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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.










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