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The photographic news
- Bandzählung
- 29.1885
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- 1885
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- Bandzählung
- No. 1393, May 15, 1885
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The photographic news
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Band
Band 29.1885
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- Register Index III
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May 15, 1885. J THE PHOTOGRAPHIC NEWS. 309 ROBERT’S RULING MACHINE. BY JOHN MAYALL, JUN., F.R.N.S., F.7.s.* The ruling machine of the late Herr F. A. Nobert, in so far as its special applications for the production of microscopical test plates, interference-plates, and diffraction gratings are concerned, must, I think, be regarded as of essentially original design. Herr Nobert was extremely reticent as to the methods he em ployed in the production of his rulings. So far as I have been able to learn, he did not fully communicate his methods to any one competent to convey the information to others. The de scription, therefore, which I shall present to your notice will consist almost wholly of notes made by Dr. Schroder and myself, after- minutely examining every part of the machine, together with a few gleanings from the memorandum book, in which Herr Nobert jotted down, from time to time, fragmentary observations and data, hardly to be understood by anyone but himself. But, first, for the information of those of my hearers who may never have heard of Nobert's rulings, permit me to make a few introductory observations as to the meaning and purpose of fine rulings, so as to lead up gradually to a comprehension of the problems which Nobert set himself to solve, and the methods he adopted towards their solution. In this way I hope not to have to draw too much on your imaginations when I come to speak of the difficulties of his work, of the mechanical resources he developed in grappling with these difficulties, and of the success he attained. I have here an ivory scale divided to hundredths of an inch, the divisions of which are filled with wax and graphite, so that the lines appear black, and the interspaces white. Everyone present, who has normally good sight, will have no difficulty in resolving the lines ; that is, in seeing by the unaided eye the spaces between the consecutive lines, provided the scale be suit ably illuminated. Those among you who have not yet tried the experiments, will be surprised to observe how much more per ceptible are the lines if illuminated by a condenser. If ivory scales were accurately ruled, so that each line was equal in breadth to the interspace, some of you would resolve, without much difficulty, lines as high as 150 to 200 to the inch with the unaided eye, whilst here and there exceptionally powerful sight would resolve lines even closer. These figures would be slightly augmented if the lines were ruled on glass, and viewed under the most favourable circumstances by transmitted light. I have met with one instance where lines on glass, slightly closer than 250 to the inch, were resolved by the unaided eye. I have here a series of stage-micrometer rulings on glass, com mencing with fifty to the inch, and progressing to 10,000 to the inch. They probably belonged formerly to the possessor of a microscope in the days when the measurements of objects were generally made by direct inspection and comparison under the microscope, and before the introduction of the modern methods of measuring by screw micrometers, either on the stage or in the eye-piece. These old rulings are of fair quality, and I show them with a view of fixing in your minds a stand-point from which the fineness of Nobert’s rulings may be appreciated. Early in the century, Fraunhofer gave a great impetus to fine rulings by the production of his diffraction gratings, in which he aimed at and achieved a standard of accuracy in division and perfect similarity as to depth, breadth, and symmetry of angles of the furrows, such as had never before been formulated. He succeeded in ruling lines much closer than those contained on the highest of the series of micrometers I exhibit—as high, indeed, as 30,000 to the Paris inch ; but, according to Sir John Herschel, these fine lines were not sufficiently accurate to pro duce pure spectra, nor could he go beyond 8,200 lines to the inch when they were submitted to this most searching test of accuracy. Sir David Brewster mentions that Barton produced rulings on steel up to 10,000 to the inch, which gave excellent spectra in his hands. Some of you probably have seen the so- called “Barton’s buttons,” which were stamped from his ruled steel dies. The foundation of Nobert’s machine is a dividing engine, calculated to produce parallel divisions far finer than could be marked by any ruling point yet discovered. The division-plate is 12 inches in diameter, and has twenty rows of “ dots,” by means of which two bauds of silver imbedded near the circum ference have been graduated with extremely fine lines to every five minutes