Page:The American Cyclopædia (1879) Volume XI.djvu/529

 MICROMETER 511 MICROMETER (Gr. /u//cpof, small, and measure), an instrument applied to telescopes j and microscopes for measuring minute spaces and objects. Telescope Micrometers. A paper by Mr. Townley in the " Philosophical Trans- actions " for 1667 describes a micromenter vith 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 ac- count of his invention, it only became known through Mr. Townley, into whose possession one of the telescopes' had fallen. This instru- ment was afterward improved by the cele- brated Dr. Hooke, who is said to have added parallel hairs. A similar micrometer was af- terward 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 refer- FlG. 1. FIG. 2. ring 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, fc and ?, are moved in a plane per- pendicular 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 objec- tive. 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 be 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 554 VOL. xi. 33 w, fig. 2, which being held by the arms x x, 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 a t bisects the two stars. The milled heads m ra are then turned until the webs car- ried by the forks bisect each a star. The dis- tance is indicated by the number of revolu- tions and parts of revolutions of the screws which separate the movable lines. The posi- tion is ascertained by a comparison of the read- ing of the position circle, g 7i, with its reading of the zero of position, which is when the lines are so placed that the image of a star will trav- erse 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 as- tronomical and geodetic circles. Huygens used a micrometer which employed a circular dia- phragm 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 ex- actly covered the disk of the planet, its diam- eter was found by a comparison with the size of the aperture. Fraunhofer's suspended annu- lar 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 con- sists 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 sus- pended in the atmosphere. It is used to estab- lish the position of 'an unknown object by com- parison 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 vari- ous 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. FIG. 8.