1894.] The Spectrum Changes in ft Lyrce. 279 



It may be remarked that the period of the light-changes of the 

 star is about twelve days twenty-two hours, and there are two 

 .approximately equal maxima of mag. 3'4, a principal minimum of 

 mag. 4'5, and a secondary minimum of 3'9, the period of variation 

 stated being that which elapses between two successive principal 

 minima. 



Professor Pickering found that during the first half of the 

 period that is, between principal and secondary minima the bright 

 lines were on the less refrangible sides of the corresponding dark 

 ones, while during the second half they were displaced to the more 

 refrangible sides. He further remarked that " the actual changes in 

 the spectra, when studied in detail, are much more complicated than 

 has been stated above, and show a variety of intermediate phases 

 and changes in the dark as well as in the bright lines." 



At Professor Pickering's request, I took up the work at Kensing- 

 ton in July, 1891, the instrument employed being the 6-inch Henry 

 object glass and prism of 7|, which I have described in a previous 

 communication.* Several photographs were taken with this instru- 

 ment, but it was not until the new 6-inch prism of 45f was employed 

 in the research that any considerable advance was made. With the 

 higher dispersion of this instrument the spectrum is depicted in 

 greater detail, and more minute changes can therefore be deter- 

 mined. 



Since my work was commenced, accounts of the photographic 

 spectrum of /3 Lyrae have been published by Belopolsky,^ Father 

 Sidgreaves, and Vogel,|| and various suggestions have been made by 

 them and others as to the conditions which bring about the varia- 

 bility. 



On this account, although the reductions of the sixty-four photo- 

 graphs which I have obtained are not yet completed, I have thought 

 it desirable to give a brief resume of the facts already acquired. 



For the complete study of the problem more photographs will be 

 required, and a considerable amount of time will be required for the 

 discussion of them. The present communication, therefore, is limited 

 to a preliminary consideration of the variation in the spectrum as 

 photographed at Kensington, and I have consequently in it omitted 

 reference to the results obtained by other workers. In a subse- 

 quent paper, however, a complete history of the subject will be 

 given. 



To facilitate references to the spectrum, thirteen photographs 



* 'Phil. Trans.,' 1893 (A), vol. 184, p. 678. 



t Ibid., p. 679. 



' Mem. Soc. Spett. Ital.,' June, 1893. 



' Monthly Notices, E.A.S.,' 1894, p. 96. 



|| ' Sitzungsberichte,' Berlin, February, 1894. 



