telescope is used". Nevertheless, Herschel completed his catalogues, and left behind him a work of imperishable value. For a hundred years the work was under-estimated by astronomers, until, at the close of the nineteenth century, the catalogues were reduced and discussed by the late Professor E. C. Pickering, of Harvard, who gave the following estimate of the value of Herschel's work: "Herschel furnished observations of nearly 3000 stars, from which their magnitudes a hundred years ago can now be determined with an accuracy approaching that of the best modern catalogues. The average difference from the photometric catalogues is only ±0·16, which includes the actual variations of the stars during a century, as well as the errors of both catalogues. The error of a single comparison but little exceeds a tenth of a magnitude."
Herschel's observations of starlight were not only visual and telescopic. In 1798 he passed the light of a few stars of the first magnitude through a prism applied to the eye-glasses of his reflectors. He found that the light of Sirius consisted of red, orange, yellow, green, blue, purple, and violet: that Betelgeux contained the same colours, but that the red was more intense than in Sirius; that Procyon contained more blue and purple, and Arcturus more red and orange than Sirius; that Aldebaran contained much orange; and Vega much yellow, green, blue and purple. Of course, Herschel was unable to discern the dark lines in these spectra, but his observation deserves to be remembered as the first application of the prism to starlight.
It is difficult to know whether to admire most the observational skill or the intellectual grasp displayed in these subsidiary researches; and we cannot but feel, with Arago, "a deep reverence for that powerful genius that has scarcely ever erred".
