308 



band $ 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 10, 11,12, and 13 of the year 1838 were 

 frosty days; yet the lines were well seen: $, i, y, C 6, C 15, C 16, 

 and D are specially mentioned. On the 1 3th, when the thermometer 

 stood at 23 F., * 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 Kirchhoff and Bunsen's paper *, 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 

 relative position of these bright rays and the dark lines in the laven- 

 der part of the spectrum, which are only visible in the sun's light 

 under the most favourable circumstances, must not be relied on as 

 accurate, since it was determined by measurement, and not by direct 

 comparison. Possible inaccuracy of adjustment will render the 

 * Pogg. Ann. ex. p. 161. 



