MICROMETER. 



MICROMETER. 



622 



The plate carrying the wire is drawn by the screw, and held back by 

 springs, which prevent any lost time. A micrometer of this kind is 

 now generally applied to circles, transits, and theodolites, in addition to 

 the fixed wires, which of course are always necessary. There are two 

 verifications : first, the ascertaining the value of a revolution of the 

 screw ; and secondly, determining the reading of the screw-head when 

 the moveable wire c >ineides with the fixed wire. In a circle or theodo- 

 lite the micrometer wire is placed upon a sharp distant object, and the 

 divided limb read off. The screw is turned through several revolu- 

 tions, and the object is again bisected by moving the whole instru- 

 ment by its tangent screw, and the divided limb read off a second time. 

 We have then the same angle measured in revolutions of the screw 

 and in the divisions of the instrument, and by a simple proportion 

 have the value of a revolution and of a part. With a transit, the pas- 

 sage of Polaris over the micrometer wire is observed after successive 

 revolutions of the screw. The angular motion of Polaris for the inter- 

 vals is computed from the polar distance, and thus the value in arc 

 obtained for a revolution of the screw. To determine the zero position 

 of the micrometer wire, the moveable wire is brought to touch the 

 fixed wire, first on one side and then on the other, and the screw-head 

 read off each time. The mean of the two readings will be that when 

 the two wires are exactly superimposed. 



The pntition irire micrometer is much used for observations of double 

 tars, and is the wire micrometer proper for equatorials. In this con- 

 struction there are two wires parallel to each other, each moveable by 

 its own screw : the whole apparatus can also be turned round in the 

 plane of the wires, so as to place the wires in any direction, the angle 

 round which it is turned being read off by two verniers upon a small 

 circle called the petition circle. In measuring a double star the wires 

 are brought near each other, and the apparatus turned round until the 

 "two stars are either threaded on- one of the wires, or, being placed 

 between them, are judged to lie in the same direction. The division 

 of the micrometer circle is then read off, and the observation in posi- 

 tion is made. Now, by the divided circle of the micrometer turn the 

 apparatus round 90, and the wires will be at right angles to the line 

 joining the two stars. By moving the equatorial, place one wire A on 

 one of the stars, and place the other wire B, by its screw, on the second 

 star; Read off the screw-head of B, and then place A on the second 

 star by moving the equatorial, and B on the first by moving its screw, 

 and read off the revolutions and parts of B. The difference of the two 

 readings of B will give, hi revolutions and parts of the screw, twice the 

 angle between the two stare. The process may be repeated, keeping 

 B fixed and moving A. Before or after a series of observations the 

 Eero or index error of the position circle should be ascertained. Place 

 the instrument nearly in the meridian, and make a star run along one 

 of the wires from end to end. Read and note the position circle, 

 which should mark 90 and 270, and the difference from this is the 

 correction to be applied to all 'the angles of position observed during 

 the evening. The value of a revolution of the screw may be deter- 

 mined by separating the two wires a given number of revolutions, and 

 observing a series of transits of known stars over them. As large 

 equatorials are always carried by a clock movement, we should recom- 

 mend fixing the position micrometer upon a slipfriny-piece, by which a 

 small motion up or down or to the right or left can be given to the 

 wires without meddling with the clock or the equatorial. With this 

 apparatus the measurement of double stars is perfectly easy. The wire 

 micrometer requires illumination for seeing the wires, and the light 

 thus admitted often obliterates faint and ill-defined objects. 



The micrometer microscope, for reading off the divisions of graduated 

 circles, depends upon the same principle aa the wire micrometer. An 

 enlarged image of the divisions of the limb of the circle is formed, 

 and this image is measured by the revolutions and parts of a screw. 

 [CIRCLE.] 



2. The divided object-ylass micrometer and keliometer. If an object- 

 glass be cut across so as to form two semicircles, and the semilenses be 

 separated by sliding one beyond the other, each portion will form its 

 proper image, and theae will retreat from each other as the semilenses 

 are moved. The semilenses are mounted on slides, and the quantity of 

 separation is read off upon a scale. In Bessel's heliometer, a very fine 

 and perfect instrument of this class, the focal length of the object-glass 

 is eight French feet, and the aperture nearly six French inches. A 

 description and plate will be found in the ' Astronomische Nachrichten," 

 No, 189. The only objection which can be made to this species of 

 micrometer, besides the extreme difficulty of constructing it, is, that 

 stars are not seen so round and well defined as in an entire object- 

 glass. Bessel's measures of double stars are, however, so far as we can 

 judge, the most accurate that have yet been made. Suppose a double 

 star is to be measured with the heliometer : the whole of the object 

 Mid is turned round, until four stars appear in a right line, and the 

 semilenses are separated until the stars appear to be exactly at 

 the same distance from each other, when the scale is read off. The 

 semilenses are then shifted in a contrary direction, sliding the two 

 images over each other, until they again appear to be at equal dis- 

 tances, and the scale i '.II'. The separation of the scale is 

 four tim&-i the angular distance between the stars. There is a position 

 circle, on which the direction of the stars is read off. In measuring 

 the diameters of the nun, planets, Ac., the two images are made to 

 touch on opposite sides ; and in observations of Halley's comet it was 



made to coincide with the neighbouring stars. The divided object- 

 glass micrometer is on the same principle as the heliometer ; a cap 

 containing the divided lenses is placed over the object-glass of the 

 telescope. A similar micrometer may be obtained by dividing one of 

 the lenses of an eye-piece; and it seems probable that, with large 

 telescopes, this micrometer may be of considerable use in measuring 

 small angles. There is a good deal of colour in the images, but not at 

 the point of contact. Micrometers of this class require no illumi- 

 nation. 



