The circles are divided into 100 parts, and have no determinate value in angular measurement, but their value is de termined experimentally by observing through the telescope ; it is applied to the diameter of the sun, or any other body, whose angular measure has been pre viously and accurately determined by some other divided instrument, and from this the angle given by each observation is calculated.
The micrometer has not only been ap plied to telescopes, and employed for as tronomical purposes, but there have been various contrivances for adapting it to microscopical observations. M. Leeu wenhoek s method of estimating the size of small objects, was by comparing them with rains of sand, of which one hun dred in • line took up an inch. These grains he laid upon the same plate with his objects, and viewed them at the same time. Dr. Jurin's method was similar to this; for he found the diameter of a piece of fine silver wire, by wrapping it very close upon a pin, and observing how ma ay rings made an inch : and he used this wire in the same manner as Leeuwenhoek used his sand. Dr. Hook used to look upon the magnified object with one eye, While, at the same time, he viewed other objects, placed at the same distance, with the other eye. In this manner, he was able, by the help of a ruler, divided into inches and small parts, and laid on the pe destal of the microscope, as it were, to cast the magnified appearance of the ob ject upon the ruler, and thus exactly to measure the diameter which it appeared to have through the glass ; which being compared with the diameter, as it appear ed to the naked eye, easily sheaved the degree in which it was magnified. A lit tle practice, says Mr. Baker, will render this method exceedingly easy and plea sant.
Mr. Martin, in his Optics, recommends such a micrometer for a microscope as had been applied to telescopes ; for he advises to draw a' number of parallel lines on a piece of glass, with the fine point of a diamond, at the distance of one-fortieth of an inch from one another, and to place it in the focus of the eye glass. By this method Dr. Smith con trived to take the exact draught of ob jects viewed by a double microscope ; for he advises to get a lattice, made with small silver wires or squares, drawn upon a plain glass by the strokes of a diamond, and to put it into the place of the image formed by the object-glass. Then, by transferring thh parts of the object, seen in the squares of the glass or lattice, upon si m liar correspondi ng squares drawn on paper, the picture may be exactly taken. Mr. Martin also introduced into
compound microscopes, another micro meter, consisting of a screw.
A very accurate division of a scale is performed by Mr. Coventry, of South wark. The micrometers of his construc tion are parallel lines drawn on glass, ivory, or metal, from the lOth to the 10,0.0th part of an inch. These may be applied to microscopes for measuring the size of minute objects, and the magnify ing power of the glasses ; and to teles copes for measuring the Use and distance of objects, and the magnifying power of the instrument. To measure the Use of an object in a single microscope, lay it on a micrometer whose lines are seen magnified in the same proportion with it, ithey give, at one view, the real size of the object. For measuring the magni fying power of the compound microscope, the best and readiest method is the fel lowing : On the stage, in the focus of the object-glass, lay a micrometer, consisting of an inch divided into 100 equal parts, count how many divisions of the micro meter are taken into the field of view ; then lay a two-foot rule parallel to the micrometer : fix one eye on the edge of the field of light, and the other eye on the end of the rule, which move till the edge of the field of light and the end of the rule correspond ; then the distance from the end of the rule to the middle of the stage will be half the diameter of the field. Er. gr. If the distance be 10 inches, the whole diameter will be 20, and the number of the divisions of the micrometer contained in the diameter of the field is the magnifying power of the microscope.
Mr. Adams has applied a micrometer, that instantly shews the magnifying pow er of any telescope.
In the Philos. Trans. for 1791, a very simple scale micrometer, for measuring small angles with the telescope, is de scribed by Mr. Cavallo. This micrometer consists of a thin and narrow slip of moth er-of-p e arl, finely divided, and placed in the focus of the eye-glass of a teles cope, just where the image of the olAject is formed, whether the telescope is a reflector or a refractor, provided the eye-glass be a convex lens. This sub stance, Mr. Cavallo, after many trials, found much more convenient than either glass, ivory, horn, or wood, as it is a very steady substance, the divisions very easy marked upon it, and when made as thin as common writing paper, it has a very useful degree of transparency.