RED PROMINENCES SEEN DURING TOTAL ECLIPSES OF THE SUN. 465 
3. On the Diminished Brightness of the Sun’s Disc towards its Edges. 
That the sun is surrounded by some medium capable of absorbing his light, 
has been considered to be satisfactorily proved by the rapidly diminishing bright- 
ness of his disc towards the edges; which, Sir Joun Herscuen remarks, ‘“ can 
only arise from the circumferential rays having undergone the absorptive action 
of a much greater thickness of some imperfectly transparent medium (due to 
greater obliquity of their passage through it) than the central rays.”* He fur- 
ther states, that if the sun had not an atmosphere capable of reflecting light, the 
sky ought to appear completely dark at a total eclipse of the sun. The existence 
of the corona round the moon is therefore a proof, he adds, that the sun has an 
atmosphere capable of reflecting, and therefore of absorbing light. 
Now, the absorbing medium indicated by the corona is evidently an extremely 
diffuse atmosphere, extending to a great distance from the sun’s surface; for the 
breadth of the corona is certainly not less than the sun's radius. It can, however, 
be easily shewn, that the darkness of the sun’s limb compared with his centre, if it 
arises from the absorption of light by the solar atmosphere, must be occasioned, 
almost exclusively, by the absorptive power of those portions of the atmosphere 
which are near the surface of the sun. 
It will be sufficiently accurate, for this purpose, to assume that the sun is a 
sphere, of which AB, fig. 11, represents the surface, and a00/q@' a stratum of 
the solar atmosphere, of so small thickness that its absorptive power may be 
regarded as uniform. As the whole absorption, in passing through such a thin 
stratum, will be very small, we may safely assume that the light, in traversing a 
certain thickness of the absorbent medium, will thereby acquire no additional fa- 
cility for penetrating the remaining portions of it; equal aliquot parts of the in- 
cident light will therefore be absorbed in passing through successive equal thick- 
nesses of the stratum. 
Suppose all the sun’s atmosphere excepting a 0 U/ a’ to be removed. 
_ Let 1 = the number of intromitted rays, 





= = the number of rays lost by absorption and dispersion when the light has 
traversed a unit of thickness, 
M=1— = 
t= the thickness aa’ of the stratum; 
Then the number of rays that escape absorption after passing to the eye in the 
direction aa’, perpendicularly through the stratum, will be 
(a _— =) fe ui’. 
Mm. 
* Outlines of Astronomy, par. 395. 
VOL. XX. PART III. 61 
