Art. XII.—Description of a Reflecting Telescope made in Wellington
[Read before the Wellington Philosophical Society, 23rd October, 1872.]
The instrument which I exhibit is a Newtonian model, with a silvered-glass speculum, and with the exception of the eye-pieces has been wholly made in Wellington by following the directions given in a paper by Mr. W. Purkiss,
in the “English Mechanic,” Vol. 10, pp. 208, 330 and 357. The diameter of the reflecting speculum is 5.⅞ in., and the focal length 4 ft. 9.¾ in. The total length of the tube, which is made of galvanized iron, being 5 ft. 9 in., and its diameter 6.¼ in. The telescope is mounted on an equatorial counterpoise stand, with simple rack motion worked by hand, and on clear nights gives definition up to 300 diameters. The instrument was constructed in spare time, extending over eighteen months, but the real time employed if it was steadily worked at would occupy about six weeks. A second instrument, on the same principle, but with a 9 in. speculum, and 7 ft. focus, has also been constructed with the same machinery by Mr. J. Widdup, the speculum of which is also exhibited. The chief portion of the time was occupied in making the speculum, the remainder of the work being of an ordinary mechanical character. In making the speculum there are five distinct processes: rough-grinding, fine-grinding, polishing, parabolizing and silvering.
(a.) The first step is to construct an iron tool having the required curvature, the model from which this is cast being made in the following manner:— With a radius of double the length of the proposed focus curved gauges or templates were cut in zinc, from which the wooden model is turned, and the casting made. The tool was then fixed firmly on an upright pillar, and covered with coarse emery powder. The glass for the speculum, which in this case was originally the side light of a steamer ¾ in. thick, was then fixed to a board and laid on the tool with a 28 lbs. weight on the top, and ground by a pulling and pushing motion round the pillar until it conformed to the test of the convex gauge.
(b.) Fine Grinding.—To effect this the surface of the convex iron tool was cut by cross grooves which divide its surface into squares, the object of which is to diffuse the fine emery evenly over the surface, and so produce accuracy of figure. Before proceeding, however, the curve of the tool was perfected by what is termed file-testing. A few drops of coloured oil were first spread on the surface of the rough ground speculum, which was then laid gently on the tool so as to leave a mark on those portions which required filing—a delicate process, which had to be repeated over and over again till every part of the surface of the tool was equally marked with the oil. Before proceeding to fine-grind the speculum it was necessary to make what is termed the test-bar, to be used in an after process. This is a slip of cardboard, with parallel edges fixed on a rod, the surface of which is made accurately to fit the curve of the speculum. To proceed with the fine-grinding the tool and speculum were mounted on a machine contrived so as to give four distinct motions:—1. A slow rotation to the grinding tool. 2. Direct back and forward stroke. 3. A transverse stroke. 4. Free rotation to the speculum on its own axis. This machine is a cheap, but quite serviceable, form of that which was designed by
Lord Rosse for grinding his metallic speculum. Flower emery was used for fine-grinding, made into the thickness of cream with water, and spread evenly over the surface of the squares into which the surface of the tool was divided. The machine was then driven at about sixty revolutions per minute, the emery that escaped being collected, washed, and precipitated. This process was repeated four times, a finer degree of emery being obtained by allowing it a longer time to settle in each instance; the last time the emery being so fine that it took twenty-four hours to settle from the water, which was then drawn off by a glass syphon. When the fine-grinding was completed the surface of the speculum looked like plate-glass covered with a milky film, no grain being visible with a magnifying glass.
(c.) Polishing.—To effect this the iron tool was faced with pitch ¾ in. thick, divided into squares by grooves in a similar manner to the iron tool itself. The soft surface of the pitch when warm was moulded by the glass speculum so as to fit correctly, and then covered with rouge purified from grit by washing and settling in the same manner as the emery. The polishing was then done on the same machine as the fine grinding, and occupied six hours of continuous work, so that no change in the form of the pitch surface should take place through great alteration of temperature. The utmost care had to be exercised to prevent any dirt settling on the tool during this part of the process.
(d.) Parabolizing.—The curve of the speculum produced by the above process was such that on being mounted on the tube it gave imperfect definition. To correct this, and to give the surface the perfect curve, was the most important and delicate part of the whole process, and formerly was effected only with great difficulty. The use of the test bar already alluded to, which is the invention of Mr. Purkiss, enables it to be done with comparative facility. The strip of cardboard was fixed with the curved wood on the face of the speculum. A star was then viewed with the eye-piece out of focus, so as to get an enlarged disc of light crossed by a black band caused by the testbar. The edges of this band were found to be curved instead of straight, and all that was required to produce a proper curve was to modify the polishing surface of the tool by repeated trials until it imparted such a form to the speculum that the image of the test bar had parallel edges. This was done by scraping down the squares of the pitch on the surface of the tool so as to make them proportionally larger or smaller, at the centre or margin, as experience required. By this simple process a correction of the curve amounting to only one-millionth of an inch can be made.
(e.) Silvering.—The speculum having been thus polished to a true curvature could be used for observation in the same manner as a metallic speculum, but there was, of course, a great loss of light owing to the transparency of the glass.
To obviate this the reflecting surface was covered with an extremely delicate film of pure metallic silver by what is termed Browning's process, produced by the decomposition of nitrate of silver by sugar of milk. After a few trials on another glass surface this process was successfully applied to the speculum, when the silver film having been polished to a fine surface by a wash-leather rubber and fine rouge, the speculum was ready for mounting. The mounting requires to be very accurate in order to give equal support so as to avoid the slightest flexure of the glass, but this was effected in a very simple manner by laying it on six freely-balanced points so arranged as to give equal support.