552 
ME. W. HUGGINS ON THE SPECTEA 
a note from M. Faye, 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 Eoyal Society, vol. xv. p. 256. 
