Licence: Public Domain Mark
Credit: On the modes of measuring microscopic objects / by Harting of Utrecht. Source: Wellcome Collection.
Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.
3/12 (page 3)
![fully measured1 and marked off upon the thicker wire. The thickness of the harpsichord wire was consequently millim. = -13359 nullim. — '0052695 inches; and as the total turns of the coil were counted with certainty to within one-half of a turn, the greatest possible error could not amount to more than i th of the whole length of the coil, nor consequently affect the obser ved dia- meter of the wire by more than T4^nrth of a millim.=about ,ot ?.n inch The figure found might hence be regarded as quite correct even in the fourth decimal place. The difference2 between the diameters of the outer sur- face of the wire and of the surface at right angles to it, measured successively with a screw-ocular-micrometer, proved to be ‘00299 millim. — ‘000118 inches. If, now, the thickness of such a wire he used as a standard, not only can we compare with it the values of the divisions of glass, and screw-micrometers, but we may even assume, with much probability, that similar results are true of all micrometers made by the same workman with the same dividing-machine. Ii5 then, those who record their micrometric observations take care to mention the source from which they obtain the instrument employed, or the standard on which their measurements are based, the true measurements may be deduced from their figures by multiplying them by a certain co-efficient, determined by pieiious examination .of a micrometer by the same maker, thus a caieful testing of the following micrometers has taught me that their indications must be corrected by multiplying by the fraction which follow's each, in order to deduce the true value as given by the metallic thread above-mentioned :— Screw-micrometer, by Schiek, 0‘937 Glass-micrometer, by Oberlneuser, 0‘959 Screw-micrometer, by Pow'ell, ... 0 967 Glass-micrometer, by Ch. Chevalier, 0 969 Glass-micrometer, by Dollond, 0‘9S1 Screw-micrometer, by Ploszl, 0‘991 Glass-micrometer, by Ploszl, 1‘067 —Vol. ii., p. 293. Micrometric Nomenclature. [In his remarks on this subject, Harting proposes the use of a standard scale in which TJ(75th of a millimetre, or one micromillimetre, is regarded as unity. This method would certainly have the advantage of enabling us to dispense with the zeros prefixed to the significant decimal places in observed measurements, and would give a more distinct relative idea of the size of an object than is attainable by the use of any other nomenclature. But in this country the Eng- lish inch is usually employed as the standard, and as we write chiefly for the instruction of our English readers, who are not likely to change a system which has been long used among them, for another which, though more convenient, is neither more accurate nor more generally understood, it is unnecessary to re- produce here the arguments by which Harting endeavours to recommend his new method.]—Trans. On the Instruments used in Micrometry. The number of micrometric methods is considerable; hut in the following sections we shall content ourselves with the description of those at present in general use, reserving for the last volume of this work the description of those 1 To measure this I used an iron metre, which had been tested by Swinden at Paris, and is now preserved in the museum of objects pertaining to natural philosophy at Utrecht. 2 This difference of thickness, which occasions a certain degree of flattening of the wire, is here, as in other cases,— thr example, the hair of the head,—the cause of the curling (krulltn) of such a fibre.](https://iiif.wellcomecollection.org/image/b28043133_0005.jp2/full/800%2C/0/default.jpg)
