spaced 
at very nearly equal intervals along a normal spectrum. 
Whether the lines are in reality spaced at equal distances 
is a matter of considerable importance, since theoretical 
treatments of the radiation emitted by electron systems 
disturbed in various ways can only be verified by the test of 
experiment. Prof. Larmor has pointed out to me in a letter 
that a non-radiating system of electrons in steady orbital 
motion, when disturbed by the absorption of a radiation 
corresponding in frequency to the frequency of one of the 
electrons, should then emit radiations giving us a series of 
lines equally spaced along a normal spectrum. We must 
bear one fact in mind, however. The light emitted by the 
vapour is obliged to pass through a certain thickness of 
cooler vapour before reaching the spectroscope, and since 
the absorption-lines are very closely packed together, it is 
