Sliding Plate-holder.
My brother, Mr. Daniel Draper, to whose mechanical ingenuity I have on several occasions been indebted for assistance in the manifold difficulties that have arisen while constructing this telescope, continued these experiments at intervals. He presented me on my return with a slide and sand-clock, with which some excellent photographs have been taken. He had found that unless the slide above mentioned was made ungovernably long, the same trouble continued. He then ceased catching the sliding frame h, Fig. 32, by two opposite sides, and made it run along a single steel rod a, being attached by means of two perforated plates of brass b, b′. The cord i going to the sand-clock, was applied so as to pull as nearly as possible in the direction of the rod. A piece of cork c, gave the whole steadiness, and yet softness of motion. The lower end of the frame was prevented from swinging back and forward by a steel pin d, which played along the glass rod e. All these parts were attached to a frame k, fitting on the eyepiece holder, and permitting the rod a to change from the horizontal position in which it is here drawn, to any angular one desired. The thumb-screw f retained it in place; g and g′ are pulleys which permit the cord to change direction.
Subsequently, a better method of examining the uniformity of the rate, than by noticing the sharpness of the photograph produced, was invented. It consists in arranging a fixed microscope, magnifying about 40 times, at the back of the ground glass plate, which fits in the same slide as the sensitive plate. By watching the granulated appearance pass before the eye, as the slide is moved by the clock, the slightest variation from uniformity, any pulsatile or jerking movement is rendered visible. By the aid of this microscopic exaggeration, it was seen that occasionally, when there had been considerable changes in temperature, the steadiness of the motion varied. This was traced to the irregular slipping of b, b′.
Fig. 33.
Frictionless Slide (front view). Sectional view.
A different arrangement was then adopted, by which a lunar crater can be kept bisected as long as is necessary, and which gives origin to no irregularities, but pursues a steady course. The principle is, not to allow a slipping friction anywhere, but to substitute rolling friction, upon wheels turning on points at the ends of their axles. The following wood-cut is half the real size of this arrangement.
A glass rod a, a′, Fig. 33, is sustained by two wheels b, b′, and kept in contact, with them by a third friction roller c, pressed downward by a spring. This rod carries a circular frame d, d′, upon which at e, e′, e″, are three glass holders and platinum catches. A spring f holds the sensitive plate in position, by pressing against its back. The circular frame d is kept in one plane by a fourth friction roller g, which runs on a glass rod h, and is kept against it by the inward pressure of the overhanging frame d. The cord i is attached to the arm k, and pulls in the direction of the glass rod a. From m to a fixed point near b, a strip of elastic India-rubber is stretched, to keep the cord tight. The ring of brass n, n′ carries the whole, serving as a basis for the stationary parts, and in its turn being fastened to the eyepiece holder, so as to allow the glass rod a to change direction, and be brought into coincidence with the apparent path of the moon. At o is a thumb-screw or clamp. Through the ring n, n′, a groove p is cut, into which a piece of yellow glass may be placed, when the actinic rays are to be shut off from the plate.