TOTAL ECLIPSE OF THE SUN, APRIL 16, 1893. 
583 
more refrangible side of their true positions with respect to the sun’s limb, and in the 
case of receding vapours there would be displacements towards the less refrangible 
end. Such distortions can be determined, if they exist, by comparing the mono¬ 
chromatic images with those photographed at the same time with the coronagraph. 
For this purpose a photograph of the eclipsed sun was enlarged to exactly the same 
size as the K ring shown in fig. 10, and the comparison could be made very exactly 
by fitting a negative of one to a positive of the other. No differences of form, how¬ 
ever, could be detected, so that the velocities in the line of sight must have been 
comparatively small. Movements across the line of sight, that is normal to the 
photosphere, would not affect the forms of the monochromatic images of the 
prominences. 
PART II.—DISCUSSION UP THE OBSERVATIONS. 
VII. The Interpretation of the Photographs. 
Having now given a description of the phenomena actually photographed, I proceed 
to consider how we are to justly interpret them. For this purpose it is necessary 
first to deal generally with the results as contrasted with those given by slit-spectro¬ 
scopes, in which an object glass is employed to form an image of the eclipsed sun 
upon the slit, and then to consider the phenomena which might be expected under 
the most probable conditions of solar structure. 
Comparison of the results to be expected from Slit and Slitless Spectroscopes. 
The considerations which led me, in 1871. to employ a spectroscope without 
collimator may be briefly stated. If in an ordinary spectroscope, the straight slit be 
replaced by a circular one, bright rings replace the bright lines which are ordinarily 
seen in radiation spectra, and since in the solar surroundings we have chiefly to deal 
with radiation phenomena, the chromosphere and corona themselves can be used 
during an eclipse as ring slits, and on account of their distance, a collimating lens 
can be dispensed with. 
In the report on the eclipse of 1875, by Dr. Schuster and myself, the principles of 
the method, as applying to photographs taken during totality, were stated as 
follows 
“ 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 the 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 
* ‘ Phil. Trans.,’ 1878, Part 1, p. 139. 
