A treatise on mathematical instruments, including most of the instruments employed in drawing, for assisting the vision, in surveying and levelling, in practical astronomy, and for measuring the angles of crystals. In which their constuction, and the methods of testing, adjusting, and using them, are concisely explained / By J.F. Heather.

  • Heather, J. F. (John Fry), -1886.
Date:
1849
Catalogue details

Licence: Public Domain Mark

Credit: A treatise on mathematical instruments, including most of the instruments employed in drawing, for assisting the vision, in surveying and levelling, in practical astronomy, and for measuring the angles of crystals. In which their constuction, and the methods of testing, adjusting, and using them, are concisely explained / By J.F. Heather. Source: Wellcome Collection.

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    tacles; but for obtaining good images with high magnifying powers a combination of lenses must be used. Small glass spheres are used as microscopes of high powers; but a thin lens composed of any more -highly refractive sub- stance is preferable; because, the focal length of the sphere measured from its center being but three semi-radii, the dis- be again tested by the mould, and carefully filled up with the muffled hammer, till the result of the lamp test is perfectly satisfactory. « The edge of the reflector is next turned over to stiffen it, and the bizzle w (Fig. l),°and back beltg (Fig. 2), having been soldered on, the final Fig. 1. Fig. 2. process of polishing may be proceeded with. This process is commenced by scouring, first with a piece of pure charcoal of hard wood, and next with a mixture of Florence oil and finely washed rotten stone, applied by means of a large ball of superfine cloth. The reflector is then cleansed with a fine flannel dipped in Florence oil, and afterwards dusted over with powder of well-washed whiting, and wiped out with a soft cotton cloth. Lastly, it is carefully rubbed by the naked hand with finely washed rouge and clean water, and wiped with a smooth chamois skin. In all the polishing and cleansing processes some skill is required in the manipulation, as the hand must be moved in successive circles parallel to the lips of the reflector, and having their centers on the axis of the generating curve.” The speculum of Lord Rosse’s great telescope is composed of 1264 parts of copper and 589 of tin, fused together and cast in a mould, the bottom of which is formed of hoop iron bound closely together with the edges up- permost. By this means the heat is conducted away through the bottom so as to cool the metal towards the top, while the interstices between the hoops, though small enough to prevent the metal from running out, are sufficiently open to allow the air to escape. After casting, the speculum is annealed in a brick oven, which is heated almost to a red heat, and shut up with the speculum in it, and allowed to cool gradually. The speculum is then placed with its face upwards upon a turning apparatus, and the grind- ing and polishing performed entirely by the aid of mechanical contrivances, so that the proper parabolic form is accurately given to it. To test the work, the dial-plate of a watch is placed upon the top of a mast at 90 feet distance from the speculum, and the image of this dial plate formed by the speculum, being viewed through an eye-glass properly placed, the distinct- ness of this image denotes the accuracy of the speculum.