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- 
