center to center. The proportionate transparent part is about three-tenths of the center-to-center measurement. The area covered by these lies is 1 by 3 inches, making about 156 lines. The photograph is transparent with dense black lines in it. The glass has been cut down to a convenient size, and this plate is mounted at the back of the first compartment with the film side of the plate toward the back. This plate is over the large opening at the back of the first compartment. The differential pattern is formed automatically by the lens on this plate. The plate is held in a fixed position with its lines nearly vertical but inclined about 12° to the lines of the sweep formed by the lens. This produces fringes more or less horizontal in direction. Varying periods are tested by changing the distance from the curve which alters the scale of the sweep while the analyzing lines are unchanged. As the scale of the sweep changes, the fringes appear to rotate about the center of the differential pattern. Immediately behind the analyzing plate are two condensing lenses described in the next topic. They bring the general beam of light to a focus about 6 inches back of the plate. For visual work a movable mirror, just back of the plate, reflects the beam outside the camera box, through an eyepiece to the eye. For photographic work a small total-reflection prism and simple lens are inserted about 5 inches back of the analyzing plate. These throw the beam outside into a special camera attachment in which ordinary films or plates may be used.
The periodogram mechanism. — The remainder of the camera is especially for the purpose of producing the periodogram from the differential pattern. Almost in contact with the analyzing plate is a condensing lens consisting of two cylindrical lenses about 2 inches in diameter and 6 inches focus; these are mounted with vertical axes and with their convex sides toward each other. The aperture of the condenser is about 0.75 inch in vertical height and 1.75 inches in length. The purpose of these condensers is to coverge the light which comes through the analyzing plate on the slit at the back. The second compartment is nearly the same size as the first, namely, about 6.5 inches long. At its front end is the analyzing plate with the condensers and at its back in the same optical axis is a vertical slit about 1 inch long and 1 mm. wide. The sides of this slit are beveled so that the slit itself is at the back. In the middle of this compartment is a powerful cylindrical lens or combination of lenses with horizontal axis. This lens is made up of 4 separate positive cylindrical lenses, each 2 inches in diameter and 6 inches focus. These all have their convex sides toward the common center. They are mounted on a movable carriage of wood which slips in place or may be removed entirely. The aperture of this lens system is about 1.5 inches long by 0.75 inch high. The effect of the condensing lens and of this cylindrical lens is to cast in the plane of the slit an area of light whose size is essentially a repro-
