Great Sun-Spot Group, August 8, 1917. The disk in the lower left corner represents the comparative size of the earth. (Photo, Mt. Wilson Solar Observatory.)
The Sun's Disk. The view shows the "rice grain" structure of the photosphere and brilliant calcium flocculi. (Photo, Yerkes Observatory.)
The Lunar Surface Visible During a Total Eclipse of the Moon, February 8, 1906. (Photo, Yerkes Observatory.)
Another research of exceptional promise will be undertaken, which is of great importance in a general study of stellar evolution; and that is the determination of the spectral-energy curves of stars of various classes, for the purpose of measuring their surface temperatures. A very few of the nebulæ are found to be variable, and their peculiarities need investigation, also special problems of variable stars and temporary stars, and the spectra of the components of close double stars which are beyond the power of all other instruments to photograph.
Such a program of research conveys an excellent idea of many of the great problems that are under investigation by astronomers to-day, and gives some notion of the instrumental means requisite in executing comprehensive plans of this character. It will not escape notice that the climax of instrumental development attained at Mount Wilson has only been made possible by an unbroken chain of progress, link by link, each antecedent link being necessary to the successful forging of its following one. In very large part, and certainly indispensable to these instrumental advances, has the art of working in glass and metals been the mainstay of research. As we review the history of astronomical progress, from Galileo's time to our own, the consummate genius of the artisan and his deft handiwork compel our admiration almost equally with the keen intelligence of the astronomer who uses these powerful engines of his own devising to wrest the secrets of nature from the heavens.