98 Mr. C. S. Hastings's Theory of 



ing of Forbes, the force of which has been shown, its absorp- 

 tion-lines ought to be strongest at the limb. This is the con- 

 dition which produces the phenomena of Class III. 



Before discussing the final class we must recall a fact fami- 

 liar to the most casual observer of the sun, namely, that lying 

 upon the photosphere 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 portions of faculse. 

 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 convenience, let us call this layer A. 



Imagine, then, a stratum of vapours B above the layer just 

 described, 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 selective absorption) suppose all beneath the two 

 spherical shells in consideration to be removed. In the spec- 

 troscope, 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 appearances which would be presented by a sun so 

 constituted, if the threefold system should be studied spectro- 

 scopically. In the centre of the projected disk, the lines 

 proper to the exterior shell (B) would be reversed, i. e. dark. 

 As we approached the edge, however, oioing to the opacity of 

 the inner shell, the conditions would approximate to what they 

 would be if the layers A and B existed alone, the central body 

 being removed, and the lines would fade ; if faint, they would 

 vanish. This is our explanation of the phenomena of Class IV. 



Every theory involves certain conditions. We finally judge 



* This supposition is not opposed to probability; for though we must 

 regard the temperature generally decreasing in passing from the photo- 

 sphere 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 distribu- 

 tion 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 diathermanous than our atmo- 

 sphere. 



