INSTRUMENTS AND TECHNIQUE 49 



the movable mirror is changed in distance. In order to get the reflec- 

 tion from the mirror properly placed, the box has to have its plate end 

 lowered when the mirror comes near. 



Cyclograph camera — By the track-and-mirror arrangement, above 

 described, the observer can stay at one point while the moving mirror 

 changes the effective distance between the curve and the lens, and 

 by changing the size of the focal image brings into view all the range 

 of periods of which the instrument is capable. 



Lens. — The lens is a Tessar II B of 6 inches focus and about 

 i-inch aperture, with a negative cylindrical simple lens of —6 inches 

 focus with horizontal axis, so that in the vertical direction it neutral- 

 izes the action of the main lens. Without the cylinder there is an 

 ordinary image at 6 inches. With the cylinder all the horizontal spacing 

 comes in as before, but there is no vertical focussing; consequently, 

 each maximum in the curve appears in the image as a vertical band 

 whose intensity is proportional to the height of the maximum. 



Automatic focus — The lens is mounted as in previous instruments 

 inside and on the base of a suspended parallelogram with hinges at 

 each angle. The length of the parallelogram extends along the axis 

 of the instrument, in line with the track. This permits a focussing 

 motion of the lens in its axial line. From the front of the parallelogram 

 a lever-arm extends downward and is attached by an adjusting-screw 

 to a horizontal rod passing forward toward the axis of the drum which 

 moves the mirror-carriage. A cross-piece on the rod bears against a 

 brass spiral mounted near the axis of the drum and turning with it. 

 This spiral is so arranged that as the drum turns, the position of the 

 lens changes and the focus is maintained in a fixed plane. 



Analyzing-plate — The analyzing-plate is fixed at the focus of the 

 lens in a brass mounting attached to the back of this front compart- 

 ment of the analyzing-box. The mounting has been elaborate enough 

 to test many details and is rather more complete than ordinarily 

 needed. On the fixed plate is a circular brass plate which can be 

 rotated through 45° against a graduation in degrees. A rectangle 1 

 inch high and 2 inches long is cut through the circular plate, and on 

 this rectangle is mounted the analyzing-plate, covering a little more 

 than the rectangle. The ruled lines of the plate are vertical, that is, 

 parallel to the short side of the rectangle. In normal position the 

 circle is clamped so that the fines are inclined 12° from the vertical, 

 and therefore 12° from the vertical bands in the image. 



The plate itself is made of two screens accurately ruled 50 lines 

 to the inch, face to face, one fixed and the other with a slight motion 

 controlled by a screw. The purpose of this is to change the relative 

 size of the transparent part of the ruling without changing the distance 

 from center to center of the fines. In each screen the opaque ruling 

 is equal in width to the transparent space between. So by moving 



