TOTAL ECLIPSE OF THE SUN, APRIL 16, 1893. 
585 
parts is superposed, with the result that there is a greater tendency to masking of 
bright lines. The use of greater dispersion in the case of the prismatic camera would 
tend, to remove its deficiency in this respect. 
(3.) The slit spectroscope will give well-defined spectra, due to the solar light 
reflected by the corona, as observed by Janssen in 1871, while, in the prismatic 
camera, the corresponding dark rings will be too diffuse to show themselves, for the 
reason that they come from so large an area. 
Chromosphere and Prominences. 
The advantages of the slitless spectroscope, in the case of the chromosphere and 
prominences, may be summarized as follows :— 
(1.) Production of actual pictures. Unlike the slit spectroscope, it does not give 
the spectrum of one section only of the corona and prominences, but combines the 
functions of a telescope with those of a spectroscope, and gives actual views of the 
whole of the solar surroundings in each radiation strong enough to produce an image. 
Any chemical differences which there may be between different regions will be shown 
by the limitation of some of the spectral features to certain segments of the rings. 
(2.) Prominences will be localised by the prismatic camera, so that their separation 
from the normal corona spectrum will be greatly facilitated. The superposition of 
such a spectrum upon those due to other sources, when a slit spectroscope is employed, 
prevents their recognition as local phenomena. 
(3.) Elimination of the light from the prominences and corona scattered by the 
dust particles in our air. This light, though producing false lines in the spectrum of 
the corona, and even across the dark body of the moon itself when a slit spectroscope 
is employed, cannot by any possibility produce more than a general illumination of 
the field when viewed through a prismatic camera. Images of the corona can only he 
depicted by true coronal radiations, or by radiations reflected by the corona itself 
from the lower parts of the sun’s atmosphere, if such reflected radiations be possible. 
(4.) There is a great saving of light due to the absence of condensing lens, 
collimating lens, and slit, so that photographs may be obtained with shorter 
exposures, and, therefore, at a greater number of stages of the eclipse. 
Comparison of Methods in the Case of Atmospheric Layers. 
In a Paper communicated to the Society in 1882, # I pointed out the importance of 
considering the conditions under which we observe the phenomena of the sun’s atmos¬ 
phere. It was shown that whether the sun’s atmosphere be composed of concentric 
layers of different composition, or of vapours, all of which rest on the photosphere 
and thin out at different heights, the phenomena observed with a slit spectroscope 
* ‘Roy. Soc. Proc.,’ vol. 34, p. 292. 
4 F 
MLCCCXCVL —4. 
