438 The Bev. Walter Sidgreaves [Jan. 22, 



darkenings would be unequal. It was reserved to Professor 

 Pickering to draw this conclusion from Mrs. Fleming's discovery, on 

 the Harvard spectrograms, of both bright and dark hydrogen lines 

 in juxtaposition. These were found to exchange places periodically, 

 and synchronously with the light changes, the bright lines being 

 found on the red sides of their dark companions during the first half 

 of the light cycle, and on the violet sides in the second half. This 

 woukl be the result of the relative velocity of the components of a 

 close binary, revolving on the orbit described, one giving the bright 

 lines, the other the dark lines ; and Pickering's measure of the 

 greatest separations of the pairs of lines gave the relative velocity at 

 about 300 miles in the second. This explanation was simple and 

 attractive ; but the simplicity did not last long. Professor A^ogel's 

 examination of a goodly series of photographs at Potsdam resulted in 

 perplexing variations ; and he pointed out the uncertainty of a 

 velocity deduced from the separation of centres which cannot be 

 accurately pointed, on account of their unknown positions on broad 

 and overlapping lines. 



It was to facilitate the study of these new difficulties in their 

 relation to the light curve that the spectrographic chart was compiled 

 in the order now shown on the screen. The enlarged photographs 

 were originally mounted in one column on a long cardboard, here in 

 two columns, in the order of their time-intervals from the preceding 

 chief minimum, regardless of their absolute dates. These intervals, 

 called periodic dates, were written, in days and hours, opposite the 

 spectrograms in one margin, and the civil dates of the preceding chief 

 minima in the other margin. The phases of the light variations are 

 thus followed throughout the entire cycle by corresponding phases of 

 the changing spectrum, at intervals averaging about six hours. 



Mounted m this order, the enlargements were found to be a great 

 help in the examination of the original negatives. Each of these was 

 examined on the micrometer stage in connection with its enlarged 

 positive, and then the details, better seen on the negatives, could be 

 safely recognised on the positives without the aid of a magnifier. 

 Comparisons then were easily made ; and the chart became a guide, 

 directing attention to the parts of the spectrum which called for 

 closer examination on the original plates. 



I will first draw your attention to some general features in the 

 progressive changes of the spectrum. And the one which claims the 

 first mention is the fidelity with which the same spectral phase recurs 

 at the same phase of the light cycle. This was important to me, 

 because my arrangement of the photographs on the chart had not 

 found favour with eminent German astronomers ; and it is the corner- 

 stone of my superstructure. I do not pretend that no excej^tions 

 will be found in the future ; indeed, the regularity of a pendulum 

 would be too precise tor some of my propositions. But it is worthy 

 of notice that amongst the 54 plates which fill in the light cycle 

 there is no case of going back upon the continuity of succession in 



