TOTAL ECLIPSE OF THE SUN, APRIL 16, 1893 
89 
Fig. 12. 
Illustrating comparative lengths of arcs and lines photographed with prismatic camera and slit- 
spectroscope respectively, at beginning or end of totality. 
(2.) The radiations of such vapours may be studied by means of photographs taken 
shortly before and after totality, for which no readjustment of the instrument is 
required. The bright arcs at the cusps are of greater length than the lines in similar 
photographs, which might be taken with slit spectroscopes. 
Interpretation of Photographs taken during Totality. 
The photographs taken during totality show the spectrum of the chromosphere and 
prominences as they appear at different heights above the photosphere. 
Thus, the metallic prominence shown in the upper right-hand quadrant in Photo¬ 
graph 7, Plate 11, appears also in Photographs 8, 9, 10, 11, the lower parts being 
gradually eclipsed, so that the last only shows the spectrum of the tip of the pro¬ 
minence. 
In the case of the spectrum of the chromosphere, different parts of the arcs photo¬ 
graphed correspond to the spectrum at different distances above the photosphere. 
Thus, at a position angle corresponding to the point of contact at the beginning or 
end of totality, the edge of the moon will reveal the chromosphere to a greater depth 
than at adjacent parts, as shown in fig. 13. If the inner circle represents the boun¬ 
dary of the photosphere, and the circle concentric with it represents the chromosphere, 
the edge of the moon, at the moment of contact, will be in some such position as c a b. 
At the point a we should at that moment get the spectrum of the base of the chromo¬ 
sphere, while at b and c we should only get the spectrum of the higher reaches. 
In case the chromosphere consisted of concentric shells of vapour, the spectrum seen 
at the point a would be the integration of the spectra of all the shells of vapour, but 
at c and b only the outer shells would be effective in producing a spectrum. 
A point which it is important to bear in mind, when attempting to interpret the 
photographs taken during totality, is the production of rings by a purely continuous 
