different parts of sun-spots and from other regions of the sun's surface. Visual studies of the solar details under the best atmospheric conditions, and direct photographs made in the ordinary way, will also be essential. From such a mass of observations, if systematically made and studied, a considerable increase in our knowledge of the solar constitution might reasonably be expected to follow.
It would be beyond the province of my immediate subject to discuss the methods by which the study of the physical constitution of the nebulæ and stars may be expected to throw light on the past and future of the sun. But I can not refrain from remarking that through recent improvements in reflecting telescopes, and through the further improvements which are promised in the immediate future, a great advance in this department of astrophysical research may confidently be expected. It thus appears that if the powerful instruments required for these investigations can be provided, the opportunity should exist during the next quarter of a century to make important additions to our knowledge of the origin and development of the sun, and at the same time to throw new light on the great problem of stellar evolution.