140 
MR. J. N. LOCKYER AND DR. SCHUSTER ON 
obtain photographs of these rings, and the instrument employed has been termed a 
prismatic camera. 
The great advantage of the prismatic camera is that it combines the functions of 
a telescope with those of a spectroscope, that is to say, it does not present us with 
the spectroscopic view of one section only of the corona, but of the whole chromosphere 
and corona. The instrument which was in use in Siam consisted of a camera of 
5 feet focal length. The object glass, which belonged to Mr. Lockyer, had an aperture 
of 3f inches. In front of the object glass was placed a prism with a refracting angle 
of about 8 degrees. 
Supposing that the corona and chromosphere only send out the same homogeneous 
light, one image only will appear on the sensitive plate, the only effect of the prism 
being to displace the image. As far as protuberances are concerned we know they 
give a spectrum of bright lines, and we expect therefore to find on the plate each 
protuberance represented as many times as it contains lines in the photographic 
region. The different protuberances would be arranged in a circle round the sun, 
and these circles would overlap or not, according to the dispersive power of the prism 
and the difference in refrangibility of the fines. Fig. 1, Plate 9, represents a series of pro- 
tuberances such as we might expect to find if the dispersive power is small. Fig. 2, 
Plate 9-, the same protuberances if the dispersive power is large. The dotted fine repre- 
sents the edge of the sun. As far as the corona is concerned we know very little. If 
the conclusion which Tennant and Stone derived from recent eclipse observations is 
correct, and the higher regions of the corona send out continuous light, we shall find 
no distinct outline of the corona as we do in ordinary photographs, but we shall find 
this image drawn out along the fine of dispersion. 
Thus a corona which would appear in an ordinary photograph as fig. 3 will be 
drawn out as represented in fig. 4. Such an image would present a striated 
appearance, each irregularity being drawn out by the prism. If such an irregularity 
were confined on an ordinary photograph to a mathematical point it would appear in 
the plates exposed in the prismatic camera as a mathematical fine. As a rule, 
however, the irregularity will cover a surface of measurable extent. The visibility of 
such a local irregularity depends on the relative brightness of the surrounding objects, 
which will overlap, and, if bright enough, mask the irregularity. The continuous 
spectrum will be most easily observed, if, as actually happens, it is confined to the 
lower part of the corona. The moon in this case will form a sharp edge to the 
irregularity, and prevent any overlapping from the side on which it is situated. If 
the corona give a series of bright fines instead of the continuous spectrum we shall 
find a series of outlines on the photograph similar to that corresponding to the pro- 
tuberances. We might indeed have the two cases combined, and then we should find 
a series of images standing out of the continuous band. If we find that part of the 
corona gives a continuous spectrum that part alone will be drawn out into a band. 
We thus see that the prismatic camera not only gives us an indication of the spectrum 
