Micrometer (Gr. , small, andmeasure), an instrument applied to telescopes and microscopes for measuring minute spaces and objects. - Telescope Micrometers. A paper by Mr. Townley in the "Philosophical Transactions " for 1667 describes a micromenter with a movable wire which was constructed by a Mr. Gascoigne in England about 1640, and used by him in measuring the diameter of the moon and some of the planets. Gascoigne was killed in the civil wars in 1644, and as he left no account of his invention, it only became known through Mr. Townley, into whose possession one of the telescopes had fallen. This instrument was afterward improved by the celebrated Dr. Hooke, who is said to have added parallel hairs. A similar micrometer was afterward made by Azout, and another by the marquis of Malvasia. The instrument now in use under the name of the filar micrometer is constructed upon the same principles as either of the above, and may be understood by referring to figs. 1 and 2, drawn from instruments by Troughton of London. Fig. 1 represents a section transverse to the axis of the telescope. Two forks, k and l, are moved in a plane perpendicular to the axis of the telescope by means of the fine micrometer screws o and p.
Each of these forks has stretched across it a spider's web which is placed in the focus of the objective. These webs are parallel, and being made to embrace any object, as the disk of a planet or the distance between two stars, the number of turns of the screws, which may bo read by the graduated circles u u, will indicate the space measured, the value of a revolution of the screws having been ascertained by the time occupied by a known star in passing from one line to the other when placed at the distance of a certain number of revolutions, or by the measurement of some known space. Another web is stretched across the centre of the field, perpendicular to the other two. The position of these lines may be revolved about the axis of the telescope by means of the endless screw fig. 2, which being held by the arms x a-, attached to the box holding the spider webs, turns by means of the fixed toothed circle g h. The instrument is used as follows. Suppose it is desired to measure the angles of position and distance of two stars. The telescope is set on the objects, and the screw w is turned until the line s t bisects the two stars. The milled heads m m are then turned until the webs carried by the forks bisect each a star.
The distance is indicated by the number of revolutions and parts of revolutions of the screws which separate the movable lines. The position is ascertained by a comparison of the reading of the position circle, g h, with its reading of the zero of position, which is when the lines are so placed that the image of a star will traverse the field from side to side, bisected by one of the lines, the telescope being at rest and the star passing by the diurnal motion of the earth. A modification of the filar micrometer is the most useful and accurate adaptation for reading the divisions on the limbs of large astronomical and geodetic circles. Huygens used a micrometer which employed a circular diaphragm in the focus of the eye glass, and whose angular value was ascertained by the time it took a star to pass across it. By using wedge-shaped plates of brass, some part of which exactly covered the disk of the planet, its diameter was found by a comparison with the size of the aperture. Fraunhofer's suspended annular micrometer is an ingenious, accurate, and convenient instrument, much used for objects, such as comets, faint stars, and asteroids, which will not bear the illumination necessary to render visible the lines in a filar micrometer.
It is shown in fig. 3, and consists of a disk of plate glass having in its centre a round hole about half an inch in diameter, to the edges of which a ring of steel is cemented, and afterward turned true in a lathe. When the disk is mounted in a brass tube and adjusted in the focus of the eyepiece of the telescope, the steel ring is alone visible, and appears as if suspended in the atmosphere. It is used to establish the position of an unknown object by comparison with one whose place is known, the transits of each being taken by turns, as it passes across the ring. - There are several other forms of micrometers, constructed upon various principles, and with reference to particular uses. The more important of these are the double-image micrometers, in which two single refracting lenses or semi-lenses are made to produce double images. When the centres of two images of the sun, moon, or any of the planets are separated so that the disks touch each other, the separation of the lenses will indicate the diameter of the object.
Roemer is said to have been the first to suggest such an instrument, which had two whole lenses.
Dollond's improvement consists in placing two half lenses side by side upon a sliding frame, and was called the divided object-glass micrometer. Other double-image micrometers are made of double-refracting crystals. This invention is ascribed to the abbe Rochon, rock crystal being the substance preferred by him on account of its transparency and hardness. Arago employed Rochon's micrometer in taking more than 3,000 of the diameters of the planets. A micrometer with double images was devised by Porro of Paris in 1842, which consists of a parallel plate of glass placed within the telescope so that part of the rays from the object pass through it and another part beyond it. An inclination of the plate will produce two images of the object, whose diameter may be measured by the amount of rotation given to the glass to produce certain alterations in the position of the image. A recent method for taking the positions and distances of stars, double or in clusters, is to photograph the telescopic field in collodion on glass, and then to measure the impression on the stage of an independent micrometer constructed for the purpose The great advantages of this method are that all the careful micrometer work can be done by an assistant who is not a professional astronomer; that it can be done during day or night, and repeated at will, the photographs being imperishable; and that when done it is most accurate.
An instrument of this kind, which has undergone several stages of improvement, was designed and constructed by Lewis M. Rutherfurd of New York several years ago. A sketch of the instrument as now in use at his private observatory is given in fig. 4. It stands upon a tripod, which may be accurately levelled by the milled head screws attached to the feet. The photographic plate, p, to be measured (five inches square), is clamped upon the circular glass stage k, which is supported by and revolves with the graduated position circle g, the verniers being read by the reading glasses h h. The compound microscope a is directed perpendicularly to the plate, and can be moved in two directions at right angles to each other and parallel to the plate, on the slides e and d. The quantity of such movement is read by the microscopes c and b upon glass scales of equal parts, m. (The scale read by i is not seen in the figure.) The fractions of a division are read by the filar micrometer seen at the eye piece of c. The eye piece of the microscope a contains a cross of spider lines by which it is centred upon the image of any star to be measured. - Microscope Micrometers. Most telescope micrometers may be used for microscopes when the eye glass has considerable focal length.
Objects of known diameter are used, as lycopodium seed, or wire whose diameter has been measured by winding it many times around a cylinder, and dividing the length of the cylinder by the number of turns. A convenient micrometer was constructed by Dr.
Fig. 4. - Kutherflird's Micrometer.
Wollaston, consisting of a scale, fig. 5, made of wires 1/50 in. diameter, which occupies the place of the object, and a lens of about 1/12 in. focal length in the cap of the instrument. The object, placed beneath this between two glass slides, is moved laterally across the field by means of the milled head a. (See Microscope).