binaries have been discovered. As the objective prisms employed do not permit the use of a comparison spectrum, the binary character is apparent only when both the components are bright. In such cases the lines of the spectrum are alternately single and double. A study of the spectra of the southern stars photographed with this telescope has been made by Miss Cannon as a part of the Henry Draper Memorial.
The focal length of this telescope is about sixteen feet, so that an arc a degree in length in the sky is represented on the photographic plate by a line more than three inches long. The scale of the instrument is thus very suitable for the details of nebulæ, and for nearly everything except the centers of the densest clusters. For long exposures on difficult objects, such as globular clusters, the telescope must follow the stars in their diurnal motion with great precision. This can only be accomplished with such an instrument by watching a star visually and keeping it constantly bisected by the lines of a reticle. Formerly, a secondary telescope was used for this purpose, but, due to the flexure between the two tubes, and perhaps for other causes, really fine photographs were not obtained with this telescope until a lens for following was inserted into the field of the main instrument, so that the other telescope was dispensed with. In all cases the mean movement of the telescope is provided for by carefully devised and well-constructed clockwork; ad in the case of small and rigid instruments this alone serves fairly well, unless the exposure is more than an hour.
More than five hundred variable stars have been discovered by the writer in the globular clusters, by means of charts made with this instrument. These constitute nearly one half of all the variable stars known, but they all occur in only one thirty-thousandth part of the sky. At the centers of some of these clusters, the stars are packed together so densely that there are one hundred stars to the square minute. If the stars were equally dense over the whole sky, their number would exceed ten billions, and the sky would be so luminous that there would be no real night. In one of these clusters. Messier 3, one hundred and thirty-two variables were found. These are all situated within a circle whose area is one fourth of a square degree, or only one one-hundred-and-sixty-thousandth part of the sky. In this cluster one star in seven is variable. The photographs used for this investigation must be made with the greatest care, and must then be enlarged, or else examined by a microscope, since the images of the stars on the original plates resemble thickly scattered grains of dust.
The duration of exposure employed varies enormously, according to the instrument and the object to be attained. They have been made from one second up to twenty-four hours. With the great Bruce lens, an exposure of one second is sufficient for the brightest stars, while an exposure of four hours, or more than fourteen thousand times as long,
