386 



Dr. W. Huggins on the Motion of 



[June 13, 



motion of the nebula in the line of sight. With spectroscope B and eyepiece 2, 

 the lines appear to be coincident with those of hydrogen. In consequence 

 of the uncertainty of the character of the first line, which is single, while 

 that of nitrogen is double, this determination can now only be made by 

 means of the comparison of the third line with that of hydrogen. This 

 third line becomes very faint from the great loss of light unavoidable in 

 a spectroscope that gives a sufficient dispersive power, and the comparison 

 can only be attempted when the sky is very clear and the nebula near the 

 meridian. 



(Received June 12, 1872.) 

 § 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 refrangibility which I had 

 observed in a line in the spectrum of Sirius as compared 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 in pos- 

 session of the necessary instrumental means to resume this inquiry, and 

 since this time the prevalence of bad weather has left but few nights suffi- 

 ciently 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 substances, 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 com- 



1872, fand ich wiederum, so wie es Huggins friiher gesehen hat, unzweifelhaft (2}>(3). 

 Die relative Intensitat der drei Lichtarten babe ich mehrfach in Zahlen geschatzt und 

 erhielt z. B. in den letzten Nachten : 



Marz 6. Marz 13. 



(1) 100 100 



(2) 58 63 



(3) 49 52" 



* 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 Eev. John Michell, 

 entitled " On the means of discovering the Distance, Magnitude, &c. of the Fixed 

 Stars, in consequence of the Diminution of the Telocity 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 hundredth, 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 velocity 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." 



