THE GREAT MELBOURNE TELESCOPE.

Otago Witness, Issue 1179, 4 July 1874, Page 21

THE GREAT MELBOURNE TELESCOPE.

( A ustralasian Sketcher. ) The Melbourne telescope is perhaps of greater value to the scientific world, and is better situated for making contributions to the scientific work of the age, than any other instrument in the world. Besides its scientific status, it has also a popular position, and that one of great eminence. It is one of the sights of Melbourne, and since its erection numberless are the persons who have pleasantly spent an evening in a visit to the Observatory, for the purpose of looking at some of the wonders of the heavens in the broad mirror of the great telescope. We purpose here giving a description of what is to be seen on such a visit, and also alluding to some of the lines of observation in which the great instrument has been employed. The telescope was made by the celebrated optician, Mr Grubb, of Dublin, and arrived in this Colony in 1868. It is constructed on the " Cassegrain principle ; that is to say, the object to be viewed is reflected on a large concave mirror, 4ft. in diameter, at the base of the tube, the image being thrown thence upon a small convex mirror, Bin. in diameter, fixed at the higher end of the tube, and is then viewed through a magnifying eye-glass, which looks through a large circular hole in the centre of the large mirror. It is mounted equatorially, and can with ease be directed to any part of the visible heavens in a few seconds. Although the moving parts of the immense telescope weigh eight tons, so admirable is the method of mounting, so accurate ia the finish of the working parts, and so perfect the system of counterbalance, that a slight pressure of the hand is sufficient to turn it in any direction. The tube of the telescope is not a close tube, it is simply formed of lattice- work, free passage of the air being allowed through it, and the danger of currents, to which a complete tube would be liable, being obviated. The only purpose of the tube in any telescope is to afford a framework for the optical parts. In this one it is merely wanted to support the small mirror at the requisite distance from the larger one, and this the lattice- work does with equal rigidity and firmness to a complete tube. The most important part of the telescope is the great mirror, or speculum. This is made of what is known as speculum metal, a mixture of tin and copper, and with its containing-box weighs 2^ tons. It is hardly necessary to say that it must be perfectly accurate in form, or the image of a star would lose all distinctness and become a mere shapeless blotch, or that it must be brought to a very high polish, or its reflecting power would be imperfect. To secure absolute correctness of form, it is further necessary to provide for so distributing the bearings under the speculum upon which it rests as to furnish support at a great number of points, and to see that each carries only its due share of the weight. This is managed by resting it upon a combination of levers balancing each other. The sensitiveness of the speculum to any inequality in the support is so great that the slightest unfairness of pressure would be sufficient to cause an inflexion of the mirror that would exhibit itself optically, by distorting or blurring the image of a star.. The mirror in time requires repolishing, owing to the dimming effect of the atmosphere, and this is provided for by a polishing machine, worked by a small steam-engine, which stands in a ropm adjoining the telescope-room. The mirror is lifted out of the telescope, laid on a kind of truck, placed in the machine, and is then polished by the action of a heavy circular polishing tool, which grinds over the face of the mirror with a curious round- and-round motion. The final polish is given with rouge, a singular application to useful purposes of a material manufactured for a very different object. As the polishing progresses, the " truth " of the mirror is tested by bringing it to bear on a small object, such as a watch face, erected at a long distance off, an image of which is then thrown from the mirror upon a screen. When a clear and perfect image is thrown from all parts of the mirror, and the polish is sufficiently fine, the speculum is ready for work. The repolishing is a process which does not require frequent repetition, as with care, to prevent tarnishing, the mirror will work for some years. The mirror being replaced in the telescope, the great instrument is ready for work. The first step necessaryis to uncover it to the sky, which is effected by running back the roof of the room in which it is lodged over the other half of the building. This operation is performed with ease, the roof being mounted on wheels, which run on iron rails. The telescope is then directed to the part of the heavens which it is desired to observe, and the observing astronomer takes his position to note the i pj?tiw.»l impressions that the telescope, 1

with its broad bright eye, is receiving from the depths of space. His position, of course, depends on the altitude of the object. If it is near the zenith, he is compelled to lie on his back ; but if about midway between this and the horizon, he can sit comfortably in the observingchair. He is enabled by suitable mechanism to adjust the telescope with ease and precision while seated at the eye-glass, and the star to be watched having been brought in the centre of the field of vision, the moving machinery of the telescope is then connected with the clock, which stands in the stone pedestal of the instrument, and by this means an equatorial motion is imparted to it, which causes it to follow the movement of the star, and so keep it in sight and position. The equatorial mounting i 3 fitted with large fine-graduated reading circles, made of a mixture of palladium and silver, to resist tarnishing, and by these the position of the object in right ascension and declination is read off with ease and accuracy. The Melbourne telescope is> admirably situated for rendering important service to science. Besides eujoying the advantage of a favourable climate, it has in the southern stellar heavens a field of investigation all its own. In southern latitudes there is no observatory so complete and ! well-appointed as that of Melbourne, and possessing as it does one of the finest and I most powerful instruments ever constructed, its observations are of the utmost importance to the scientific world. One great branch of work has been the observation of the southern nebulte. The observer sits at the telescope, and carefully and laboriously draws with pencil or crayon shading the pale cloudy patch of thin diffused light which he finds projected on the black background of his mirror. This is a work of great nicety, and has to be gone over and over again till a correct transcript has been produced of the relative intensity of the light in different parts of the nebula, together with a faithful representation of the general form of the whole. When complete, the drawing stands as a record of the appearance of the nebula at that date, and serves for comparison with earlier or later drawings, for the purpose of showing whether any and what change has taken place in its aspect. Some careful and highly important observations of this nature have been made on the well known variable star Eta Argus and its accompanying nebula, which have tended to demonstrate the existence of rapid and enormous changes in this mysterious object. With respect to the principal nebulae, the Cape observations of Sir John Herschel, made in 1834-1838, and the drawings made by the great astronomer, are invaluable as elements of comparison. The changes which have been thus proved to have taken place in the nebula of this star are stated by Mr Macgeorge, in notes read by him before the Royal Society in 1871, to "have no parallel in astronomic record." The great telescope is used mainly for observations of this nature, where physical aspects and changes, appearances of the planets, phenomena of eclipses, &c. , have to be watched, than for observations of differential position. But it has also been applied to work of the latter kind, and has recently been used, in conjunction with some European observatories, to ascertain the parallax of the planet Flora. It has also been used for measuring the distances of double stars, work for which it is well fitted by its power of definition. There is, however, another class of work for which the great telescope is employed, that requires some modification of its arrangements. The smaller mirror at ' the end of the tube is removed, and a photographic apparatus is fitted there. The telescope being then directed at an object, the great mirror becomes the ob- J ject lons of the photographic instrument, and the negative image of the object to be photographed is fixed on the sensitive plate. In this way many admirable photographs have been taken of the moon, some of which were declared by Mr De la Rue, the great authority on celestial photography, to be the best astronomical photographs that were ever taken. The beautiful silver sheen of the moon as seen in a telescope, and a faithful representation of its enormous volcanoes and gigantic craters, are admirably given in these exquisite photographs. There ia still another use to which the great telescope is applied, and with a notice of that we must conclude this sketch. It is an object of great interest with the public, and on the evenings about the first quadrature of the moon it is visited by a large number of people, who are desirous of seeing the best view of our satellite that is obtainable. In conclusion, wa need only say that the great telescope cost about £5000, in addition to the sum of £1500 for the house, machinery, &c. It is to the credit of the Colony that, amidst its prevalent utilitarianism, it remembered and recognised the claims of science to the degree implied in the purchase and. support of so noble m instrument,