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 _[alpha] Lyræ_; his Type II. stars like _Arcturus_ and _Aldebaran_; his Type III. stars like _[alpha] 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, purple, and violet. _[alpha] 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. _[alpha] Lyræ_ contains much yellow, green, blue, and purple."