142 Royal Society ;— Dr. W. Huggins on the Motions of 



the vapours of these substances existing there, and that we may there- 

 fore safely take any small displacement of either set of lines to show 

 a motion of the star towards or from the earth. The lines due to 

 sodium are perhaps more intense, but are as narrow and defined as 

 the lines D x , D 2 in the solar spectrum : they fall, however, within a 

 group of very fine lines ; this circumstance may possibly account for 

 the nebulous character which has been assigned to them by some ob- 

 servers. 



The bright lines of sodium were compared with spectroscope B and 

 eyepiece 3 ; they appeared to fall very slightly above the pair in 

 the star, showing that the stellar lines have been degraded by the 

 star's motion from the earth. The amount of displacement was esti- 

 mated at about one fifth of the distance of J) 1 from D 2 , which is 

 probably rather smaller than the true amount. This estimation would 

 give a velocity of separation of 37 miles per second. At the time 

 of observation the earth' was moving from the star at about 15 miles 

 per second, leaving 29 miles to be due to the star. 



When magnesium was compared, a shift in the same direction, and 

 corresponding in extent to about the same velocity of recession, was 

 observed ; but in consequence of other lines in the star at this place, 

 the former estimation, based on the displacement of the lines of 

 sodium, was considered to be more satisfactory. 



Rigel. — The lines of hydrogen are strong in the spectrum of this 

 star, and are suitable for comparison. 



The line H/3 is not so broad as it appears in the spectrum of 

 Sirius, but is stronger than F in the solar spectrum : this line was 

 compared by means of spectroscope C and eyepieces 2 and 3. The 

 line of terrestrial hydrogen falls above the middle of the line in the 

 star ; the star is therefore receding from the earth. The velocity of 

 recession may be estimated as rather smaller than Sirius, probably 

 about 30 miles per second, the earth at the time of observation 

 moving from the star with a velocity of 15 miles, leaving about 15 

 miles as due to the star. This estimate is probably rather smaller 

 than the true velocity of the star. 



Castor. — The spectra of the two component stars of this double 

 star blend in the spectroscope into one spectrum. The line H /3 is 

 rather broad, nearly as much so as the same line in the spectrum of 

 Sirius. 



The narrow line of rarefied hydrogen was compared in spectro- 

 scope B with eyepiece 3 ; it appeared to fall on the more refran- 

 gible side of the middle of the line in the star, leaving more of the 

 dark line on the side towards the red. The shift seemed to be 

 rather greater than that in Sirius, and may probably be taken at 

 from 40 to 45 miles per second ; but the earth's orbital motion was 

 nearly 1 7 miles from the star, thus leaving about 25 miles for the 

 apparent velocity of the star. This result rests at present on obser- 

 vations on one night only, but they seemed at the time to be satis- 

 factory. 



Regulus. — The line at F rather broad. The corresponding line of 

 hydrogen falls on the more refrangible side of the middle of the dark 



