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 1 mm. on the drum represents 42.7 mm. or 

 1.7 inches on the track. This makes a 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. 



