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Updated: May 7, 2025
If it were measured it must be by an indirect progress, which I accomplished thus: The diameter of the body of B. termo, i.e., from; side to side, may in different forms vary from the 1/20000 to the 1/50000 of an inch. That is a measurement which we may easily make directly with a micrometer.
A new micrometer, a dividing screw, a new surveying-quadrant, problems for clearing the observed distance of the moon from a star of the effects of refraction and parallax, a drawing-machine, a copying-machine for sculpture anything and everything he used or saw seems immediately to have been subjected to the question: "Cannot this be improved?" usually with a response in the affirmative.
The dial on the front of the instrument is intended to indicate the exact amount of physical expansion or contraction of the strip. This is ascertained by means of a micrometer screw, S, which moves a needle, T, in front of the dial. This screw engages with a second and similar screw which is so arranged as to move the strip of vulcanite up or down.
With such an eyepiece the image viewed is beyond the farther or field lens instead of between the two lenses, and as this fact renders it easier to adjust wires or lines for measuring purposes in the focus of the eyepiece, the Ramsden construction is used when a micrometer is to be employed.
The magnification is eight diameters, so that the squares are here each two-thirds of an inch in diameter. I have a number of these micrometers of different scales, and I find them invaluable in examining cheques, doubtful signatures and such like. I see you have packed up the camera and the microscope, Polton; have you put in the micrometer?"
The boys who have progressed furthest do fine micrometer work, and they do every operation with a clear understanding of the purposes and principles involved. They repair their own machines; they learn how to take care of themselves around machinery; they study pattern-making and in clean, well-lighted rooms with their instructors they lay the foundation for successful careers.
These observations were made by adjusting a micrometer to a very powerful telescope, and were data sufficient for the determination of the orbit of the revolving star, should it be a binary system.
But there is one instrument of great value, the heliometer, which is not used at Greenwich. This serves the purpose of a double image micrometer, and is made by dividing the object-glass of a telescope along a diameter. Each half is mounted so as to slide a distance of several inches each way on an arc whose centre is the focus. The amount of the movement can be accurately read.
For a telescope which has no micrometer, the Huyghenian or negative eye-piece, as it is commonly called, is the best. As made by Huyghens, it consists of two plano-convex lenses, with their plane sides next the eye, as shown in the figure. So far as we have yet described our telescope it is optically complete.
But, granted for the moment that we could devise and successfully apply exact and accurate standards of measurement for human beings, our work would be only partially done. Any mechanic knows that it is a sad waste of time and pains to standardize tenons, with micrometer and emery paper, to a thousandth of an inch, so long as the mortises are left unstandardized.
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