140 Royal Society : — Dr. W. Hug-gins on the Motions of 



§ 2. On the Motions of some Stars towards or from the Earth. 



In the early part of 1868 I had the honour of presenting to the 

 Royal Society some observations on a small change of refraugibility 

 which I had observed in a line in the spectrum of Sirius as com- 

 pared with a line of hydrogen, from which it appeared that the star 

 was moving from the earth with a velocity of about twenty-five miles 

 per second, if the probable advance of the sun in space be taken 

 into account *. 



It is only within the last few months that I have found myself i?i 

 possession of the necessary instrumental means to resume this 

 inquiry, and since this time the prevalence of bad weather has left 

 but few nights sufficiently fine for these delicate observations. 



Some time was occupied in obtaining a perfectly trustworthy method 

 of comparison of the spectra of stars with those of terrestrial sub- 

 stances, and it was not until I had arranged the spark within the tube, 

 as described at the beginning of this note, that I felt confidence 

 in the results of my observations. 



It may be well to state some circumstances connected with these 

 comparisons which necessarily make the numerical estimations given 

 further on less accurate than I could wish. Even when spectroscope 

 C, containing four compound prisms, and a magnifying-power of 16 

 diameters are used, the amount of the change of refrangibility to be 

 observed appears very small. The probable error of these estimations 

 is therefore large, as a shift corresponding to five miles per second 

 (about iJg- of the distance of D 1 to D 2 ), or even a somewhat greater 

 velocity, could not be certainly observed. The difficulty arising from 

 the apparent smallness of the change of refrangibility is greatly in- 

 creased by some other circumstances. The star's light is faint when 

 a narrow slit is used ; and the lines, except on very fine nights, cannot 

 be steadily seen, in consequence of the movements in our atmosphere. 

 Further, when the slit is narrow, the clock's motion is not uniform 

 enough to keep the spectrum steadily in view ; for these reasons I 

 found it necessary to adopt the method of estimation by comparing 

 the shift with a wire of known thickness, or with the interval be- 

 tween a pair of close lines. I found that, under the circumstances, 

 the use of a micrometer would have given the appearance only of 

 greater accuracy. I wish it, therefore, to be understood that I re- 

 k gard the following estimations as provisional only, as I hope, by 



* Phil. Trans. 1868, pp. 529-550. As a curious instance in which later 

 methods of observation have been partially anticipated, a reference may be made 

 to an ingenious paper in the Philosophical Transactions for 1783, vol. lxxiv.,by 

 the Rev. John Michell, entitled " On the means of discovering the Distance, 

 Magnitude, &c. of the Fixed Stars, in consequence of the Diminution of the 

 Velocity of their Light." The author suggests that by the use of a prism " we 

 might be able to discover diminutions in the velocity of light as perhaps a hun- 

 dredth, a two hundredth, a five hundredth, or even a thousandth part of the 

 whole." But he then goes on to reason on the production of this diminished ve- 

 locity by the attraction produced on the material particles of light by the matter 

 of the stars, and that the diminutions stated above would be "occasioned by 

 spheres whose diameter should be to the sun, provided they were of the same 

 density, in the several proportions of 70, 50, 30, and 22 to 1 respectively." 



