72 Royal Society. 



the common constituents of the atmosphere. It is well known that 

 some gases when placed hefore a continuous spectrum produce lines 

 of absorption ; among these are bromine and iodine vapour ; yet the 

 dark lines caused by these two halogens * do not coincide with the 

 bright lines into which Pliicker found the light of Geissler's tubes 

 containing bromine and iodine to be resolved by the prism ; nor 

 have I succeeded in reversing them by bringing these substances into 

 a very hot but little luminous flame. Bright lines were discerned, 

 but in other positions. In connexion with this subject, it may be 

 worth noting that a prismatic examination of the sun's rays passing 

 through three inches of mercury vapour at above 300° C, did not 

 afford the least indication of the reversal of the bright rays that 

 appear when that metal is rendered incandescent. 



From the fact that the atmospheric lines do not always present 

 the same appearance when the sun is on the horizon, and that the 

 band 8 has been observed during a shower, and the most prominent 

 lines during a fog, it has been sometimes supposed that the aqueous 

 vapour in the atmosphere is the cause of them. Yet this can 

 scarcely be. They seem not to be due to little vesicles of condensed 

 vapour; for the sun's rays when passing through the edges of a cloud 

 do not exhibit them, unless, of course, near the horizon. And they 

 seem not to be due to gaseous water ; for they appear near sunset 

 when the aqueous vapour in the atmosphere is reduced to a minimum 

 by frosty weather, though they are not seen when the sun is higher 

 up in the heavens on a damp warm day. From Sir David Brewster's 

 notes, it appears that February 1 0, 1 1, 1 2, and 1 3 of the year 1 838 were 

 frosty days; yet the lines were well seen: 8, t, ij, C6, C 15, C 16, 

 and D are specially mentioned. On the 13th, when the thermometer 

 stood at 23° F., i is said to have been the most prominent. 



In the paper referred to at the commencement, we, in common 

 with most of those who worked on the subject before the appearance 

 of Kirchhorf and Bunsen's paper f, ascribed a bright line coincident 

 with D to other spectra than that of soda. This was no doubt owing 

 to the almost universal trace of that substance. 



The electric lights produced between charcoal points by Professor 

 Holmes's magneto-apparatus, and by a galvanic battery with 

 M. Serin's lamp, were found to be identical, when subjected to pris- 

 matic analysis. Each exhibited a continuous spectrum, and not 

 those variations of bright and dark which other observers, as well as 

 myself, have noticed in electric lights from charcoal points of but 

 inferior quality. The spectrum extended at both ends beyond that of 

 ordinary direct sunlight, and the only lines which I could discern were 

 bright ones in the violet or lavender region. The following refractive 

 indices were determined for the magneto-electric light ; and they are 

 compared with the refractive indices of the nearest principal dark 

 lines of the solar spectrum as determined with the same prism. The 



* See " Experiments and Observations on some cases of lines in the prismatic 

 spectrum produced by the passage of light through coloured vapours and gases, 

 and from certain coloured flames," by Prof. W. A. Miller. Phil. Mag. August 

 1845. 



t Pogg. Ann. ex. p. 161. 



