METHODS OF PERIODIC ANALYSIS. 93 
bevel attached to a vertical axis with a worm gear at the top, which the 
electric motor drives with a belt connection. In order to aid the motion 
of the camera, a cord passes from its back to the outer end of the track 
and by a system of pulleys and weights exerts a slight constant force. 
The motor is so connected that the camera travels away from the curve. 
The details here described may be seen in plate 10. 
The differential pattern mechanism.—The camera is divided into 
three separate compartments, to each of which access is obtained by a 
sliding door moving in grooves on the side. The front compartment 
produces the differential pattern. It is about 7 inches long by 5 inches 
wide in the clear and 4 inches high. It is nearly divided into two parts 
by a partition which comes down from the top at about 2 inches from 
the front end. This partition does not go down to the floor of the com- 
partment, but leaves a space of about an inch. A hole 1.5 inches in 
diameter is cut through the front of this compartment a little above its 
center, and another hole of the same size to match is cut through this 
partition, while at the back of this compartment a large opening is 
made a little over 2.5 inches wide and about 2 inches high. The lens 
is carried on a special carriage consisting of a horizontal and a vertical 
part. The vertical piece has a hole 1.5 inches in diameter cut in it, 
and the lens is mounted over the hole. The lens now in use consists 
of a spherical lens concavo-convex 2 inches in diameter and 12 inches 
in focus placed on the inside, and a positive cylindrical lens of the same 
size and focus placed on the outside with axis vertical. The convex side 
of each lens is placed outward. The lens carriage is placed partly under 
the partial partition and the lens in its holder comes directly between 
the two holes mentioned. When the sliding door of the compartment 
is down, the compartment is sufficiently light-tight to fulfill all the 
requirements of a camera. The movable carriage of the lens is mounted 
on two small glass tubes and runs between guides. A spring at its back 
end pulls it toward the position of focus for distant objects, where its 
motion is stopped by a pin. A long screw is passed through a hole in 
the bottom of the camera box and enters the bottom of this lens car- 
riage, so that an automatic arrangement outside and underneath the 
camera can regulate the focus. This consists of a vertical axis with 
two lever arms. The upper lever arm is a short one connected to the 
screw which comes from the lens board. The lower lever arm is some 
4 inches below the upper and goes off in a direction nearly at right 
angles; it carries on its end a wheel in a horizontal position. This 
wheel is so placed that it runs on an especially arranged track attached 
to the side of the center rail of the main track. By varying the eleva- 
tion of this special focussing track in different parts of the main track, 
the focus of the lens can be automatically controlled. 
At the back of this first compartment is the analyzing plate, the same 
plate used in previous work. The spacing of its lines is 0.5 mm. from 
