The advantages of sun spot photography in ascertaining the sun's rotation, keeping count of the spots, and in a permanent record for measurement of position of the sun's axis and the spot zones, are obvious. In direct portrayal of the sun's corona during total eclipses, photography has offered superior advantages over visual sketching, in the form and exact location of the coronal streamers; but the extraordinary differences of intensity between the inner corona and its outlying extensions are such that halation renders a complete picture on a single plate practically impossible. The filamentous detail of the inner corona, and the faintest outlying extensions or streamers, the eye must still reveal directly. In solar spectrum photography, research has been especially benefited; indeed, exact registry of the multitudinous lines was quite impossible without it. Photographic maps of the spectrum by Thollon, McClean and Rowland are so complete and accurate that no visual charts can approach them. Rowland's great photographic map of the solar spectrum spread out into a band about forty feet in length; and in the infra-red, Langley's spectrobolometer extended the invisible heat spectrum photographically to many times that length. At the other end of the spectrum, special photographic processes have extended the ultra-violet spectrum far beyond the ocular limit, to a point where it is abruptly cut off by absorption of the earth's atmosphere. On the same plate with certain regions of the sun's spectrum, the spectra of terrestrial metals are photographed side by side, and exact coincidences of lines show that about forty elemental substances known to terrestrial chemistry are vaporized in the sun. [Illustration: A VIEW OF THE 100-FOOT DOME IN WHICH THE LARGEST TELESCOPE IN THE WORLD IS HOUSED. (_Courtesy, Mt. Wilson Solar Observatory._)] [Illustration: MOUNT CHIMBORAZO, NEAR THE EQUATOR. An observatory located on this mountain would make it possible to study the phenomena of northern and southern skies from the same point. (_Courtesy, Pan-American Union._)] [Illustration: LICK OBSERVATORY, ON THE SUMMIT OF MT. HAMILTON, ABOUT TWENTY-FIVE MILES S. W. OF SAN JOSE, CALIFORNIA. It contains the famous Lick telescope, a 36-inch refractor.] [Illustration: NEAR VIEW OF THE EYE-END OF THE YERKES TELESCOPE. The eyepiece is removed and its place taken by a photographic plate.]