In 1873 the Astronomer-Royal of England commenced at Greenwich Observatory to photograph the Sun daily. This work has been carried on there by _Edward Walter Maunder_ (born 1851), and Greenwich Observatory possesses a photographic record of sun-spots. At the Meudon Astrophysical Observatory, near Paris, Janssen has, since 1876, secured photographs of the solar surface, which were comprised in a great atlas, published by him in January 1904. These photographs have revealed a remarkable phenomenon—the “réseau photospherique,” the distribution over the solar surface of blurred patches of light, which Janssen considers are inherent in the Sun. The Greenwich records of sun-spots and of magnetic disturbances have been made use of by Maunder in his remarkable studies, promulgated in 1904, of the connection between sun-spots and terrestrial magnetism. Maunder finds that on the average magnetic storms are dependent on the presence of sun-spots, and on the size of the spot. The magnetic action, he finds, does not radiate equally in all directions from the sun-spots, but along definite and restricted lines. Herschel’s hypothesis of a dark and cool globe beneath the solar photosphere was seen to be untenable after the introduction of the spectroscope. The first important theory as to the solar constitution was that advanced in 1865 by the French astronomer, _Hervé Faye_ (1814-1902). Numerous other theories were afterwards advanced by Secchi, Zöllner, Young, and others, but a complete description of the various developments in solar theorising cannot be given here. There is no complete “theory” of the exact constitution of every part of the Sun, but the unpretentious “Views of Professor Young on the Constitution of the Sun,” which appeared in April 1904 in ‘Popular Astronomy,’ represent the latest ideas of the foremost solar investigator. Professor Young regards the reversing layer and the chromosphere as “simply the uncondensed vapours and gases which form the atmosphere in which the clouds of the photosphere are suspended.” He says that the contraction theory of Helmholtz,—explained in another chapter,—advanced to explain the maintenance of the Sun’s heat, is true so far as it goes; but that it is all the truth is now made doubtful by the discovery of radium, which “suggests that other powerful sources of energy may co-operate with the mechanical in maintaining the Sun’s heat.”