144 CLIMATIC CYCLES AND TEEE GROWTH 



top mirror is on a level with the light source, and the lower one is on a level 

 with the object glass of the analyzing box, running the carriage out or in 

 has the sole effect of changing the object distance without disturbing the 

 optical axis. The carriage runway is calibrated in terms of standard scale 

 (see p. 44). 



(3) The Analyzing Box — Facing the movable mirror is found the analyzing 

 box, which contains most of the working parts of the instrument. The 

 objective is a Tessar II B lens of 6 inches focal length, directly in front of a 

 negative cylindrical lens of 12 inches focal length, whose axis is horizontal. 

 Therefore all rays in the horizontal planes come to a focus at 6 inches from 

 the objective, while rays in vertical planes focus at 12 inches. The image 

 formed by this system at the 6-inch focal plane consists then of a reproduction 

 of the holes in the cycleplot with respect to horizontal spacing of the centers 

 of gravity of the holes, but with the images in the form of parallel vertical 

 bars of light ; the width of any bar depends upon the width of the correspond- 

 ing maximum and its light intensity upon the amplitude of the maximum. 

 This image of parallel bars is called a "sweep" or a cylindrical pattern. 



In the focal plane there is placed an analyzing plate consisting of two 

 glass plates having alternate transparent and opaque rulings, whose grating 

 constant is 0.02 inch. Superposing the two plates makes possible any size 

 of transparent line up to 0.01 inch, the total spacing of a pair of successive 

 transparent and opaque lines being constant at 0.02 inch. As arranged at 

 present, the width of a transparent line is closely 0.001 inch. The analyzing 

 plate is inclined at an angle of 17 degrees to the vertical sweep lines. 

 It is then easily seen that the analyzing plate breaks up every vertical bar of 

 light into segments, some of which are permitted to pass through, and some of 

 which are held back. The resulting pattern is a series of light images or dots 

 in several duplicate horizontal bands or sets. With a total effective rec- 

 tangular aperture behind the focal plane of 0.4 by 1.5 inches, the longer side 

 horizontal, an inclination of 17 degrees, with the analyzing plate used, yields 

 almost exactly five of these horizontal bands in the pattern. A thread is 

 placed in the focal plane to indicate horizontal direction. 



Behind the analyzing plate is a condensing lens system consisting of two 

 cylindrical lenses of 6 inches focal length with their axes vertical. Near 

 this system is a small mirror set at an angle of 45 degrees, which throws the 

 light to one side to an observer who views the pattern through an eye lens. 

 The observer then has the cycleplot window to his left and the movable 

 mirrors to his right, while he is out of the direct line of the rays. For photo- 

 graphy, the small secondary mirror is swung out of the way and the light 

 allowed to proceed directly to a camera compartment. 



When the mirror carriage is moved, the object distance changes and, of 

 course, the focal plane will suffer a corresponding slight change. To keep 

 the position of the focal plane constant, the lens system of the analyzing 

 box, consisting of Tessar lens and negative cylinder, is mounted on a movable 

 frame which receives a slight motion from an arm resting against a spiral 



