In 1863 Mascart undertook a complete photographic investigation of the ultra-violet portion of the solar spectrum, a work of no mean magnitude. He, however, did not employ a train of prisms for producing the spectrum, but a diffraction grating, using the light reflected from the first surface. The first photograph of the spectrum of a star was secured by Henry Draper, the son of Dr. J. W. Draper, one of the pioneers in photography in 1872. It was not till the introduction of dry plates in 1876 that the photography of the fainter celestial objects or of their spectra was possible, as a long exposure was naturally required. Stellar spectra were photographed by Huggins in 1879, and in the next year Draper photographed the nebula of Orion. As the dry plates became more rapid, and as longer exposures were employed, revelation followed revelation; the nebulæ as seen by the naked eye, and even some stars, were found by the Henrys, Roberts, Max Wolf, Barnard, and others, to be but the brighter kernels of large nebulous patches. This new application of photography, depending upon long exposures (the longest one I know of has extended to forty hours), had an important reflex action on the mechanical parts of the telescope; it was not only necessary to keep the faintest star exactly on the same part of the plate during the whole of the exposure, but night after night the stellar image must be brought on to the same part of the plate so that the exposure might be continued. A system of electric control of the going of the driving-clock of the telescope by means of a sidereal clock was introduced, the simplest one being designed by Russell, of Sydney; a most elaborate one by Grubb, of Dublin. Another application of the method of long exposures has been the discovery of minor planets by the trails impressed by their motion among the stars on the photographic plates on which the images of both are impressed. A complete spectroscopic survey of the stars by means of photography was commenced in 1886 at Harvard College, as a memorial to Draper, who died while he was laboring diligently and successfully in securing advances in astrophysical inquiries. To carry on this work at Harvard, Professor Pickering wisely reverted to the method first employed by Fraunhofer, and utilized by Respighi and another in 1871, of placing prisms in front of the object-glass. In the photographing of stellar spectra by means of objective prisms, the driving-clock of the telescope must _not_ go exactly at sidereal rate, but at certain speeds depending on the brightness and position of the star under examination.