On the Repulsion of an Electrical Current on ttself. 247 
The sun consists of a luminous nucleus, which would by itself 
produce a continuous spectrum, and of an incandescent gaseous 
atmosphere, which by itself would produce a spectrum consisting 
of an immense number of bright lines characteristic of the nu- 
merous substances which it contains. The actual solar spectrum 
is the converse of this. Were it possible to observe the spectrum 
belonging to the solar atmosphere with all its attendant bright 
- lines, vo one would be surprised to hear that, from the existence 
of the characteristic bright lines of sodium, potassium, and iron 
in the solar spectrum, the presence of these bodies in the sun’s 
atmosphere has been ascertained. According to the proposition 
which I have just laid down, there can, however, be just as little 
doubt concerning the truth of this assertion, as if we saw the 
real spectrum of the solar atmosphere. 
I will, Jastly, mention a phenomenon which, although appa- 
rently trivial, was of peculiar interest to me, because I foresaw it 
theoretically, and afterwards verified it by experiment. Accord- 
ing to theory, a body which absorbs more rays polarized in one 
direction than in another, must also emit those rays in the same 
proportion. A plate of tourmaline cut parailel to the optical 
axis absorbs, at common temperatures, more of those rays falling 
perpendicularly, whose plane of polarization is parallel to the axis 
of the crystal, than of those whose plane is at mght angles to the 
axis. At temperatures above a red heat, tourmaline also pos- 
sesses this same property, although in a less marked degree. 
Hence the rays of light which the plate of tourmaline emits per- 
pendicular to its surface must be partially polarized ; and, more- 
over, they must be polarized in a plane perpendicular to the plane 
of polarization of the rays which have been transmitted by the 
tourmalme. This theoretical conclusion is borne out by ex- 
periment. 
XXXVITI. Remarks on Ampére’s Experiment on the Repulsion 
of a Rectilinear Electrical Current on itself. By Mr. Jamus 
CroLu, Glasgow*. 
q* reference to Dr. Forbes’s “ Notes on Ampére’s Experiment 
on the Repulsion of a Rectilinear Electrical Current on 
itself,’ which appeared in the Philosophical Magazine for Fe- 
bruary, the followmg remarks may perhaps be acceptable. 
I have long been under the impression that Ampére’s experi- 
ment, although successful, does not prove the thing intended, 
namely, that the different parts of a rectilinear electrical current 
are mutually repulsive; for the motion of the wire is evidently 
* Communicated by the Author. 
