The time occupied by the sun's disc in passing before the slit gives the value of a second of time in minutes of arc. This, combined with the length of the lines estimated in the same unit, gives the means for a graphic representation of the protuberance.
The application of this method to the study of the solar region just mentioned as seen on this occasion, showed a protuberance extending over about thirty degrees (or one twelfth) of the sun's circumference, ten of which were east of the vertical diameter, and twenty west. Near the extremity of the western part, a cloud was lying, distant one and a half minutes, or one twentieth of the sun's diameter, from its limb. This cloud, about two minutes long and one high, was parallel to the limb. One hour afterward, a new drawing showed that the cloud had risen rapidly, and taken a globular form. But its movements soon became still quicker; for ten minutes later, at eleven o'clock, the globe was enormously extended in a direction perpendicular to the limb and to its previous position. A little mass of matter was also detached from the lower part, and hung between the sun and the main body of the cloud. Thick weather coming on prevented further observations.
To resume our remarks. Considered in regard to its principle, the new method is based upon the difference of the spectral properties of the protuberances and of the photosphere. The light of the latter emanates from solid or liquid particles, which are incandescent, and is incomparably brighter than that of the former; so that these have hardly been visible hitherto, except during eclipses. But the case is quite altered when we use the spectra of these bodies. For the solar light is spread over the whole extent of its spectrum, and thus much weakened; while that of the protuberances, on the contrary, is condensed into a few lines whose intensity bears some proportion to that of the corresponding solar ones. Hence their lines are quite easily seen in the field, together with those of the sun, though their ordinary images are entirely effaced by the dazzling light of the photosphere.
Another very fortunate circumstance for the new method comes to the support of the one just mentioned, namely, that the bright lines of the protuberances answer to the dark ones of the solar spectrum. Hence they are not only more easily seen in their own proper field, outside and on the edge of the solar spectrum, but they can even be followed into the interior of the latter, and by this means the protuberances can be traced upon the globe of the sun itself.
As regards the determination of chemical composition, the methods followed during total eclipses always carried with them some uncertainty; since, in the absence of the sun's light, graduated scales had to be employed to fix the position of the lines. The new method enables us to compare the two spectra directly.
As to the results obtained during the brief period in which this method has been used, they are as follows:
1st. That the luminous protuberances observed during total eclipses belong unquestionably to the circumsolar regions.
2d. That these bodies are mainly or entirely composed of incandescent hydrogen gas.
3d. That they are subject to movements of which no terrestrial phenomenon can give us any idea; since, though they are masses of matter having several hundred times the volume of the earth, they change completely their form and position in the course of a few minutes.
Such are the principal facts arrived at. I hope, notwithstanding the state of my eyes—fatigued by protracted experiments upon the subject of light—that I shall be able to continue my labors, and have the honor of submitting the results to the Bureau.