Page:Climatic Cycles and Tree-Growth - 1919.djvu/119

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METHODS OF PERIODIC ANALYSIS.

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 particular 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.