586 
MR. J. NORMAN LOCKYER ON THE 
will, in the main, be the same in. both cases, for the reason that we have to deal with 
the projection of a sphere and not with a section. The only criterion is that if the 
vapours rest on the photosphere, the lines will thicken towards the base, whereas, in 
the case of a separate higher layer they would not widen or brighten towards the 
base, but really be thickest at the top, if we do not take account of effects of tempera¬ 
ture. Taking temperature into consideration, as the lines will be less bright as the 
distance from the sun is increased, and, therefore, the temperature is reduced, the lines 
produced by the higher layers will be of equal brightness throughout, and dimmer 
than the others. Accordingly the slit spectroscope can only give information as to 
the distribution of vapours in the sun’s absorbing atmosphere by means of these very 
delicate observations of brightness, or widening of the bright lines observed. 
Somewhat similar difficulties are met with in the case of the prismatic camera, 
when we attempt to distinguish between the two kinds of layers. First, consider 
the effects during totality. In the case of a vapour extending down to the photo¬ 
sphere, we should obtain spectrum rings decreasing in intensity as we pass outwards 
from the moon’s limb (this is exemplified by the ring obtained in the case of 1474 K, 
to which reference will be made later, and the same appearance is seen in the case of 
the other coronal rings). The apparent internal diameters of such rings upon the 
photographs would be equal in every case, but their heights would depend upon the 
intensities of the radiations producing them. There should be no difficulty in detecting 
such rings unless they are of very nearly equal brightness, and very numerous, or 
unless they do not extend far enough to be visible beyond the moon. 
There is a very definite way in which the photographs taken with the prismatic 
camera may indicate the presence of layers of vapours concentric with the photo¬ 
sphere, but not reaching down to it. At a certain height above the photosphere, 
the chromosphere spectrum in a photograph of the chromosphere visible at any one 
instant beyond the edge of the moon, will show arcs with certain relative intensities. 
As the moon advances and gradually uncovers the base of the chromosphere, the 
same arcs will remain visible, but those produced by a layer which does not extend 
lower down will be reduced in intensity as compared with arcs produced by vapours 
which do reach lower down ; the latter will continue to get brighter, while the others 
remain of the same absolute intensity. As the lowest part of the chromosphere is 
shown in the photographs taken immediately after totality, or exactly at the end, 
it is only necessary to compare the relative intensities of the arcs in different photo¬ 
graphs in order to investigate the general question as to the existence of layers. A 
large number of photographs taken in rapid succession with instantaneous exposures 
during the visibility of the chromosphere spectrum, either near the beginning or end 
of totality, would further enable us to determine the order of such layers in proceed¬ 
ing outwards from the photosphere. 
Another way in which the prismatic camera may possibly help to determine the 
presence of layers is as follows. A layer concentric with, but separated from, the 
