330 M. A. J. Angstrom's Optical Researches. 



other influence than that some of its own lines render, by their 

 brightness, some of the others less perceptible. 



2. By shining lines peculiar to each metal. These lines are 

 easily distinguished from the foregoing by the fact, that when 

 the condenser is not altogether too strong, and consequently the 

 charge too feeble, they do not form complete transverse lines, 

 but appear to proceed from both edges of the prism, and to be 

 extinguished before they reach the centre. They are no less 

 distinguished from the former lines by their brighter light, and 

 seem, as it were, to stand forth against the dull ground on which 

 the said lines are drawn in undiminished intensity through each 

 point of the transverse section. This predominating brightness, 

 particularly with strong charges, is doubtless the reason why 

 Masson, in all the spectra which be has drawn, has not found 

 more than four lines identical. 



To explain the production of the first spectrum, I assume with 

 Faraday and Masson that the electric spark is produced by a 

 current which propagates itself across, and by means of, ponde- 

 rable matter, which it heats in the same manner, and according 

 to the same laws, as a voltaic current heats a metallic wire. The 

 other spectrum, on the contrary, is produced by the heating and 

 dispersion of the particles of the conductor which proceed simul- 

 taneously from both the poles, but on the way lose both their 

 temperature and their luminous properties. 



Among the constant lines there are two which were observed 

 by Fraunhofer, and which exceed the others in brightness, 

 namely, a double line in the yellow and one in the green light. 

 In Masson' s discovery, and also here in fig. 1, Plate II., they are 

 denoted by 7 and 8*. It is possible that these two lines are 

 sometimes common to both spectra, and that their brightness is 

 thus increased ; I will not venture to deny this. In the case of 

 bismuth, at least, this is the case with the line y. 



To render the comparison between the solar spectrum and 

 that of the electric spark more evident, I have drawn both of 

 them, side by side, in fig. 1. Both drawings are made from 

 nature, an angle of one minute embracing a millimetre and a 

 half of the drawings. The difference between the spectra is, that 

 to form a correct notion of the upper or electric spectrum, the 

 black lines must be regarded as luminous ones ; luminous with 

 the colour which corresponds to their position in the spectrum. 

 For these observations, a flint-glass prism of Merz, with a refract- 

 ing angle of 46° 3 A' 57", was made use of. 



The observations on which the drawing of the electric spec- 



* In Masson's memoir, however, the line § for antimony is incorrectly 

 given ; in like manner the line 3, which is in general very weak, is in the 

 case of copper confounded with the line which I call D. 



