There are many other micrometers, but they are not in such general use as to demand any notice here. The reader will find them very fully and elaborately described in Pearson's ' Practical Astronomy,' vol. ii. See also Herschel and South's `Observations of 330 Double and Triple Stars' (pp. 22, 23), containing tables of the values of Troughton's screws.
Since the above article was written by the Rev. B. Sheepshanks, micrometry, like all other branches of science, has put forth new shoots. In the original micrometer of Gaseeigne, nicely ground parallel edges of brass plate were used, and Dr. Hooke made the important Improve.
meat of substituting parallel hairs. Such an Instrument embodies the principle of the best and most recent micrometers. The most as well as important part of this instrument is the screw or screws by which the forks are moved. Their threads must not only be at the same distance from each other, but have their inclination equal all round. In the screw of Troughton's micrometer there are about 103 threeds in an inch.
The usual method of finding the value of a revolution of the screw has been already given. The method of Professor Gauss of Gottingen is thus stated by Sir David Brewster (' Encyclopaedia Britannica,' art. ' Micrometer') :—" He employs for this purpose a standard telescope, with a micrometer, the value of whose scale has been accurately deter mined. Since the wires of a telescope-micrometer adjusted to distinct vision of the stars or planets are accurately in the focus of parallel rays falling on the object-glass, it follows that rays issuing from the wires and falling on the inside of the object glass will emerge from it perfectly parallel. Now if we place the object-glaas of the standard telescope close or near to that of the first telescope, the parallel rays formed by , those issuing from its wires will be refracted to the focus of the standard telescope, and a distinct image of the wires will be there formed. The observer therefore when he looks into the standard telescope will see distinctly the wires of the first telescope, and by means of his micrometer he will be able to measure exactly the angular distance of these wires at whatever distance they may happen to be placed. This angular distance, divided by the revolutions and parts of a revolution which are necessary to bring the wires of the first telescope to the zero of their scale, will give the value of one revolution of the screw, or of one unit of the scale." The parallel fibres of a micrometer must be extremely fine, and of uniform diameter. Since Hooke's time, wires and fibres of silk have been used instead of hairs. Troughton introduced the spider's line, which he found to be sufficiently fine, opaque, and elastic to answer the purpose: the stretcher, or long line which sustains the web, being alone adapted. The fibres of spun glass, of melted sealing-wax, of
asbestos, and the minute crystals of mezzolito have also been used; but Wollaaton's fine platinum wire (DUCTILITY] has of into years been preferred. These wires offer another advantage in the method of illu. minating them, which in the transit instrument is generally done by means of a lamp, the light of which passes along the horizontal axis to a small speculum which reflects it upon the wire. The new method is to make the wires red hot by transmitting a voltaic current through them. Arago (who first adopted, if he did not first suggest the idea) feared that the images of objects beyond and apparently near the heated wire would undulate, but such was not found to be the case. This method of illumination has been used for the wires of the microscopic micro meters attached to the Oxford heliometer [II KLIOMETElt]. M. Porro's mode of illuminating the wires is to mount them on glass plates, the edges of which are formed into suitable curves, so as to introduce a thin vein of light perpendicularly to the axis of the telescope, so that the wires appear of a Silvery white in a dark field. Brewster also pro posed to obtain micrometer lines by placing in the field of view the reflected images of a fixed or moveable system of wires, or of lucid discs, attached to the side of the eye-piece. The same distinguished optician has also proposed what he calls photographic micrometers. The reader is of course aware of the ease with which microscopic photo graphs can be impressed upon a film of collodion—as many as 10,000 portraits having beeu introduced into a square inch. "If a system of opaque or transparent lines be impressed upon the collodion photo graphically, when reduced to the minutest size, from a system of large and sharply defined lines, we shall have the most perfect mierometrical scale that can be conceived, the portion of the collodion that contains no nitrate of silver being as transparent as if the dark spaces were solid wires or metallic plates, placed in the focus of the eye-glass." The plan of opening and shutting a pair of parallel wires optically instead of mechanically, and of using it as a general principle in micro meters, has also been applied; but for the details we must refer to the treatise above quoted, in which will also be found minute details respecting various forms of double-image micrometers, hi which two singly refracting lenses, seml-leines, or prisms, are separated by screws, or where the two images are separated optically, or in which the two images are formed by double refraction, and many others.
For a description of micrometers for microscopes see Idtcnoscorit.