( 581 ) 
proceeding from B is invisible to his eye. Slight irregularities of 
density in the atmosphere on the path AO will indeed deflect the 
course of the rays, but only slightly if the gases have a normal 
index of refraction. The irregularities show themselves as shallow 
elevations and depressions in the edge of the sun’s disc. In the same 
manner, rays like BO' do not materially deviate from the course 
which they would have to follow in a perfectiy calm atmosphere of 
continuously decreasing density. 
Let us now suppose unequally distributed sodium vapour to be 
present near A above the limit ZZ' (the photosphere). We suppose 
this vapour to be hardly luminous, if at all. The greater part of the 
shaft of white light BO' is only slightly irregularly refracted in it, 
just as in the other gases to be found there; but those rays whose 
wavelength differs but slightly from Ap, or Ap, are much more 
deflected, and they may even follow the course indicated by the 
dotted line Bh O. Then from O at a small distance Ah above A, light 
may be seen proceeding from B — a source of light with a contin- 
uous spectrum — closely resembling sodium light. A spectroscopic 
examination of this light, however, will show that it differs more or 
less from that of the D-lines. 
It might be thought that only rays with an abnormally high 
refractive index, i. e. with wavelengths rather greater than Ap, or 
Ap, can reach the observer along the path Bh O. Such, however, is 
not the case; for if above A, there were a layer comparable to a 
prism with the refracting edge perpendicular on the plane of the 
woodeut and with base turned upwards, rays with an index smaller 
than unity must be able to traverse the path Bh O. 
Accordingly in the spectrum of the light that appears outside the 
sun’s dise we can expect to find rays which are situated on either 
side of the D-lines; perhaps the probability is a little greater for 
the light on the red side of the absorption-lines, because from 4 to 
h the density is more likely to decrease than otherwise. 
It is further clear that very near the limb there is the greatest 
probability of also seeing light, that differs relatively much in 
wavelength from the sodium light; for there a less degree of abnor- 
mality of index suffices to deflect rays in the direction of 0. On 
the other hand, far above A, we can, as a rule, discern only such 
light as is hardly to be distinguished from Z-light. 
These actually prove to be the principal characteristics of the 
chromosphere-lines. Mostly they have a broad base and are arrow- 
headed. Compare the description and the diagrams to be found in 
LockyeEr’s Chemistry of the Sun pp. 109 and 111, 
