526 Professor John W. Nicholson [May 2, 



this way obtain contours for a pair of close components which cannot 

 be detected visually as a pair, and the actual interval can be deduced 

 by a series of measurements of the joint contour, consisting of two 

 overlapping parabolas. We calculate the position of the vertex of 

 the inner one, and thence the separation, which can, in R^^ be 



o 



determined within about O'OOl of an Angstrom unit. The actual 



o 



separation in this line is as small as 0-030 A., and the present method 

 could measure accurately a much smaller separation. 



AYe pass now from the phenomena of structure and intensity of 

 a single line to those involving a comparison of different lines. 

 Here the behaviour of the plate for different wave-lengths must be 

 dealt with. But it so happens that every plate can be calibrated by 

 throwing on to it not only the whole spectrum under examination, 

 but the radiation — a continuous spectrum — from the positive crater 

 of the carbon arc. The energy distribution in this case is known 

 from Wien's law when the temperature of the arc is known, as it is 

 very closely. On the slide you will notice the curious contour 

 bounding this spectrum, largely due to vagaries in the sensitivity of 

 the plate. Above it is the helium spectrum on the same plate. To 

 obtain an absolute scale of intensities down the helium spectrum, 

 independent of all sources of error due to apparatus, we only need 

 to compare the heights of the lines with the corresponding heights 

 directly below them in the carbon-arc spectrum. It is in fact 

 logarithms of intensity which the heights represent, and the differ- 

 ences of height represent powers of a definite factor entering into 

 the intensities, so that the photographs give no visual impression of 

 the enormous differences of intensity which occur. For example, 

 the line of wave-length X 3888, a Principal line in the helium spec- 

 trum, appears quite short on the photograph, but is actually the 

 most intense in the spectrum. Absorption in the apparatus is strong 

 in this region. One of our conclusions, in fact, is that Principal 

 Series deserve their name even in elements which appeared hitherto 

 to be exceptions, in that they do contain, for the visible region, a 

 preponderant part of the radiated energy. 



It is not necessary to use the carbon arc in every subsequent 

 experiment. We can by its means calibrate the helium spectrum 

 under conditions easily reproduced, and subsequently take this as 

 our standard, especially when the work projected is the variation of 

 the helium spectrum under changing conditions of excitation. Some 

 of the remaining slides indicate the unexpected character of some of 

 these variations It would not be possible in this Discourse to give 

 anything like a complete account of the phenomena of this class 

 already investigated, and I shall therefore confine myself to some of 

 those which are most striking. In the first place we may notice the 

 spectrum of a mixture of hydrogen and helium or neon. The funda- 

 mental phenomenon which this method has detected is what we have 



