It is generally possible so to arrange the method of observation as to eliminate the effect of an error in “the reading for coincidence of the webs” from the results. This excellent time-saving contrivance has also been used in Gill’s apparatus for measuring astrographic plates (see below).
Ghost Micrometer.—C. E. Burton and Sir Howard Grubb (Monthly Notices, xli. 59). after calling attention to J. von Lamont’s paper (Jahrbuch der K. S. b. München, p. 187) and K. L. von Littrow's paper (Proc. of Vienna Acad. of Sciences, xx. 253) on a like subject, proceed to describe a most ingenious form of “Ghost Micrometer,” in which the image of a fine line or lines ruled in (or rather cut through) a silver film deposited on glass is formed at the common focus of an object-glass and eyepiece of a telescope. A faint light being thrown on the outside of the silvered plate, there appear bright lines in the field of view. We have not had an opportunity of testing this, nor Grubb’s more recent models; but, should it be found possible to produce such images satisfactorily, without distortion and with an apparatus convenient and rigid in form, such micrometers may possibly supersede the filar micrometer. Their absolute freedom from diffraction, the perfect control of the illumination and thickness of the lines, and the accuracy with which it will be possible to construct scales for zone observations will be important features of the new method.
The Astrographic 'Micrometer or Measuring Machine.—The application of photography to exact astronomy has created the necessity for new forms of apparatus to measure the relative positions of stellar and planetary images on photographic plates, and the relative positions of lines in photographic spectra.
Especially important has been the problem of measuring the “catalogue plates” of the international Carte du ciel—a work that implies the determination of the positions of some millions of stars—that is to say, of all stars to the 11th or 12th magnitude. The problem has been how to accomplish this work with the minimum of labour consistent with the desired, accuracy. The adoption of a réseau photographed upon the plate has greatly facilitated the procedure. A plate of parallel-surfaced glass has a film of silver deposited upon it. On this film is ruled a system of lines 5 mm. apart, and another similar system of lines at right angles to the first, thus dividing the silvered surface of the plate into squares 5 mm. on the side. The cutter employed to rule these lines removes the silver in fine lines from the surface of the glass, Thus, if a photographic plate, before it is exposed in the telescope, is placed with its sensitive surface nearly in contact with the silvered surface of this réseau, and if parallel light, normal to the surface of the plate, is allowed to fall on the silvered film through the glass on which the film has been deposited, that light will pass through the fine lines in the silver film where the silver has been removed by the cutter, but will otherwise be intercepted by the silver film. Thus a latent image of the “réseau-lines” will be formed on the sensitive plate, and, when the latter has been exposed to the sky in the telescope, we obtain, on development, a negative of the images both of the stars and of the réseau-lines. If the errors of the rectangular co-ordinates of these lines are known, the problem of determining the co-ordinates of any star-image on the plate becomes reduced to the comparatively simple one of interpolating the co-ordinates of the star relative to the sides of the 5 mm. square within which that image is included. This interpolation can, of course, be accomplished with the aid of a micrometer-microscope whose optical axis is normal to the plate, provided that the plate is mounted on slides which enable the observer to bring the réseau-squares successively under the microscope.
This system has an additional advantage beyond its convenience, viz. that if any distortion of the film takes place during development the same distortion will be communicated both to the star-images and to the réseau-lines, and consequently its effect will be eliminated from the resulting star co-ordinates, except in so far as the distortion within the 5 mm. square is of an irregular character; this exception is hardly worth consideration. An originally unanticipated difficulty has arisen from the fact that the réseau-lines have not been ruled on plates of optical glass with optical surfaces, and that, in consequence of irregular refraction in the glass plate, the rays do not always pass through the silver film-lines in a direction strictly normal to the silvered surface; therefore, if the sensitive surface of the photographic plate is not in contact with the silver film of the réseau, the undeveloped photographic copy of the réseau may in such a case not be an exact reproduction of the silvered réseau. It is practically impossible to work with the sensitive film in contact with the réseau-film, not only because dust particles and contact would injure the silver film, but also because the plate-glass used for the photographic plates is seldom a perfect plane. The discrepancies produced in this way are, however, very small, if care is taken to minimize the distance between the silver film and the photographic plate and to select a reasonably good piece of glass for the réseau. For very refined work, however, the irregularities in the reproduction of the réseau may be studied by comparing the measures of the original réseau with the mean of corresponding measures of a number of photographed copies of it.
At Greenwich, Oxford and several other observatories, instead of measuring the distances of the star’s image from the opposite sides of the 5 mm. réseau-square by means of a spider-line micrometer, a glass scale, on the plan shown in fig. 16, is employed in the common focus of the objective and the eyepiece. The image of the star is set upon the intersections of the lines of the central cross, and the positions of the réseau-lines are read off by estimation to 110° of a division on the glass scale. As each division corresponds to 3 sec. of arc, the nearest estimate corresponds with a nominal accuracy of ±0·3″. This involves a loss of accuracy because, with a spider-line micrometer, the accidental error of pointing is of the order of ±0·1″ of arc.
Greenwich Astrographic Catalogue, vol. i., by permission of the Controller of H.M. Stationery office.
Fig. 16.—Diagram of the diaphragm in eyepieces of the micrometer used for
measuring the plates of the Astrographic Catalogue.
In the measuring machines in general use the field of view, as in the case of the glass-scale micrometer, is sufficiently large to include the image of the 5 mm. square. The microscope or viewing telescope is fitted with a spider-line micrometer having two screws at right angles to each other, by means of which readings can be made first on one réseau-line, then on the star, and finally on the opposite réseau-line in both co-ordinates. This form of micrometer is of course capable of giving results of high precision, but the drawback is that the process involves a minimum of six pointings and the entering of six screw-head readings in order to measure the two co-ordinates of the star.
Gill’s Measuring Machine.—Sir David Gill (Monthly Notices, R.A.S. lix. 61) devised a measuring machine which combines the rapidity of the glass-scale micrometer with the accuracy of the spider-line micrometer and simplifies the reductions of the observations at the same time. The essential conditions of the instrument are:—
1. The object glass of the micrometer-microscope is placed midway between the plane of the photographic plate and the plane of the micrometer webs.
2. The micrometer is provided with a “fixed square” 5 mm. × 5 mm., the sides of this square being parallel spider webs 4″ of arc apart; the size of the square is reckoned from centre to centre of these double webs.
