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Having seen some glass, &c. micrometers with exquisite fine divisions, for the purposes of applying to microscopes and telescopes; and in accuracy, being equivalent to the micrometer just described by our author, I judge, some account of their application and uses here will be very acceptable to the curious and inquisitive reader. A particular description of these as made by the ingenious Mr. Coventry, has been already given in the Encyclopædia Britannica, Vol. XI. p. 708.

The singular dexterity which Mr. Coventry and others now possess, of cutting by an engine fine parallel lines upon glass, pearl, ivory, and brass, at such minute distances as, by means of a microscope, are proved to be from the 100th to the 5000dth part of an inch, render this sort of micrometer the easiest and most accurate means of obtaining the exact natural size of the object to be magnified, and how many times that object is magnified. Mr. B. Martin, and other opticians, many years ago applied divided slips of glass, ivory, and horn to the body, in the focus of the eye glass of microscopes; but the thickness of the whole medium of the glass was found to diminish the distinct view of the object: ivory and horn, from their variable texture, were found to expand and contract too readily to be commodious. It is therefore to Mr. Cavallo that we are indebted for the happy thought of adapting slips of divided pearl to telescopes, to ascertain their power, &c. which substance the opticians now find to be the best for microscopical micrometers. It possesses a sufficient degree of transparency, when made about the thickness of writing paper; is a steady substance; admits very easily of the finest graduations, and is generally made in breadth about the 20th part of an inch.

Fig. 9. Plate II. A, is a representation of this scale, with divisions of the 200ths of an inch, every fifth and tenth division being left longer than the others, which only go to about the middle. If the eye glass of the microscope or telescope, to which this micrometer is to be applied, magnify very much, its divisions may be proportionably minute.

To measure by this micrometer the size of an object in a single microscope, nothing more is required than to lay it on the micrometer, and adjust it to the focus of the magnifier, noticing how many divisions it covers or coincides with. Supposing the parallel lines to be the 1000dths of an inch, and the object covers two divisions, its real size is the 500th of an inch; if five, 200th of an inch, &c.

To find how much the object is magnified, is not so easily done by the single, as by the compound microscope, as has been before explained. The following simple method has been adopted by Mr. Coventry, and which may be considered tolerably accurate. Adjust a micrometer under the microscope, suppose 100th of an inch of divisions, with a small object on it, if square, the better; notice how many divisions one side of the object covers, suppose ten; then cut a piece of white paper something larger than the magnified appearance of the object; fix one eye on the object through the microscope, and the other at the same time on the paper, lowering it down till the object and the paper appear level and distinct: then cut the paper till it appear exactly the size of the magnified object; the paper being then measured, suppose an inch square: now, as the object under the magnifier, which appeared to be one inch square, was in reality only ten hundredths, or the tenth of an inch, the experiment proves that it is magnified ten times in length, one hundred times in superficies, and one thousand times in cube, which is the magnifying power of the glass; and in the same manner a table may be made of the power of all the other glasses.

In using the compound microscope, the real size of the object is found by the same method as in the single; but to demonstrate the magnifying power to greater certainty, adopt the following method. Lay a two-feet rule on the stage, and a micrometer level with its surface, (an inch suppose, divided into 100 parts:) with one eye see how many of those parts are contained in the field of the microscope, suppose 50; and with the other, at the same time, look for the circle of light in the field of the microscope, which with a little practice will soon appear distinct; mark how much of the rule, from the center of the stage, is intersected by the circle of light, which will be half the diameter of the field. Suppose eight inches; consequently the whole diameter will be sixteen. Now, as the real size of the field by the micrometers appeared to be only 50 hundredths, or half an inch, and as half an inch is only one 32d part of 16 inches, it shews the magnifying power to be 32 times in length, 1024 superficies, and 32768 in cube or bulk. For accuracy, as well as for comparative observations, the rule should always be a certain distance from the eye; eight inches in general is a proper distance.

Another way, and the most easy for finding the magnifying power of compound microscopes, is by using two micrometers of the same divisions; one adjusted under the magnifier, the other fixed in the body of the microscope in the focus of the eye glass. Notice how many divisions of the micrometer in the body are seen in one division of the micrometer under the magnifier, which again must be multiplied by the power of the eye glass. Example: Ten divisions of the micrometer in the body are contained in one division under the magnifier; so far the power is increased ten times: now, if the eye glass be one inch focus, such glass will of itself magnify about eight times in length, which, with the ten times magnified before, will be eight times ten, or 80 times in length, 6400 superficies, and 512000 cube.

Fig. 10. Plate II. A, represents the field of view of the compound microscope, with the pearl micrometer, as applied to the aperture in the body, called the eye stop; and a magnified micrometer that is laid on the stage, shewing that one of the latter contains ten of the former.

A set of ivory and glass micrometers, about six in number, besides one or two pearl ones for the eye stops, are generally packed up with the best sort of microscopes made by Messrs. W. and S. Jones, Opticians, Holborn. They are divided into lines and squares, from the 100th to the 1000dth parts of an inch; and, besides measuring the magnifying powers of microscopes, are generally found useful in measuring the diameters, proportions, &c. of opake and transparent objects, even of the minutest kind. The smallest divisions of the glass micrometer to be useful, are those divided into the 4000dth part of an inch; and as these may be crossed again with an equal number of lines in the same manner, they form squares of the SIXTEEN MILLIONTH part of an inch surface, each square of which appearing under the microscope true and distinct. And, even small as this is, animalculæ are found so minute as to be contained in one of these squares!

Glass micrometers with squares, applied to the solar microscope, divide the objects into squares on the screen in such a manner, as to render a drawing from it very easy; and are employed with great advantage in the lucernal microscope.

The micrometers are constructed with moveable frames or tubes, so as to be either applied or taken away in the readiest manner.

For the uses of the pearl micrometer as applied to the telescope, see Mr. Cavallo’s pamphlet descriptive of its use, 8vo. 1793, and the Philosophical Transactions for 1791.