454 
PROFESSOR FORBES ON THE SUPPOSED ORIGIN OF 
It occurred to me that an experiment might be made upon the light coming from 
different parts of the sun’s surface which should decide this question. For supposing 
the sun to be surrounded by an atmosphere which his light must traverse, it is clear 
that the absorptive action must be greatest upon the light which reaches us from the 
edges of the sun (those points in whose horizon the earth appears), and least for that 
which traverses his atmosphere vertically (or to which the earth appears in the zenith). 
It results from this that the light derived from the extreme circumference of the solar 
disc might be expected to present more numerous and broader bands than when ob- 
tained from its whole surface, since the more complete spectrum derived from its 
central parts would fill up the gaps left in the spectrum of the lateral rays and con- 
ceal their deficiencies. 
As the occurrence of the annular eclipse of the 15th of May suggested the inquiry, 
so it also afforded a very satisfactory mode of putting it to the test of experiment. 
With a view to fix the aspect of the spectrum more accurately on my memory, 
I examined it the day before very carefully with the telescope of a theodolite placed 
about thirty feet from a vertical slit about one fiftieth or one sixtieth of an inch wide, 
upon which the solar rays were thrown by a heliostat. In front of the telescope was 
a flint-glass prism by Dollond, which exhibited the lines very satisfactorily. This 
apparatus was also arranged previously to the eclipse, and I satisfied myself that no 
minute changes of adjustment in the parts of the apparatus, such as the angle of 
incidence on the prism, the distance from the slit, the breadth and vertically of the 
slit, the quantity of light reflected by the heliostat, whether from single or double re- 
flecting surfaces, &c., made any serious difference in the distinctness or general appear- 
ance of the spectral lines *. As the eclipse proceeded, and consequently the proportion 
editors of the London and Edinburgh Philosophical Magazine (December 1836) have, however, referred me to 
the mention of it in Sir John Herschel’s writings, particularly his Elementary Treatise on Astronomy, from 
which I extract the following remarkable passage. “The prismatic analysis of the solar beam exhibits in the 
spectrum a series of fixed lines totally unlike those of any known terrestrial flame. This may hereafter lead 
us to a clearer insight into its origin. But before we can draw any conclusions from such an indication, we 
must recollect that previous to reaching us it has undergone the whole absorptive action of our atmosphere, as 
well as of the sun’s. Of the latter we know nothing, and may conjecture everything It deserves in- 
quiry whether some or all of the fixed lines observed by Wollaston and Fraunhofer, may not have their 
origin in our own atmosphere. Experiments made on lofty mountains or the cars of balloons on the one hand, 
and on the other with reflected beams, which have been made to traverse several miles of additional air near 
the surface, would decide this point. The absorptive effect of the sun’s atmosphere, and possibly also of the 
medium surrounding it (whatever it be), which resists the motion of comets, cannot be thus eliminated.” 
Herschel’s Astronomy, p. 212 note. See also his Essay on Light, Encyclopaedia Metropolitana, art. 505. 
The object of the experiment now described is to show a method of elimination which applies, at least, to the 
sun’s atmosphere. 
* An erroneous impression seems to have been prevalent both as to the magnitude of the apparatus employed 
by Fraunhofer and as to the imperious necessity of these minute adjustments. Though the philosopher of 
Munich used a telescope of four inches aperture, and a prism of the same diameter, for observing the spectra 
of faint objects, such as stars, it appears that his map of the solar spectrum was made with a theodolite tele- 
