520 



SCIENCE. 



\_S. S. Vol. I. No. 19. 



ball of Saturn rotates as a solid bodj', aud 

 the ring as an assemblage of particles, each 

 of which moves -nith a velocity determined 

 by Kepler's thii-d law, the expressions for 

 the ball and for the planet are very dif- 

 ferent, the former being linear, and the lat- 

 ter an equation of a degree higher than the 

 second. I have determined these expres- 

 sions for the special case above mentioned. 

 Thejr are still further simplified by assum- 

 ing that plane A also passes through the 

 center of the planet. 



Now, if we bring the image of Saturn, 

 formed by a telescope, upon the slit of a 

 spectroscope, with the slit in the intersect- 

 ing plane A, the expressions above referred 

 to are also the equations to the curves of 

 which the lines in the spectrum of the 

 planet are a part, referred to an undis- 

 placed spectral line and the perpendicular 

 line through its center as axes ; for, in these 

 curves, x is proportional to the perpendicu- 

 lar distance fii'om plane B, and, by Dop- 

 pler's principle, y is proportional to the ve- 

 locity in the line of sight. The simplest 

 case, is, of course, that in which the slit co- 

 incides with the major axis of the ring ; this 

 is also the condition for which the differen- 

 tial velocity of points on the surface of the 

 ring is a maximum, and it is one which can 

 be approximately realized in observation. 



Hence the laws of rotation of the com- 

 ponent parts of the sj^stem can be determined 

 (within certain limits) by the form of the 

 special lines, and the form can be determin- 

 ed with very considerable accuracj^ by 

 photographing the spectrum with a suitable 

 instrument. 



According to the assumptions which have 

 been made above, and which represent the 

 accepted hj^pothesis, lines in the spectrum 

 of the ball are straight, but inclined; as 

 compared with their direction the general 

 inclination of the (theoretically) curved 

 lines in the spectra of the opposite sides of 

 the ring is smaller, and it is reversed. The 



actual aspect of the lines on my photo- 

 graphs is in exact accordance with that re- 

 quired bj' the hypothesis. 



If the ring rotated as a whole, the lines in 

 its spectrum would be straight, and theii- 

 direction would pass through the origin ; 

 they would be very nearly prolongations of 

 the planetary lines. Such an aspect of the 

 lines as this could be recognized on my 

 photogi-aphs at a glance. 



The direction of a line fi-ee from displace- 

 ment was obtained by photographing the 

 spectrum of the full moon on the same plate, 

 on each side of the speeti'um of Saturn. 



For further details, with the numerical 

 results of measurement of the plates, I must 

 refer to the May number of the Astrophysi- 

 cal Journal, in which I have described these 

 observations at some length. 



James E. Keeler. 



Allegheny Obseevatoey. 



A GENERAL SUBJECT-INDEX TO PEEIODICAL 

 SCIENTIFIC LITERATURE. 



The Editor of Science — My Dear Sir: 

 I notice that you are printing in Science 

 various replies to the circular of the Royal 

 Societj' of London relating to the matter 

 of a general subject-index to all scientific 

 publications. Your correspondents have so 

 far been in favor of such an undertaking. 

 As I do not believe it to be practicable, it 

 may be of interest to some of your readers 

 to see my own replj^ which I venture to 

 send here^vith. I have made a few trifling 

 changes in the copy which I enclose. 

 I am, very respectfully, 



Edward S. Holden. 

 The Lick Obseevatoey, 



March 30, 1895. 



Mount Hamilton, April 24, 1894. 

 To Professor M. Foster, Secretary B. S., 



Cliairman of the Committee on a Subject-In- 

 dex, 'etc., etc. 



My Dear Sir : I beg to acknowledge receipt 

 of the circular of April 6 relatiug to a pro- 



