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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 contrivances 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 protuberance. 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.]
8. The third object which I had proposed to myself was to seek to gain from an examination of the spectra of the umbrae and penumbrse 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 a
a note from M. Fate, in which he suggests to me the prismatic examination of solar spots, since, according to his theory of the constitution of the sun, the spectrum of the umbra of a spot should be compound, consisting of a continuous spectrum with dark lines, and a second spectrum of bright lines. My first observations were made with a direct-vision spectroscope of Hofmann, which was so arranged that the image of the sun was formed upon the slit, after the light had been enfeebled by reflection from a prismatic solar eyepiece.
When, by means of the finder, a spot was brought upon the slit, the feeble light from the umbra appeared as a narrow dark band upon the bright solar spectrum. The lines of Fraunhofer appeared stronger and thicker in the spectrum of the umbra.
In October 1866, Mr. Lockyer, who had independently made similar observations, presented a paper to the Royal Society*, in which he states that he observed the lines of absorption of the solar spectrum to appear thicker where they crossed the spectrum of the spot. He also states that he saw no indication of the presence of bright lines.
It was not until April 15, 1868, that a favourable opportunity occurred to examine a large spot with the new spectroscope described in this paper.
The presence of some haze in the atmosphere permitted the spectroscope to be applied directly to the telescope, and the slit to be placed at the focus of the object-glass. The slit was rotated so that its length was in the direction of the length of the spot, and when the middle of the umbra fell upon the slit, its spectrum appeared as a feebly illuminated band upon the bright solar spectrum. The band appeared divided into two parts by the spectrum of the bright prominence, which extended nearly across the umbra.
The phenomenon of an increase of thickness of the lines of Fraunhofer, which I had previously observed, was very marked.
It was obvious that a part only of the light which appeared to form the spectrum of the umbra came from that particular region of the sun. The imperfect transparency of our atmosphere causes it to become strongly illuminated when the sun shines upon the earth; and the brilliant light which is seen to be radiated by it near the sun’s limb, is also radiated by that portion of the atmosphere which is between the observer and the sun. It might be, therefore, that the whole of the light which appeared to come from the umbra was really due to the illuminated intervening atmosphere. That such was not the case, and that some part of the light to which the spectrum was due came from the umbra, became evident when the telescope was moved so as to bring the sun’s limb across the slit; for then the spectrum of the light from our atmosphere was less bright than the spectrum of the umbra of the spot.
In order to obtain some estimate as to how much of the apparent light from the umbra really came from it, I made use of a graduated wedge of neutral-tint glass. The spectrum of our atmosphere at the sun’s limb became so dark that the lines could not be distinguished when the part of the wedge marked 10 was before the eye. To bring
* Proceedings of the Royal Society, vol. xv. p. 256.