Page:Researches on Irritability of Plants.djvu/42

Rh afterwards easy to reproduce this record by contact-print on a photographic paper.

It is sometimes necessary to have the recording-point brought two or more times to the same place, in order that the successive records may be rendered the more strictly comparable. This is accomplished by a rack and pinion to adjust the height of the platform carrying the plant. When the platform is lowered, the petiole, which is attached to one arm of the recording-lever, pulls it down, and the recording-point is moved to the left. When the platform is raised, then by the action of the counterpoise attached to the other arm of the lever the index is moved in the opposite direction. In this way the recording-point can be brought to any position that is desired. The same end is secured through adjustments of a micrometer by which the carrier of the writing-index is raised or lowered.

Another adjustment that is necessary is the bringing of the recording-point near to the writing-surface without actual contact; so that, when the index is set in a state of resonance, it may trace a dotted line. The necessary adjustment is brought about by means of a micrometer-screw at the top of the instrument, by which the lever can be made to approach or recede from the writing-surface. When the speed of the plate is slow, the successive dots may be so close together as to appear like a continuous line.

More troublesome is the adjustment necessary to render the plane of movement of the index exactly parallel to the writing-surface. If this be omitted, the writing-point in one part of the record, say to the right, will be too far away to strike the surface, whereas in another part, say to the left, it will press against the plate and lose its freedom. This difficulty I have been able to overcome by mounting the vertical rod, carrying the writer, inside another tube. An attached tangent-screw, T, then causes a very slow rotation