6d6 



NATURE 



[October 22, 1891 



with the stellar line in all these pictures? The answer is, either 

 the star is moving towards or from the earth in the line of sight, 

 or the earth is moving from or towards the star. But in De- 

 cember the earth in its motion round the sun is moving at right 

 angle; to the direction of a Aurigse : why then doe? not the 

 stellar hydrogen line a^ree in position with the terrestrial 

 hydrogen line ? The simple explanation is that a Aurigse is 

 moving with respect to the sun. 



In what way is it moving ? Well, that also is clear ; the 

 stellar line is displaced towards the red end of the spectrum — 

 that is to say, the star light is redder than it should be in con- 

 sequence of a motion of recession ; this proves that the star is 

 moving away from u^, and measures of the photograph show the 

 rate of this motion to be 154 miles per second. We also know 

 that in October the earth, in its milion round the sun, is mov- 

 ing towards o Aurigoe nearly at the same rate as we have just 

 seen that a Aurigae is running away from the sun. Conse- 

 quently, at that time, their relative motions are nearly in- 

 sensible, because both are going at the same rate in the same 

 direction, and we find accordingly in October that the positions 

 of the stellar and artificial hydrogen lines perfectly correspond. 

 Finally, in March, the earth, in its motion round the sun, is 

 moving away from a Aurigse, and as a Aurigae is also running 

 away from the sun, the star-light becomes so much redder than 

 normal that the stellar hydrogen line is shifted completely to 

 one side of the hydrogen and artificial line. 



The accuracy of these results may be proved as follows : — 



If we measure all the photographs of a. Aurigse which Dr. 

 Vogel has obtainel, we can derive from each a determination of 

 the relative velocity of the motion of the star with respect to our 

 earth. 



Of course these velocities are made up of the velocity of 

 motion of a Aurigas with respect to the sun (which we may 

 reasonably assume to be a uniform velocity) and the velocity of 

 the earth due to its motion round the sun. But the velocity of 

 the earth's motion in its orbit is known with an accuracy of 

 about one five-hundredth part of its amount, and therefore, 

 within that accuracy, we can allow precisely for its effect on 

 the relative velocity of the earth and a, Aurigse. When we 

 have done so we get the following results fjr the velocity of 

 the motion of a Aurigse with respect to the sun. You see by 

 the following table how beautifully they agree in the Potsdam 

 a-esults, and how comparatively rough anil unreliable are the 

 results obtained by the older method at Greenwich : — 



o Aurigce — Potsdam. 



I believe that in a few years — at least, in a period of time 

 that one may hope to see — we shall not be content merely to 

 correct our results for the motion of the earth in its orbit onlyj 

 and so test our observations of motion in the line of sight, but 

 that we shall have arrived at a certainty and precision of work- 

 ing which will permit the process to be reversed, and that we 

 shall be employing the spectroscope to determine the velocity 

 of the earth's motion in its orbit, or, in other words, to deter- 

 mine the fundamental unit of astronomy, the distance of the 

 sun from the earth. 



I will take as another example one recent remarkable spectro- 

 scopic discovery. 



Miss Maury, in examining a number of photographs of stellar 

 spectra taken at Harvard College, discovered that in the spec- 

 trum of /3 Aurigse certain lines doubled themselves every two 

 days, becoming single in the intermediate days. Accurate Pots- 

 dam observations confirmed the conclusion. 



The picture on the screen shows the spectrum of 3 Auriga 

 photographed on November 22' and 25 of la^t year. In the first 

 the lines are single ; in the other every line is doubled. Mea- 

 sures and discussion of a number of these photographs have 

 shown that the doubling ( f the lines is perfectly accounted for 

 by the supposition of two suns revolving round each other in a 

 period of four days, each moving at a velocity of about 70 miles 

 a second in its orbit. 



When one star is approaching us and the o;her receding, the 

 lines in the spectrum formed by the light of the first star will be 

 moved towards the blue end of the spectrum, those in the spec- 

 trum of the second star towards the red end of the spectrum. 

 Then, as the two stars come into the same line with us, their 

 motions become at right angles to the line of sight, and their 

 two spectra, not being affected by motion, will perfectly coin- 

 cide ; but then, af'.er the stars cross, their spectra again sepa- 

 rate in the opposite direction, and so they go on. 



Thus by means of their spectra we are in a position to watch 

 and to measure the relative motions of two objects that we can 

 never see .tpart — nay more, we can determine not only their 

 period of revolution, but also the velocity of their motions in 

 their orbits. Now, if we know the time that a body takes to 

 complete its revolution, and the velocity at which it moves, 

 clearly we know the dimensions of i's orbit ; and if we know the 

 dimensions of an orbit we know what attractive force is neces- 

 sary to compel the body to keep in that orbit, and thus we are 

 able to weigh these bodies. Tlie components of $ Aurigse are 

 two suns, which revolve about each other in four days ; they are 

 only between 7 and 8 millions of miles (or one-twelfth of 

 our distance from the sun) apart, and if they are of equal 

 weight they each weigh rather over double the weight of 

 our sun. 



I have little doubt that these facts do not represent a per- 

 manent condition, but simply a stage of evolution in the life- 

 history of the system, an earlier stage of which may have been 

 a nebular one. 



Other similar doub!e-stars have been discovered both at Pots- 

 dam and at Cambridge, U.S., stars that we shall never see sepa- 

 rately with the eye aided by the most powerful telescope ; but 

 time does not permit me to enter into any account of them. 



I pass now to another recent result that is of great cosmical 

 interest. 



The Cape photographic star charting of the southern hemi- 

 sphere has been already referred to. In comparing the existing 

 eye-estimates of magnitude by Dr. Gould with the photographic 

 determinations of these magnitudes, both Prof. Kapteyn and 

 myself have been greatly struck with a very considerable sys- 

 tematic discordance between the two. In the rich parts of the 

 sky — that is, in the Milky Way— the stars are systematically 

 photographically brighter by comparison with the eye-observa- 

 tions than they are in the poorer part of the sky, and that not 

 by any doubtful amount, but by half or three fourths of a mag- 

 nitude. One of two things was certain — either that the eye- 

 observations were wrong, or that the stars of the Milky Way 

 are bluer or whiter than other stars. But Prof. Pickering, of 

 Cambridge, America, has lately been making a complete photo- 

 graphic review of the heavens, and, by placing a prism in front 

 of the telescope, he has made pictures of the whole sky like thi^:. 

 [Here two examples of the plates of Pickering's spectroscopic 

 Durc/uimsterung were exhibited on the screen.] He has dis- 

 cussed the various types of the spectra of the b ighter stars, as 

 thus revealed, according to their distribution in the sky. He 

 finds thus that the stars of the Sirius type occur chiefly in the 