3. Reticle and circular micrometer. The micrometers hitherto de- 

 scribed are applied to the accurate measures of small angles ; the 

 present class, though very useful in certain cases, are of much lower 

 pretensions. The reticle, or diaphragm, as it is sometimes called, is 

 any fixed arrangement of wires or bars which can be applied to a 

 telescope for the purpose of measurement. They are chiefly used when 

 an object will not admit of illumination, or where the astronomer has 

 no accurately divided instrument at his disposal, or, as in the case of 

 La Caille at the Cape of Good Hope, when the object is to fix approxi- 

 mately a greater number of stars than could be done in the same time 

 with ordinary instruments. Suppose a cross like an X or V to be cut 

 out of brass-plate and inserted in the principal focus of a telescope 

 with the axis of the letter in a meridian. A star in passing through 

 the field is occulted at its passage behind each of the bars, and the time 

 noted. The interval will show, by an easy calculation, how far it 

 passes from the vertex ; and the mean of the times, the moment when 

 it passes the axis of the diaphragm. If the true position of any one 

 star so passing is known from any other source, all the other stars can 

 be thus determined differentially with respect to it. The method is 

 not very accurate, but may often be applied advantageously and with 

 very small instrumental means. If a fine wire be drawn perpendicular 

 to the axis, and a bright star, observed with illumination, be made to run 

 along the wire, the axis of the diaphragm can be set in a meridian, and 

 that is the only verification necessary. The computation in declination 

 will be least if the angle between the bars is such that the base of the 

 triangle is equal to its altitude. This reticle is very convenient for 

 mapping, if placed in the meridian, or for cometary observation, if the 

 telescope is mounted as an equatorial, however rudely. 



The circular micrometer was introduced, we believe, by Olbers, and 

 perfected by Frauenhofer (' Astron. Nachricht.' iv. 22 ). A metal ring 

 is set in the centre of a perforated glass plate, and the outer and inner 

 edge of the ring is turned true. The plate is fixed in the focus of a 

 telescope, and the appearance is that of a ring suspended in the heavens. 

 The telescope is pointed, and the observer notes the time when a star 

 disappears at the outer ring, re-appears on the inner ring, disappears 

 again, and finally re-appears. If two stars be thus observed, it is clear 

 that when a mean is taken of the disappearances and re-appearances of 

 each, that the difference between the two means will be the difference 

 of right ascension between th"e two stars, and therefore that if one be 

 known, the other is determined. Again, if the diameter of the ring 

 has been determined, and the declination of the stars nearly known, 

 the time of describing the chord of the ring will give, by an easy com- 

 putation, the distance of the chord from the centre, and that the more 

 accurately the smaller the chord described. The sum or difference of 

 these two distances is the difference of the stars in declination. The 

 computation of the second star with its approximate declination may 

 be repeated if the stars are near the pole. It will be seen that nothing 

 is required for the circular micrometer but the power of fixing the 

 telescope for a few minutes until a known star passes the field, and 

 that no illumination is required. It is especially the apparatus for 

 determining the place of a faint comet or planet, and in the hands of 

 Olbers, Harding, and many other German astronomers, has been of 

 infinite use in cometary astronomy, and in the discovery and observa- 

 tion of the small planets. When the comet has a large motion, or 

 when the position of the star is so low as to require attention to the 

 difference of refraction, the computation is a little more complicated, 

 but generally scarcely any computation is required, and the results, in 

 right ascension at least, are good. The observations at the inner edge 

 of the ring are to be preferred. When however the object will bear 

 illumination, and the astronomer possesses a telescope so mounted that 

 he can apply a wire micrometer to it, the results from this are incom- 

 parably more accurate, and the reticle above mentioned is certainly 

 better for determining declination. Frauenhofer afterwards (' Astronom. 

 Nach.' iv. 43) proposed another ring and reticle micrometer. He cut 

 a series of rings or lines upon a piece of plane glass which he placed in 

 the principal focus of the object-glass, and then by a side lamp illumi- 

 nated the rings, leaving the rest of the field dark. It is evident 

 that for certain observations this micrometer would have great 

 advantages. 



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 380 Double and Triple 

 Stars ' (pp. 22, 23), containing tables of the values of Troughton'a 

 screws. 



Since the above article was written by the Rev. R. Sheepshanks, 

 micrometry, like all other branches of science, has put forth new shoots. 

 In the original micrometer of Gascoigne, nicely ground parallel edges 

 of brass plate were used, and Dr. Hooke made the important improve. 



