( 580 ) 
From all this we infer : 
1. Where light emitted by a source that yields a continuous 
spectrum, traverses a space in which sodium vapour is unequally 
distributed, the rays in the’ neighbourhood of the D-lines will be 
much further deflected from their course than any others. Of all 
things this holds good of those rays whose wavelength differs so 
little from Ap, and Ap, that they can hardly be distinguished from 
sodium light. A pretty strong light, therefore, misleadingly resem- 
bling sodium light, but in reality owing its existence to other 
sources, may seem to proceed from a faintly luminous sodium vapour, 
in a direction deviating from that of the incident light. 
2. A spectroscopic examination of the light that has traversed, 
in a fairly rectilinear direction, the space filled with sodium vapour, 
shows, in the places where the D-lines are to be found, broad dark 
bands owing to the fact that the light of these places in the spec- 
trum has deviated sideways from its course, and has not reached 
the slit of the spectroscope. 
The former of these inferences we will now apply to certain 
phenomena in the neighbourhood of the disc of the sun; the latter 
to some peculiarities of the sun-spots. 
Let the arc ZZ' represent a part of the disc of the sun, the obser- 
ver being stationed far off in the direction of 0. This ZZ’ may be 
taken to be either the limit of the photosphere, or the critical 
sphere which in A. Scumipt’s theory of the sun plays such an 
important part. In either case, a ray emitted from any point A on 
the surface at an angle of nearly 90° will reach the point O along 
a path the curvature of which diminishes regularly, if we assume 
that the density of the sun’s atmosphere in a direction normal to 
the surface decreases continuously. 
Se 
yo 
meen Ee 
yy 
a eon cans 
<=. 
Z 
Fig. 4. 
A ray emitted from B under the same circumstances will proceed 
along BO' and does not, therefore, reach O; the observer in O will 
see A lying just within the margin of the disc of the sun; light 
