Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/264

248 MICKOMETEK [WEBBING. what he calls his " duplex micrometer, " shown in perspective in fig. 18 : " A plate of glass about 2^ inches square is ruled with twenty- one lines in one direction ^th inch apart, and two lines in the other direction 2 inches apart. The extreme lines of the set therefore form a perfect square of 2 inches. These lines are ruled with exceeding accuracy and care, but provision is left for ascertaining any errors that remain either as to distance or want of perfect squareness. Along one side of the square is mounted a micro meter frame in the ordinary way, actu ated by a screw of one hundred threads to the inch. This micrometer frame carries eleven line.- corresponding exact ly to each alternat< line in the gla.s.- reticule, so that when the first spider line is made coincident with the first dia mond line on the glass the last spider line will be coincident with the last line on the glass, and each of the spider lines will be coincident with all the odd numbers of diamond lines, 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21. Over this glass plate is placet! a brass cap in which two eye-pieces are mounted, one sliding in a groove at right angles to the other, so that, while one has its journey backwards and forwards on the horizontal line, the other has its journey on the vertical line, according to how the cap is placed, for this cap is capable of rotation to meet various circumstances. " How to Use the Instrument. 1. The two stars are brought on the horizontal line, and the distance measured from centre to centre along that line. This distance is measured by counting the number of spaces on the glass, adding the residue as measured by the micrometer screw. Thus the screw is never used for larger measures than ^Vth inch, and therefore errors of screw and temperature errors are much reduced. In bisecting, one star is brought into the field of one eye-piece, and a bisection is made with one of the diamond lines by moving the micrometer by one or other of its slipping piece screws. Then the other eye-piece is moved till the second star is seen, and a bisection is made with the nearest spider line by moving the micrometer head. Then the eye can be moved back to the first eye-piece, and the bisection checked, and again back to the other eye-piece. When it is seen that both are satisfactory the measure can be read off. 2. The micrometer is turned round till the horizontal line becomes parallel to the path of apparent motion of the star. This is easily found by stopping the clock and allowing the star to run along the horizontal wire. Now the other star will be found to cross the vertical line somewhere, while the first star is on the horizontal line. This second star is then bisected on the vertical line, while the first star is bisected by one of the spider lines ; thus the difference in right ascension is found. We then have two sides of a right-angled triangle and of course all the elements are known. " To Ascertain tlie Errors (if any] of the Distance of the Lines. Of course, the usual plan of taking transits can be adopted, and to ascertain if the lines be perfectly at right angles a special additional eye-piece is provided, so that transits can be taken across each diagonal of the square." This instrument has great advantage over Clark s in ease of adjustment and use, and has done good work at the University Observatory, Oxford (Mem. 11. A. 8., vol. xlvii. pp. 5-12). Professor Pritchard claims too much when he estimates its work as equal in accuracy with that of the heliometer at least the published results do not confirm such a view. But it is a very valuable instrument for measuring objects too faint for the limited aperture of most heliometers, and which at the same time are farther apart than the field of view of an ordinary eye-piece. The accuracy of the duplex micrometer would be very greatly increased if Clark s idea (above mentioned) of viewing both widely separated webs in one eye-piece of high power could be reduced to a convenient practical form. Method of Wcbbiny the Filar Micrometer. The webbing of a micrometer is a process that should be familiar to all practical astronomers. English opticians usually proceed as follows. A spider (the variety is marked by a cross on the back, and is found in English gardens about decayed wood) is caught, and placed on a wire fork. The insect immediately attaches a web to the wire and begins to lower itself by a web to the ground. This web is wound up on the fork till ten or twelve turns, separated by a convenient space, have been secured. A brush with varnish is then passed along the prongs ; the webs are thus securely fixed to the fork. The parallel prongs of the fork must be sufficiently far apart to allow the web-frame of the micrometer to pass between them. The frame to be webbed is placed on a flat dull black surface between the prongs of the fork, the latter being carefully arranged so that one of the webs lies nearly in the furrow ruled in the frame for its reception. As the web-frame is generally thicker than the fork, the web will now be stretched across the former, with a certain amount of tension, and is brought into the furrow with a finely pointed piece of soft wood. If the surface of the frame is well polished, and the furrows sharply cut, without "burr," the web should leap sharply and decidedly into its place. Each end of the web is then secured by a drop of shellac varnish, which should be allowed to harden thoroughly before the frame is touched. The webs can be very readily so handled against a black back ground, with the aid of a hand lens of 2 or 3 inches focus. In experienced hands this method gives good results, but the following, which is generally followed on the Continent, is preferable. A web, about 2 inches longer than the widtli of the frame, is unwound from a cocoon, 1 and small pieces of lead are attached to its extremities by beeswax. One end of the web, with its attached lead, is laid on a piece of cork floating in a tumbler of water ; the other end is allowed to hang down in the water, where it becomes thoroughly saturated and untwisted. It is then laid across the fork, and dropped into its furrows in the manner above described, the little lead weights exerting a definite tension. Varnish 2 is immediately applied to secure the webs, and the frame is not touched till it is dry. The bevel-edge of the web-frame introduced by Eepsold (type D) offers great facilities for accurate webbing, and should be cm- ployed in all future micrometers. Illumination of Micrometers. When micrometer observations are made by night it is necessary to have some mode of rendering the webs visible, either by rays of light at right angles to the axis illuminating the webs, or by rays nearly coincident with the axis of the telescope. In the former case we get bright webs in a dark field, in the latter dark webs on a bright field. In the older telescopes bright web illumination is produced by small lamps with nozzles that enter the tubes L, L (fig. 9). The illumination is regulated in colour and intensity by wedges of coloured or darkened glass passing through slides in the nozzles. But it is inconvenient to have lamps so near the observer s eye, and it is at least very difficult to obtain a perfectly dark field when the wires are illuminated in this way. The Clarks, in their micrometer of the great Washington tele scope, have made the end of box T (fig. 15) transparent, and light is thrown on the webs from a lamp held by an assistant. Holden has very recently applied a lamp ingeniously hung so as to preserve its vcrticality and the constant direction of its light in a similar way, adding a plain silvered mirror inside the box and opposite the lamp, so as to illuminate the webs symmetrically. In the Clarks and Holden s methods it is only the webs at right angles to the screw that are illuminated. For illumination of the field, in very old telescopes, light was thrown on a small ivory reflector fixed outside the object-glass in the axis of the telescope by an arm fitting on the cell of the lens. This involved the aid of an assistant to direct lamplight on the ivory reflector, or the very frequent change of a lamp support. After wards the light from an attached lamp was intro duced through a hole in the telescope-tube and thrown upon an ellipti cal plane (generally dull- gilt) having its centre part cut away sufficiently to avoid interruption of the cone of rays from the object-glass. Many in genious modes of sus pending the lamp have been invented for the pur pose of securing a con stant direction of its light coupled with vertically of the lamp, to Cooke, is shown in fig. 19. Fig. 19. One of the best of these, due L is the lamp, P a prism to reflect 1 It is asserted that webs from cocoons are more elastic, better shaped, and more durable than those obtained during an effort of the insect to escape. The best webs we have seen were from a cocoon obtained in Holland, but we have been unable to ascertain the name of the variety of spider. 2 Arpelander used to apply two drops of varnish at each end of his webs. He first fixed each extremity by a drop of shellac varnish, and after that had dried he applied a drop of copal varnish nearer the centre of the frame ; the latter took

a long time to harden, but gave ultimately a much stronger attachment.