420 MR WILLIAM SWAN ON THE PRISMATIC SPECTRA OF THE 
imperceptible, shines out in great brilliancy; while the lines 6, y, 0, and ¢ remain 
totally unchanged in position, colour, and intensity. 
While the line a is thus exceedingly variable in its brightness, the lines 8, +, 6, 
and zg, on the other hand, are perfectly steady; and being never absent in 
carbohydrogen spectra, there is every reason to believe that they are really 
characteristic of the body undergoing combustion. Beyond a on the less refracted 
side there is a faint trace of red light, which, as it becomes so feeble as almost to 
disappear when the light is derived from the lowest point of the flame of the 
Bunsen lamp, is probably due to the exterior envelope of the flame, and not to 
the interior cone. The line a is separated from @ by an extremely dark space, 
almost destitute of light. The line @ is of a faint yellowish green colour, but well 
defined, and is accompanied by four almost equidistant lines 6, @,, &c., which 
diminish in brightness as their distance from ( increases. After another very 
dark interval, the extremely beautiful line y follows, which is exceedingly bril- 
liant, and of such absolutely definite refrangibility as, like a, to form a perfectly 
sharp image of the slit through which the light passes. Its colour is a fine 
slightly bluish or pea green, and it is accompanied by a fainter line y,. The 
next line 6 is the less refracted edge of a broad band of light containing four 
fine lines. This group, which is of a pale ashy colour, is separated by dark in- 
tervals from Y and %. The line Z belongs to a brilliant but not very well defined 
band of a fine purple tint, which is accompanied by a fainter line e. 
I have completed observations of the minimum deviations for the lines a, £, +, 
8, and Z; and also for the principal lines of the solar spectrum, which are given in 
Series 1, Tables II. and III., pp. 427, 428. From an examination of these tables it 
appears, that while several lines in the carbohydrogen spectrum coincide nearly in 
position with remarkable lines in the solar spectrum; yet in no case, if we except 
the line a, has the observed coincidence been exact. The observations, therefore, 
rather tend to prove that the bright lines of the carbohydrogen spectrum coincide, 
not with the dark lines, but with the bright spaces of the spectrum of sun light. 
Postscript added since the preceding Paper was read.* 
From the well known coincidence discovered by FRAUNHOFER, to exist between 
the line R, in the spectrum of a lamp, and D of the solar spectrum, taken in con- 
nection with similar phenomena, which have since been observed, it might be 
inferred, as a general law of the spectra of flames, that their bright lines always 
coincide with dark lines of the solar spectrum. 
The result of the investigations which have now been detailed, is obviously 
unfavourable to such a conclusion. In publishing observations bearing on a ques- 
tion of so much interest and importance, I was anxious, if possible, to leave no 
* Printed by permission of the Council. 
* 
