The English micrometer still retains the essential features of
Troughtons original construction above described. The later
English artists have somewhat
Fig. 4.
changed the mode of communicating
motion to the slides, by
attaching the screws permanently
to the micrometer head and
tapping each micrometer screw
into its slide. Instead of making
the shoulder of the screw a flat
bearing surface, they have given
the screw a spherical bearing
resting in a hollow cone (fig. 4)
attached to the end of the box.
The French artists still retain
Troughton’s form.
Fraunhofer’s Filar Micrometer.—The micrometer represented in fig. 5[1] is the original Merz micrometer of the Cape Observatory, made on Fraunhofer’s model. S is the head of the micrometer screw proper, s that of the screw moving the slide to which the so-called “fixed web” is attached, s′ that of a screw which moves the eyepiece E. C is the clamp and M the slow motion in position angle.
Fig. 5.
L, L are tubes attached to a larger tube N; the latter fits loosely on a strong hollow cylinder which terminates in the screw V. By this screw the whole apparatus is attached to the telescope. The nozzles of small lamps are inserted in the tubes L, L, for illuminating the webs in a dark field; the light from these lamps is admitted through apertures in the strong hollow cylinder above mentioned (for illumination, see p. 385). In this micrometer the three slides moved by S, s, and s′ are simple dovetails. The lowest of these slides reposes upon a foundation-plate pp, into one end of which the screw s is tapped. In the middle of this slide a stiffly fitting brass disk is inserted, to which a small turn-table motion may be communicated by an attached arm, acted on by two fine opposing screws accessible to the astronomer; and by their means the “fixed web” may be rendered strictly parallel with the movable one. Another web is fixed parallel to the axis of the screw, as nearly as possible in the same plane with it and passing through the axis of rotation of the micrometer. For the internal structural details of the micrometer the reader is referred to the article “Micrometer” in the 9th edition of the Encyclopaedia Britannica.
To use the instrument, it is well first to adjust the web moved by the screw S, so that its point of intersection with the web (commonly called the “position-web”), which is parallel to the axis of the screw, shall be nearly coincident with the axis of rotation of the micrometer box. For this purpose it is only necessary to direct the telescope to some distant object, bisect that object with the movable wire, and read the number of revolutions and parts of a revolution of the screw; now reverse the micrometer box 180° and repeat the observation; the mean of the two readings will be the point required. Now direct the telescope to a star near the equator and so that the star’s image in its diurnal motion shall pass across the intersection of the two webs which mark the axis of rotation of the micrometer box. Then, as the diurnal motion causes the star-image to travel away from the axis of rotation, the micrometer box is rotated till the image of the star when at a considerable distance from the axis is bisected by the position-web. The micrometer is now clamped in position-angle by the clamp C, the star again brought back to the axis, and delicate adjustment given in position-angle by the slow-motion screw M, till the star-image remains bisected whilst it traverses the whole length of the position-web by the diurnal motion only. This determines the reading of the position-circle corresponding to position-angle 90° or 270°.[2]
The position-angles of double stars are reckoned from north through east, the brighter star being taken as origin. To observe the position-angle of a double star it is only necessary to turn the position-web so that it shall be parallel to the line joining the centres of the components of the double star. To test this parallelism the single web must be made to bisect the images of both components simultaneously, as in fig. 6, because it is evident that if the two components of the double star are not exactly equal in magnitude, there will be great tendency to systematic error if the web is placed on one side or other of the stars.
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| Fig. 6. |
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| Fig. 7. |
To avoid such error Dawes used double wires, not spider webs, placing the image of the star symmetrically between these wires, as in fig. 7, and believed that by the use of wires, much thicker than spider webs, the eye could estimate more accurately the symmetry of the star-images with respect to the wires. Other astronomers use the two distance-measuring webs, placed at a convenient distance apart, for position wires. This plan has the advantage of permitting easy adjustment of the webs to such a distance apart as may be found most suitable for the particular observation, but has the disadvantage that it does not permit the zero of the position-circle to be determined with the same accuracy; because, whilst by means of the screw s (fig. 5) the eyepiece can be made to follow the star for a considerable distance along a position-web parallel to the screw, the bisection of the web by a star moving by the diurnal motion at right angles to the micrometer screw can only be followed for a limited distance, viz. the field of the eyepiece. But, as the angle between the position-web and the distance-webs is a constant, the remedy is to determine that angle (always very nearly a right angle) by any independent method and employ the distance-webs as position-webs in the way described, using the position-web only to determine the instantaneous index error of the position-circle.
To measure distances with the Fraunhofer micrometer, the position-circle is clamped at the true position-angle of the star, and the telescope is moved by its slow motions so that the component A of the star is bisected by the fixed wire; the other component B is then bisected by the web, which is moved by the graduated head S. Next the star B is bisected by the fixed web and A by the movable one. The difference between the two readings of S is then twice the distance between A and B.
The great improvement now introduced into all the best micrometers is to provide a screw s, which, not as in the Fraunhofer micrometer, moves only one of the wires, but which moves the whole micrometer box, i.e. moves both webs together with respect to the star's image in the direction of the axis of the screw. Thus the fixed wire can be set exactly on star A by the screw s, while star B is simultaneously bisected by the movable wire, or vice versa, Without disturbing the reading for coincidence of the wires. No one, unless he has previously worked without such an arrangement, can fully appreciate the advantage of bringing up a star to bisection by moving a micrometer with a delicate screw-motion, instead of having to change the direction of the axis of a huge telescope for the same purpose.
Fig. 8.
When it is further remembered that the earlier telescopes were not provided with the modern slow motions in right ascension and that the Struves, in their extensive labours among the double stars, used to complete their bisections of the fixed wire by a pressure of the finger on the side of the tube, one is puzzled whether more to wonder at such poor adaptation of means to ends or the patience and skill which, with such means, led to such results.[3] Dawes, who employed a micrometer of the English type (figs. 1, 2 and 3), used to bolt the head of one of the screws, and the instrument was provided with a slipping piece, giving motion to the micrometer by screws acting on two slides, one in right ascension, the other in declination, so that “either of the webs can be placed upon either component of a double star with ease and certainty” (Mem. R.A.S. xxxv. 1 9).
The micrometer shown in fig, 8 was made by
Repsolds for the Cape Observatory. Fig. 9 represents the same- ↑ When it is remembered that the measurements of the Struves, Dembowslci, Secchi, the Bonds, Maclear and of most modern European astronomers have been made with Fraunhofer or Merz micrometers it is not too much to say that fig. 5 represents the instrument with which a half of the astronomical measurements of the 19th century were made.
- ↑ For the corrections applicable to measures of position-angle in different hour angles, on account of errors of the equatorial instrument and of refraction, see Chauvenet’s Practice and Spherical Astronomy, ii. 392 and 450.
- ↑ Professor Watson used to say, “After all the most important part of a telescope is the man at the small end.”
