OF ARTS AND SCIENCES. 147 



but, modify them as we will, within the bounds of probability, the 

 reasoning remains the same. 



Suppose now a certain vapor which is confined to the upper stratum 

 of the photosphere, or, rather, one of which the lower limit is thus re- 

 stricted ; then, according to the reasoning of Forbes, the force of which 

 has been shown, its absorption lines ought to be strongest at the limb. 

 This is the condition which produces the phenomena of Class III. 



Before discussing the final class, we must recall a fact familiar to the 

 most casual observer of the sun, namely, that lying upon the photo- 

 sphere is a stratum producing a very strong general absorption, so 

 strong indeed that the disk is probably less than a fourth as brilliant 

 near the edge as at the centre. This layer is very thin, as proved 

 by the great difference in brilliancy between the upper and lower por- 

 tions of faculfe. 



Since the difference of absorption at the two levels is very great, 

 the conclusion follows, because the facula itself is so low that it rarely, 

 if ever, appears as a projection on the limb of the sun. For conven- 

 ience let us call this layer A. 



Imagine then, a stratum of vapors, B, above the layer just de- 

 scribed, which are not represented at all in the photosphere, and 

 which are of nearly the same temperature as this layer A.* Then 

 (for the sake of simplicity regarding this layer as having no elective 

 absorption) suppose all beneath the two spherical shells in considera- 

 tion to be removed. In the spectroscope, light from such a source as 

 the two layers A and B would yield a continuous spectrum ; for the 

 inner shell (A), radiating only white light, would be robbed of nothing 

 not supplied in equal quantity by radiation from the outer shell (B), 

 since they are of the same temperature. If such layers as these 

 really do exist about the sun, we can now readily state the appear- 

 ances which would be presented by a sun so constituted, if the three- 

 fold system should be studied spectroscopically. In the centre of the 

 projected disk, the lines proper to the exterior shell (B) would be 



* This supposition is not opposed to probability, for though we must regard the 

 temperature as generally decreasing in passing from the photosphere outward, 

 it does not follow that this decrease is continuous. A similar general law may 

 be stated for our own atmosphere, but in a clear night the air in the immediate 

 vicinity of the ground is colder than that just above. The explanation of this 

 phenomenon is familiar in the theory of dew and hoar-frost. Analogous causes 

 for irregularity in the distribution of temperature in the solar atmosphere must 

 be even more efficacious, since the layer A is probably a more vigorous radiator 

 than the earth, and the gases above it are certainly far more diathermous than 

 our atmosphere. 



