The purpose and design of the new equipment at the royal observatory, Edinburgh

The author deals with the reasons which determined the form of the telescope - a Cassegrain reflector of 36 inches aperture and 54 feet focal length, serving a spectrograph of one, two or three prisms - and indicates the problems to which it is intended to apply it. These are, generally, the intensity of stellar light for different parts of the spectrum. It is pointed out that even an empiric treatment of this question led Adams to a method that had doubled our knowledge of stellar distances. A theoretical treatment, comprising a discussion of the behaviour of the photographic plate, led to fixing the temperature sequence. A slit spectrograph is required in order to deal with the lines. The state of the lines, combined with that of the continuous spectrum, conveys all the information that reaches us of the star's constitution and atmosphere. Subsidiary but highly interesting questions are those of selective absorption of light by the atmosphere of the earth. Behind these are the theoretical and laboratory investigations of the relation between intensity of light and the density of silver deposit on the photographic plate. The whole presents an attractive field.