. 1869.] On some Recent Spectroscopic Researches. 219 



sphere by which F is produced, Tliis conclusion is in accordance 

 with the presumably enormous mass of Sirius, as suggested by its 

 great intrinsic splendour, which, according to some calculations, is 

 393 times that of our sun. Some more recent comparisons of its 

 light by Alvan Clark, however, would give a somewhat smaller pro- 

 portion. 



The platform from which the astronomer observes is itself in 

 rapid motion. The earth is moving through space, in its orbit 

 round the sun, at about 19 miles per second. The part of the 

 earth's motion, which will be in the direction of any star, either 

 towards it or from it, will be different for different seasons of the 

 year, and according to the star's position relatively to the plane in 

 which the earth moves. 



At the time the observations on Sirius were made, the earth was 

 moving from the star with rather more than half its orbital velocity ; 

 this would leave about 30 miles due to the star itself. 



Another correction has to be apphed. It appears probable that 

 the whole solar system is in motion towards the constellation 

 Hercules, with a velocity of about 4 or 5 miles per second. As 

 Sirius is situated in a part of the heavens opposite to Hercules, its 

 motion must be further reduced to about 26 miles per second, the 

 remaining (about) 15 miles of separation between the earth and 

 Sirius being due to the two causes just assigned. 



The true motion of Sirius will consist of this radial motion from 

 the earth, compounded with the transverse motion which is shown 

 by a small displacement of the star relatively to the neighbouring 

 stars, and is known as its " proper motion : " this apparent motion 

 represents a velocity of the star from 24 to 40 miles per second, 

 according to the value which is assigned to the parallax of the star, 

 that is, according to the distance at which we suppose it to be from 

 the earth. 



There are in the proper motion of Sirius certain ]}eriodic 

 inequalities which led to the prediction, that there existed a body 

 very near to Sirius, a jDrophecy which received a remarkable fulfil- 

 ment by the discovery of a companion star to Sirius by Alvan Clark. 

 We now pass to some observations on that strange and mysterious 

 order of heavenly bodies, the Nebulm. The writer's former observa- 

 tions showed that the prevalent opinion, that all these objects were 

 swarms of bright stars too remote to be separately distmguishable, 

 could no longer be maintained. A large part of these objects 

 give a remarkable spectrum (Fig. 2, Plate), which appears to be 

 peculiar to the nebulae. It consists essentially of three bright lines, 

 though in some objects a continuous spectrum and a fourth hne are 

 also present. The writer showed that within the limits of the 

 apparatus he then employed, the brightest of the hnes was coincident 

 with the brightest of the lines of nitrogen, and the third line with 



