David Gill

“ Instruments have been invented by Alvan Clark and Sir Howard Grubb for measuring with the spider-line micrometer angles which are larger than the field of view of the eyepiece. In both cases two eyepieces are employed, one to view each separate web. One drawback to this form of instrument is that the two webs cannot be viewed simultaneously, and therefore the observer must rely on the steadiness of rate of the clockwork and uniformity in the conditions of refraction whilst the eye is moved from one eyepiece to the other. ”

“ 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. ”

“ 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 1/10° of a division on the glass scale. ”

“ In delicate researches two divisions of the scale should always be read, not merely for increased accuracy but to obtain the corrections for “run” from the observations themselves.
Repsolds employ for the micrometers of their reading microscopes the form of construction shown in fig. 9, omitting, of course, the motion of the whole micrometer box given by the screw s for those cases in which the axis of the micrometer is supposed to remain constant in position, as, for example, in the case of the reading microscopes of transit circles (see Transit Circle) . ”

“ Suppose now the solar spectrogram to be viewed in the focus of O1, and the converging rays to be reflected by the prisms P1 and P3, till an image is formed in the focus of the eyepiece at the point where the axis of the eyepiece intersects the upper face of the prism P3. Then if the prism P4 is cemented to P3, a sharp image of such lines of the solar spectrograph as are visible in the field of view will be seen in the eyepiece. ”

“ The micrometer-screw S has a pitch of 0·5 mm., its head is divided into 100 parts. Two spiral springs underneath press the plate B1 with its agate end-bearing against the rounded end of the screw S. The whole number of revolutions of the screw is read by the scale X (fig. 23) . The whole stage T, carrying both spectrograms, can be moved from right to left on the steel cylinder Z, by turning the head K, on the axis of which is a pinion that gears into a toothed rack attached to the lower side of the cylinder Z. A scale N on the cylinder Z serves for setting the slide to any required position. ”

“ Its fundamental principle is that, by a combination of glass scales with a micrometer screw, “the chief part of the distance to be measured is read off on the scale; the fractional part of the scale space is not estimated but measured by the screw.” Hinks claims that thus never more than one- or two-tenths of a revolution of the screw need be used in making the measure, and little time is lost in running the screw backwards and forwards. ”

“ In this way the scale can be viewed by a microscope of much higher magnifying power than can be employed for the photographed spectrum. Indeed, if the scale were subdivided to 1/10 mm. the power employed might only be limited by the sharpness of the division-lines. But for refined work this would imply the investigation of too many divisions of the scale; it is therefore more usual to divide the scale into single millimetres or half-millimetres and to provide a micrometer which subdivides the millimetre into 1000 or, by estimation, into 10,000 parts. ”

“ Each movable web must pass the other without coming in contact with it or the fixed wire, and without rubbing on any part of the brass-work. Should either fault occur (technically called “fiddling”) it is fatal to accurate measurement. ”

“ The varnish to fix the webs is applied, not on the surface τ as is usual, but on a bevel for the purpose, [14] the position of the webs depending on their tension to keep them in their furrows. The result is that no trace of “fiddling” exists, and the movable and fixed webs come sharply together in focus with the highest powers. Under such powers the webs can be brought into apparent contact with such precision and delicacy that the uncertainty of measurement seems to lie as much in the estimation of the fraction of the division of the head as in the accuracy of the contact.
Fig. ”

“ Beyond the introduction of the spider line it is unnecessary to mention the various steps by which the Gascoigne micrometer assumed the modern forms now in use, or to describe in detail the suggestions of Hooke, [8] Wren, Smeaton, Cassini, Bradley, Maskelyne, Herschel, Arago, Pearson, Bessel, Struve, Dawes, &c., or the successive productions of the great artists Ramsden, Troughton, Fraunhofer, Ertel, Simms, Cooke, Grubb, Clarke and Repsold. It will be sufficient to describe those forms with which the most important work has been done, or which have survived the tests of time and experience. ”

“ In 1813 Wollaston proposed fine platinum wires, prepared by surrounding a platinum wire with a cylinder of silver, and drawing out the cylinder with its platinum axis into a fine wire. [7] The surrounding silver was then dissolved by nitric acid, and a platinum wire of extreme fineness remained. But experience soon proved the superiority of the spider web; its perfection of shape, its lightness and elasticity, have led to its universal adoption. ”

“ Flamsteed, in the first volume of the Historia coelestis, has inserted a series of measurements made by Gascoigne extending from 1638 to 1643. These include the mutual distances of some of the stars in the Pleiades, a few observations of the apparent diameter of the sun, others of the distance of the moon from neighbouring stars, and a great number of measurements of the diameter of the moon. Dr John Bevis (Phil. Trans. (1773) , p. 190) also gives results of measurements by Gascoigne of the diameters of the moon, Jupiter, Mars and Venus with his micrometer. ”

“ The effect of turning the pinion V is, of course, to displace the focus both of the solar and stellar spectrographs in the field of the eyepiece, but this displacement, is easily restored by focussing screws O1 and O2. By successive adjustments of this kind condition (3) can be accurately realized. These three adjustments having been made, the prisms P3 and P4 are removed and replaced by another prism in which the silvering is arranged as in fig. ”

“ 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. ”

“ It is a convenient feature in Repsolds’ micrometer that the webs are very near the inner surface of the top of the box, so that the eye is not brought inconveniently close to the plate when high powers are used. ”

“ The chief drawback to type A is that the errors of the screw are liable to change by wear, otherwise the apparatus, as made and used at Potsdam, is, on the whole, a convenient and accurate one. In determining the errors of the screw of the Potsdam form of machine it is necessary to have regard to the fact that the screw is placed at one side of the slide, as in fig. ”