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



that is to say, values which are nearly equal to those of D and E 

 in the electric spectrum. 



A direct experiment nevertheless showed that D in the electric 

 spectrum does not completely coincide with the vivid yellow line 

 formed in the spectrum of the flanie of alcohol, but lies some- 

 what nearer to the red. The yellow line coincides with the line 

 to the left of D which is common to all the metals, and which 

 I have denoted by n in fig. 2, Plate II. 



5. When the solar spectrum is compared with the electric 

 one, it is found that some of the lines, such as C, D, E, G, and 

 we may also say H, have their corresponding lines in the solar 

 spectrum ; but for the strongest, 7 and 8, this is not the case. 



A complete correspondence, however, between the solar and 

 the electric spectrum is the less to be expected, as the lines in 

 the former, as is generally assumed, are not only due to the 

 action of the atmosphere, but also to be referred to the action of 

 the sun itself. For the present we arc not in a position to sepa- 

 rate the two systems of lines from each other, although Brew- 

 ster's numerous researches on the peculiar influence of the atmo- 

 sphere have furnished interesting results. Thus the atmosphere 

 produces dark lines on the violet side of C, and on the red side 

 of D ; in like manner is produced, on the other side of D, a wide 

 band, possibly in the neighbourhood of y, &c. 



Miller states that during an approaching thunder-storm he 

 has seen dark lines between D and C, but nearer to D, which 

 would seem to coincide with the line y. In Broch's drawing of 

 the solar spectrum, taken as the sun was near rising or setting, 

 a dark zone also occurs near D, although the distance from D 

 appears to be smaller than that between y and D. Between F 

 and G is a dark zone, which possibly corresponds to the line F 

 in the electric spectrum. 



The analogy between the two spectra may, however, be more 

 or less complete when abstraction is made from all the minuter 

 details. Regarded as a whole, they produce the impression that 

 one of them is a reversion of the other. I am therefore con- 

 vinced that the explanation of the dark lines in the solar spec- 

 trum embraces that of the luminous lines in the electric spectrum, 

 whether this explanation be based upon the interference of light, 

 or the property of the air to take up only certain series of oscil- 

 lations. The first view has only one difficulty, and that is to 

 explain how the different retardations, which are necessary for 

 the effect, are produced ; this will be the more difficult for the 

 electric spectrum, inasmuch as all these retardations must occur 

 in the inconsiderable mass of air which is in direct contact with 

 the electric current. 



6. In fig. 2 I have given a sketch of the principal lines 



