METHODS OF PERIODIC ANALYSIS. 95 
duction of the aperture of the objective, namely, 1 inch high by 0.25 
inch wide, but the detail in this area of light is brought in focus by the 
cylindrical lens and integrates the horizontal lines of the differential 
pattern. When, therefore, the differential pattern shows a series of 
horizontal fringes, they become reproduced by a series of horizontal 
lines crossing the slit, while in the slit itself they appear as a series of 
dots. When a period is disclosed by proper position of the camera, it 
will produce horizontal lines on the analyzing plate. A series of black 
and white dots, therefore, go through the slit into the final compartment; 
but when the distance is such that the lines on the differential pattern 
are at some slant, then, the integration carried into the slit being still 
horizontal, the illumination in the slit is uniform. In this way the 
beaded or corrugated effect in the slit indicates a period at that partic- 
ular distance from the curve. 
In order to read off periods directly in the final result without the 
necessity of making exact measures, an automatic signal or period 
indicator is introduced in this second compartment. Above the upper 
and lower ends of the slit are placed small pieces of mirror at 45°, and 
corresponding to these there are two small holes 0.25 inch in diameter 
in the side of the box. Outside of these holes again is a mirror at 45° 
reflecting light from the curve in the window. So long as the holes are 
open, direct light from the curve is reflected by the two sets of mirrors 
through the slit on to the film beyond, as will be described. A shutter 
is placed over the outer holes in the box with a lever carried down to 
the vicinity of the central rail. On the end of the lever arm is a wheel. 
At proper intervals small pieces of wood are placed in the side of the 
track, so that as the wheel passes over them the shutter is opened and 
light passes to the mirrors and makes a dot or a line on each side of 
the film in the third compartment. In this way marks can be placed on 
the film independent of the periodogram, and yet they can be spaced 
exactly to represent the different periods tested. Special periods, for 
example 5 or 10 years, etc., are indicated by the extra length and 
density of the marks produced. These appear on the margins of the 
periodograms in plate 11. 
The final compartment at the rear contains a drum on a vertical 
axis which is slowly rotated as the whole mechanism moves along the 
track. The rear wheel resting on the center rail is connected by 
gearing to the drum, so that 1mm. on the drum represents 42.7 mm. or 
1.7 inches on the track. This makesa convenient length for the final 
periodogram. The drum can be detached, carried to a dark room to 
have a film pinned to its periphery, returned in a special light-tight 
box, and mounted on its axis for an exposure. The times of exposure 
depend on characteristics of the curve under test, but it is necessary 
to allow about 35 minutes for the range from 4 to 15 years, and several 
times that for the range from 15 to 25 years. Plates 10, 11, and 12 
illustrate the apparatus and the periodic analysis produced. 
