50 CLIMATIC CYCLES AND TREE-GROWTH 



one screen slightly across the other, the transparent part can be 

 changed from zero up to 0.01 inch. The width found advantageous 

 is 0.004 inch or two-tenths of the spacing of the lines. 



Visual compartment — From the analyzing-plate the light passes 

 into the middle or visual compartment through the condensing-lenses. 

 These are two 6-inch positive cylindrical lenses with vertical axis, so 

 that the eye placed 6 inches away may receive all the light from the 

 plate and see its whole area. It is more convenient to have the observer 

 at the side than at the end, where he may interfere with the light 

 coming from the curve beyond, so back of the condensers is a vertical 

 mirror on a hinged support. When the support is pulled forward, it 

 takes a position at 45° and throws the beam out at the side through a 

 small lens and to the eye. The lens puts the image slightly out of 

 focus to the eye, as in such condition the eye recognizes alignments of 

 blotches better. 



Photographic compartment — When the mirror-support is thrown 

 back out of the way, the beam goes straight on to a triple lens of 3 

 inches focus, which reproduces the analyzing pattern on a ground 

 glass in the third and last compartment. This last compartment is 

 held separate on a clamp by which the ground glass may be brought 

 to the most advantageous focus. A plate-holder fits in place of the 

 glass and may occupy three slightly different positions, so that three 

 exposures can be made on the same plate. 



Recent changes — The above description gives the form of the 

 instrument used in the cycle analyses in this volume. But since 

 writing this chapter added floor-space has made it possible to lengthen 

 the track to 40 feet. With this the two extra mirrors have been 

 removed, together with the automatic focussing device and scale 

 illumination, and a small convenient scale is now located directly in 

 front of the observer. 



CYCLOSCOPE 



A small portable analyzer has been constructed for exhibit pur- 

 poses, but fully equal to real analyzing work. It consists of a small 

 illuminator with a long electric light inclosed and cord to be attached 

 to a wall-socket. Curves 10 inches long may be placed in this. The 

 analyzing part is a box 12 inches long and 4 inches square, with top 

 which opens on a hinge. It carries a convex spherical and a cylindrical 

 lens at the front, with a little chance to focus by hand; then a simple 

 analyzing-plate fixed at the proper inclination; then condensing- 

 lenses and an eye-lens. One looks through it toward the illuminated 

 curve and walks nearer or farther and watches the changing pattern. 

 When a cycle is indicated by proper horizontal alignment of spots in 

 the analyzing pattern, its value may be found by a simple formula 

 after measuring the distance from the lens to the illuminated curve. 



