320 
SECOND REPORT- 1832. 
on coloured light*. Fraunhofer found that the spectrum formed 
by solar light is crossed with numerous dark lines of different 
thicknesses, while the spectrum of artificial white flames con¬ 
tains all the rays which are thus wanting. Fraunhofer counted 
about 590 of these lines; and in a fine map of the spectrum which 
he has published, he has inserted the strongest of them, amount¬ 
ing to about 354. Some of these lines he found to be entirely 
black, while others were darker than the rest of the spectrum. 
From various experiments to which he submitted them, he con¬ 
cluded that they have their origin in the nature of the light of 
the sun, and that they cannot be attributed to illusion, aberra¬ 
tion, or any other secondary cause. Sir John Herschel, taking 
a wider view of the subject, remarks, that “it is no impossible 
supposition that the deficient rays in the light of the sun and 
stars may be absorbed in passing through their own atmo¬ 
spheres ; or, to approach still nearer to the origin of the light, 
we may conceive a ray stifled in the very act of emanatiqn from 
a luminous molecule by an intense absorbent power residing in 
the molecule itself; or, in a word, the same indisposition in the 
molecule of an absorbent body to permit the propagation of any 
coloured ray through or near them, may constitute an obstacle 
in limine to the production of that ray.” 
For reasons which I may have an opportunity of explaining 
in another communication, I conceive that the original light of 
the sun is continuous from one end of the visible spectrum to 
the other, and that the deficient rays are absorbed by the gases 
generated during the combustion by which the light is produced. 
But whatever be the manner in which these dark lines are oc¬ 
casioned, it is manifest that while they are of the highest value as 
affording fixed points in the spectrum, they render the sun’s light 
absolutely unfit for experiments on absorption. We cannot, for 
* In the spectrum formed by a narrow “ beam of day-light,” Dr. Wollaston 
had, previously to the year 1802 , discovered seven lines, which he has designated 
by the letters A, B, /, C, g , D, E, the first line being, according to his observa¬ 
tions, the extreme boundary of the red rays of the spectrum, and E the extreme 
boundary of the violet rays. The correspondence of these lines with those of 
Fraunhofer, I have, with some difficulty, ascertained to be as follows: 
A, B, /, C, g , D, E, Wollaston’s lines. 
B, D, b, F, G, H, Fraunhofer’s lines. 
There is no single line in Fraunhofer’s drawing of the spectrum, nor is there 
any in the real spectrum coincident with the line C of Wollaston ; and, indeed, 
he himself describes it as not being “ so clearly marked as the rest.” I have 
found, however, that this line C corresponds to a number of lines half-way be¬ 
tween b and F, which, owing to the absorption of the atmosphere, are particu¬ 
larly visible in the light of the sky near the horizon. 
In order to have seen the lines B and H of Fraunhofer, especially the last, 
Dr. Wollaston’s “beam of day-light” must have come from a part of the sky 
very near the sun’s disc. 
