Microscopical micrometers are generally employed for measuring very minute spaces, such as the subdi disions of astronomical instruments, or the diameters of objects magnified in the field of a microscope. All the micrometers described in chapter first, may be considered as micrometers of this kind, when placed in the anterior focus of the eye-glass of a microscope.
One of the simplest micrometers for microscopes, consists of a screw with an index, fixed in the focus of the eye-glass. The point of the screw is made to move across the space to be measured ; and as the index re gisters the number of revolutions necessary for this purpose, the diameter of the object will be found by ascertaining the number of threads in one inch.
Dr. Junin measured the magnitudes of minute ob jects, by comparing them with small pieces of silver or brass wire placed beside them ; the diameter of which had been previously ascertained by coiling it round a cylinder, and observing how many breadths of the wire were contained in a given number of inches.
One of the best microscopical micrometers is the single lens micrometer, invented by Dr. Wollaston. This instrument, says Dr. Wollaston, is furnished with a single lens of about of an inch focal length. The aperture of such a lens is necessarily small, so that when it is mounted on a plate of brass, a small perforation can be made by the side of it in the brass, as near to its centre as of an inch.
When a lens thus mounted is placed before the eye for the purpose of examining any small object, the pupil is of sufficient magnitude for seeing distant ob j ects at the same time through the adjacent perfora tion, so that the apparent dimensions of the magnified image might be compared with a scale of inches, feet, and yards, according to the distance at which it mig)tt be convenient to place it.
A scale of smaller dimensions attached to the in strument will, however, be found preferable on ac count of the steadiness with which the comparison may be made ; and it may be seen with sufficient dis tinctness by the naked eye, without any effort of nice adaptation, by reason of the smallness of the hole through which it is viewed.
The construction that I have chosen for the scale is represented in Plate CCCLXXVI. Fig. 21. It is com posed of smalls wires about of an inch in diame ter, placed side by side, so as to form a scale of equal parts, which may be with ease counted by means of a certain regular variation of the lengths of the wires.
The external appearance of the whole instrument is that of a common telescope consisting of three tubes. The scale occupies the place of the object-glass, and the little lens is situated at the smaller end, with a pair of plain glasses sliding before it, between which the subject of examination is to be included. This part of the apparatus is shown separately in Fig. 23. It has a projection, with a perforation, through which a pin is inserted, to connect it with a screw represent ed at b, Fig. 22. This screw gives lateral motion to the object, so as to make it correspond with any parti cular part of the scale. The lens has also a small mo tion of adjustment, by means of the cap c, Fig. 22, which renders the view of the magnified object distinct.
Before the instrument is completed, it is necessary to determine with precision the indications of the scale, which must be different, according to the dis tance to which the tube is drawn out. In my instru ment, one division of the scale corresponds to of an inch, when it is at the distance of 16.6 inches from the lens ; and since the apparent magnitude in small angles varies in the simple inverse ratio of this distance, each division of the same scale will correspond to at the distance of 8 y inches, and the interme diate fractions 4-", &c. are found by intervals of 1.66 inch, marked on the outside of the tube. The basis on which these indications were founded in this instrument, was a wire, carefully ascertained to be of an inch in diameter, the magnified image of which occupied fifty divisions of the scale, when it was at the distance of 16.6 inches ; and hence one division= 1 1 50 x 200 10o00' Since any error in the original esti mate of this wire must pervade all subsequent measures derived from it, the substance employed was pure gold, drawn till fifty two inches in length Weighed exactly five grains. If we assume the specific gravity of gold to be 19.36, a cylindrical inch will weigh 3837 grains ; and we may thence infer the diameter of such a wire to be of an inch, more nearly than can be ascertained by any other method.