OF SOME OF THE STAES AND NEBTJLiE. 
551 
Since the diminished intensity of the marginal parts of the sun is probably due to the 
greater depth of solar atmosphere through which it had passed, it was conceivable that 
by its spectrum the light of these parts of the solar disk might exhibit some indications 
of the larger amount of absorbing medium which it had traversed. 
Two different methods of observation were employed. The telescope, armed with one 
of the spectroscopes constructed for stellar observations, was directed to the sun. By 
the aid of the finder the telescope was moved so as to bring in succession upon the slit 
different parts of the solar disk. Though the detection of any difference depended upon 
the memory of the observer, still any considerable alteration would certainly have been 
noticed. 
The other plan was to cause the sun’s light, after reflection from a plane mirror 
attached to a heliostat moved by clockwork, to fall upon an achromatic object-glass of 
6 feet focal length, by which an image of the sun was formed upon the slit of the large 
spectroscope which I employed for the examination of the spectra of the chemical elements. 
By neither of these methods have I been able to detect any difference in the spectra 
of different parts of the sun’s disk. 
2. I have made numerous observations for the purpose of obtaining a view, if possible, 
of the red prominences which are seen during a solar eclipse. The invisibility of these 
objects at ordinary times is supposed to arise from the effect of the illumination of our 
atmosphere. If these bodies are gaseous, their spectra would consist of bright lines. 
With a powerful spectroscope the light from our atmosphere near the sun’s limb would 
be greatly reduced in intensity by the dispersion of the prisms, while the bright lines of 
the prominences, if such were present, would remain but little diminished in brilliancy. 
These observations have been made with different spectroscopes, and also with other con- 
trivances arranged on the same principle, but hitherto without success. 
[The observations made in India of the solar eclipse of August 18 have shown that 
the spectra of the prominences are discontinuous. Lieut. Herschel, R.E., who had 
charge of the instruments sent out by the Royal Society, determined approximately the 
position of three bright lines. One in the red about C, one apparently coincident with 
D, and one near F. On the day after the eclipse, M. Janssen, making use of a method 
similar to that described above, succeeded in observing the solar ^protuberances. On 
October 20, 1868, Mr. Lockyer also succeeded in viewing the spectrum of a protu- 
berance. Now that the positions of the lines are known, one of the “ other contrivances ” 
mentioned above will probably succeed. It consisted of screens formed of absorptive 
media, by which light of all refrangibilities other than those corresponding to the lines 
of the flames might be absorbed. By this method the flames could be examined and 
measured. — October 30, 1868.] 
3. The third object which I had proposed to myself was to seek to gain from an 
examination of the spectra of the umbrae and penumbrae of solar spots some information 
as to the nature of these remarkable phenomena. 
I had already made some experiments in this direction when in August 1866 I received 
mdccclxviii. 4 g 
