THE COU.DE EQUATORIAL TELESCOPE.
At Paris the instrument used for photo graphing the moon is the equatorial Coude (Fig. SOS). The telescope tube is in two parts, which are bent at right angles, while at the elbow is a mirror at 45° to the incident light which reflects the rays up to the camera or eyepiece. There is also a mirror outside, which reflects the light into the tube. The object glass is corrected for photographic rays, and has a focal length of about 62 ft. The aper ture used is 56 cm. The diameter of the image on the negative, which is after wards enlarged, is about 61 in. The most rapid plates are used, and they are ex posed for nearly half a second. The tele scope is stationary, the plates being moved by clockwork. A fine photograph of the moon, kindly furnished by M. Loewy of the Paris Observatory, and taken by the instrument just described, is reproduced in Plate 41. The parts
which conic out most distinctly are those which are on the borderland between the dark and the illuminated portions of the moon's surface. When the moon is at its full, the sun is behind the earth, amid th,-3 lit-up surface is wanting in contrast effects. This corresponds to taking an ordinary view with the sun directly behind the camera. In all cases where the plate is moved to keep the image in position, advantage is taken of the fact that the focal length of a lens is prac tically the same for all rays which come to a focus on or very near the axis. The definition of the image, therefore, remains good throughout the exposure. If the axis of the lens be inclined very much to the incident light, a caustic or diffused focus is obtained.