in arc. These graduations are viewed by two com • Abstract of a paper read before the Society of Arts. pound microscopes, each provided with eye-piece screw-micro meters of special construction. The microscopes can be fixed at various points in the main iron bed of the machine, so as to check the rotation of the graduations of the division-plate ; and the eye pi see screw-micrometers in the microscopes enable the observer to subdivide the graduations by inspection, and to correct the movement of rotation, within very narrow limits. The rotati >n of the division-plate is effected by a tangent-screw acting up i a worm on the edge of the divisiou-plate. The tangent-screw is controlled by a large milled head, and a gradu ated drum shows the amount of motion. A screw adjustment is provided, by which the tangent-screw can be disconnected from the division-plate. In order to equalize the pressure of the tangent-screw against the division-plate it is mounted to swing between conical bearings, and a counterpoise on a lever arm beneath keeps it in contact with the worm. The method employed by Nobert for obtaining the minute divisions of his test-plates was, to utilise the radius of the division-plate as a lever to move the glass plate on which the rulings were made at right angles to the motion of the ruling point. For this purpose he attached to the centre of the division-plate a bent arm, on which slides a bar faced with silver, having at one end a finely- steel point which can be adjusted with a scale and vernier so as to project more or less beyond the centre of the division-plate or axis of rotation. The radius of the division-plate thus be comes the long arm of the lever, whilst the radius of the pro jection of the polished steel point beyond the axis of rotation forms the short arm, the centre of the division-plate being the fulcrum. The motion of the short arm of the lever is com municated by contact with an agate plate to a polished steel cylinder adjusted to slide at right angles to the movement of the ruling point in V-shaped bearings of agate. The steel cylinder carries a circular metal table, on which the glass plate to be ruled is fixed by wax and clamps. To diminish the friction of the steel cylinder on the agate bearings, a counterpoise is pro vided, to lift it on a roller, whilst a weight attached by a silk cord to one end, keeps the agate plate in perfect contact with motor steel point. The motion of the lever arm is, of course, in arc, and hence the divisions would not be strictly equidistant unless compensation were made for the difference in length of arc and its sine ; but since the actual space included between the first and last lines of the test-plates hardly exceeds 1-50th inch, this difference would be inappreciable. It may be assumed that Herr Nobert used the arc motion during the process of division only, and that for moving the plate over the spaces of the blank bands between the rulings he utilized the fine screw connected with the agate plate in the steel cylinder, by which a motion of the plate of about sdogth of an inch can easily be effected ; in this way he would reduce the total motion of the division-plate in arc to about one-half. It would be possible to increase or decrease the successive divisions of the bands by increasing or decreasing the length of the short arm of the lever ; but in view of the risk which such adjustments would involve, it is highly improbable that such a plan was adopted. There are other possible methods of effecting the divisions, all involving risk of large errors ; the most probable conjecture is that Herr Nobert used the greatest possible eccentricity of the short arm of the lever, so as to utilise the smallest motion in arc required for his subdivisions. The arrangement for carrying and adjusting the diamond point is specially ingenious. The questions to be solved were— (1) to provide means to adjust a diamond edge to any angle within required limits ; (2) to balance it truly so that the weight pressure for ruling could be perfectly controlled; (3) to raise and lower it strictly in one plane—that is to say, mechanically free from lateral play, so that the consecutive divisions of the ruling depended solely on the motion imparted to the glass plate by the dividing engine ; (4) to cause the diamond to oscillate freely in one plane ; (5) to control the length of the line to be ruled ; (6) to connect the whole with mechanism to insure an even rate of speed in the ruling movement of the diamond. These matters have been worked out by Herr Nobert with extraordinary perseverance, as evidenced by the elaboration of the adjustments. I should despair of making myself understood on these points by mere verbal description ; I shall, therefore, ask you to inspect the mechanism at the close of my remarks, for, to be understood, such complex adjustments need to be seen “ in the flesh.” It would be an easy matter to suggest simplifications and possible improvements in this part of the machine. But it must i bo noted that Herr Nobert had always to work with very
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