1878.] last two total solar eclipses. 211 



which, it is true, will have to be constructed for the purpose, but 

 which it is in our power to construct. 



Another instrument however has succeeded, and the photo- 

 graphs obtained by means of it, have even in many respects the 

 advantage over photographs taken by the ordinary method. I 

 speak of an instrument which is termed a prismatic camera, and 

 which consists simply of a spectroscope deprived of its collimator. 

 The parallel ray coming from different parts of the corona are 

 refracted in the ordinary wa}^ and are concentrated on the sensi- 

 tized plate by means of a lens. If the object is monochromatic 

 a single image will be formed. If the spectrum consists of a 

 series of lines, a series of images will be formed ; and, finally, if the 

 spectrum is continuous the image will be drawn out into a band. 



The advantage of this method (which is due to Mr Lockyer) 

 consists in the fact that we obtain not only information on the spec- 

 trum of a section of the corona as we would if a slit was employed, 

 but of the whole corona. The disadvantage consists in the difficulty 

 to obtain a scale by means of which we can judge in what part of the 

 spectrum the different images are situated. In the Siamese eclipse 

 of 1875 there were luckily some prominences present, which 

 gave us a partly known spectrum on the plate which could be used 

 as reference spectrum. The following results were obtained by 

 means of this instrument. 



1°. The lower parts of the corona gave a strong continuous 

 spectrum which left a photographic impression to a wave-length 

 8530, that is, beyond N and up to a height of 3 minutes from the 

 edge of the sun. 



2^ The upper parts of the corona gave a spectrum apparently 

 homogeneous and of a wave-length which seems to coincide with 

 the hydrogen line near G, and therefore most likely due to 

 hydrogen. 



In addition to this a ring is seen partly round the sun, corre- 

 sponding to this same hydrogen line. This fact is of importance 

 in connexion with a photograph obtained during the late eclipse by 

 Dr Henry Draper by means of a similar arrangement. No ring 

 shews on Dr Draper's photograph, and this is only one of many facts 

 which tend to shew that the line spectra in the corona this year 

 were uncommonly faint. Prof Young's testimony on this subject 

 is decisive. The following is taken out of a letter written by him 

 to one of the New York papers and reprinted in Silliman's Journal. 



While however there may be room to question the conclusion 

 that the corona this year was uncommonly faint, there can be no 

 question that its spectrum was profoundly modified. 



"The bright lines which come from the gaseous constituents 

 were conspicuous in 1869 and in all the subsequent eclipses until 

 the present one, but this year they were so faint as to be seen by 



