1881.] Investigations on the Spectrum of Magnesium. 195 



spectrum of magnesium given by a Rutherford grating, but attribute 

 them to a different cause. The Rutherford gratings have aperiodic in- 

 equality in the ruling, due to an imperfection of the screw of the ruling 

 machine, in consequence of which the image of every bright line is 

 accompanied by a series of fainter images at nearly equal distances on 

 either side of it, diminishing rapidly in brightness as they recede from 

 the principal line. These ghosts are so much fainter than the principal 

 lines that they are not noticed in the case of any but bright lines, and 

 except in the case of very bright lines only two, one on each side, are 

 seen to accompany each principal line. 



Fig. 5. 



Solar spectrum 

 by .Rutherford 

 grating 4th 

 order after M. 

 Fievez. 



Magnesium. 



Magnesium b 

 group with 

 ghosts pro- 

 duced bj 

 Rutherford 

 grating. 



Spectrum 

 of 4th 

 order. 



Spectrum 

 of 3rd 

 order. 



Fig. 6. 



The positions of these ghosts have been investigated by Mr. Peirce 

 in the "Mathematical Journal" of the Johns Hopkins University, 

 Baltimore, who has found theoretically, and confirmed it by actual 



