94 CLIMATIC CYCLES AND TREE-GROWTH. 



center to center. The proportionate transparent part is about three- 

 tenths of the center-to-center measurement. The area covered by 

 these lines 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 compart- 

 ment. 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. Vary- 

 ing 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 analyz- 

 ing 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 con- 

 denser 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 com- 

 partment 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- 



