216 



On some Recent Spectroscopic Researches. [April, 



simple interpretation that the substance in the star is not hydrogen 

 be preferable to the somewhat out-of-the-way hypothesis that the 

 Une has suffered a small displacement by the motion of the star? 

 Certainly it would if we had one line only for comparison. Since, 

 however, there are two strong lines in different parts of the spec- 

 trum, coincident, within the power of an ordinary spectroscope, with 

 those of hydrogen, and a tlurd line almost certainly coincident under 

 similar circumstances, the probability is so great that hydrogen exists 

 in the star, that we are, probably, justified in assuming that such 

 is the case, and therefore in ascribing any very minute want of 

 coincidence which a more powerful spectroscope may reveal, to a 

 change which the Hght has undergone since it left the star. 



The question then presented itself, how large a spreading out 

 of the spectrum would be necessary in order to detect such a 

 velocity of motion as might be expected from the observed proper 

 motions of the few stars of which the parallax is known. The 

 spectroscope which the writer had hitherto employed consisted of 

 two prisms, and was competent to show a shift of position in a 

 line equal to the distance which separates the lines D of sodium, 



which would be equivalent to a velocity of about 190 miles per 

 second. The velocities in a direction at right angles to the line of 

 sight of the few stars, of which the distance is known, lie between 

 twenty mUes and sixty miles per second. An apparatus, therefore, 



