In the paper giving the results of the experiments, the phenomena of the spurious disks are very completely described; but they did not attract the attention which they deserved, and they only became an object of especial interest to students of physics when they were again studied by the famous German optician Fraunhofer, a generation later.
Incidentally the experiments are of interest, as yielding us a measure of the excellence of Herschel's telescopes, and a measure which is quite independent of the keenness of his vision. From them we may be sure that the efficiency of the nine-inch mirror used was not sensibly less than that of the highest theoretically attainable excellence. In this connection, too, we may refer to the Philosophical Transactions for 1790, pp. 468 and 475, where Herschel gives observations of both Enceladus and Mimas seen in contact with the ball of Saturn. I have never seen so good definition, telescopic and atmospheric, as he must have had on these occasions.
Researches on the Spectra of the Fixed Stars.
The spectroscope was applied by Secchi to the study of the spectra of the fixed stars visible to the naked eye in the years 1863 to 1866. He examined the nature of the spectrum of each of the larger stars, and found that these stars could be arranged in three general classes or types. His results have been verified and extended by other astronomers, and his classification has been generally accepted. According to Secchi, the lucid stars may be separated into three groups, distinguished by marked differences in their spectra. Secchi's Type I. contains stars whose spectra are like those of Sirius, Procyon, and α Lyræ; his Type II. stars like Arcturus and Aldebaran; his Type III. stars like α Orionis.
Herschel also made some trials in this direction. In the Philosophical Transactions for 1814 (p. 264), he says:
"By some experiments on the light of a few of the stars of the first magnitude, made in 1798, by a prism applied to the eye-glasses of my reflectors, adjustable to any angle and to any direction, I had the following analyses:
"The light of Sirius consists of red, orange, yellow, green, blue, [Pg 197] purple, and violet. α Orionis contains the same colors, but the red is more intense, and the orange and yellow are less copious in proportion than they are in Sirius. Procyon contains all the colors, but proportionately more blue and purple than Sirius. Arcturus contains more red and orange, and less yellow in proportion than Sirius. Aldebaran contains much orange and very little yellow. α Lyræ contains much yellow, green, blue, and purple."
Here the essential peculiarities of the spectrum of each of the stars investigated by Herschel is pointed out, and if we were to use his observations alone to classify these stars into types, we should put Sirius and Procyon into one type of stars which have "all the colors" in their spectra; Arcturus and Aldebaran would represent another group of stars, with a deficiency of yellow and an excess of orange and red in the spectrum; and α Orionis would stand as a type of those stars with an excess of red and a deficiency of orange. α Lyræ would represent a sub-group of the first class.
Herschel's immediate object was not classification, and his observations are only recorded in a passing way. But the fact remains that he clearly distinguished the essential differences of the spectra of these stars, and that he made these observations in support of his statement that the fixed stars, "like the planets, also shine with differently colored light. That of Arcturus and Aldebaran, for instance, is as different from the light of Sirius and Capella as that of Mars and Saturn is from the light of Venus and Jupiter."
Of course, no special discovery can be claimed for him on these few instances. We can see, however, a good example of the manner in which he examined a subject from every side, and used the most remote evidence exactly in its proper place and time.