A MONSTER TELESCOPE.

Hawke's Bay Weekly Times, Volume 1, Issue 22, 17 June 1867, Page 141

A MONSTER TELESCOPE.

In the ensuing spring there will be

ready for trial one of tiie most important telescopes which have been tin offspring of the scientific and construe tive skill of this country. The Legis iature of Victoria having voted the sum fof .=£sooo for the construction of a large reflecting telescope, to be erected at Melbourne, for the purpose of effect ing a thorough survey of the nebula) ;nd multiple stars of the southern ■hemisphere, the President and Counci, of the Royal Society (whose advice and co-operation had been requested j selected Mr Grubb, of Dublin, the eminent optician, to construct- this important instrument. This gentleman, we need not remind our readers, is as well known for his various contribu tions to the optical department of scientific literature as for the photographic lenses manufactured by him, and with which his name is always associated ; and photographers will naturally feel n certain degree of pride when one who has been so long connected with the scientific branch of photography is selected for the carrying out of such a prominent work as the one we are about to describe. We have said that the telescope is a reflecting one. The impossibility of obtaining discs of glass of the requisite purity has rendered it imperative to employ catoprics instead of dioptrics —reflection rather than refraction —when a telescope of large -ize is to be constructed. The famous telescope of Lord Rosse is one of this description. An image is formed in the focus of the mirror, and is exatiined by suitable eye-pieces, a number oi Hiese being supplied of different degrees of magnifying power to suit the requirements of the observer. There are several kinds of reflecting telescopes, all on the broad principle here indicated, but constructed iu a variety of ways so as to secure the greatest possible advantage. The Gregorian telescope receives the reflected image of the object under examination upon a small convave speculum placed in the lube of the instrument and iu the axis of the large speculum, in the centre of which there is a hole through which the reflected image is trausmit ted to be examined by the eye-piece. In the Newtonian telescope the image is received upon a small plane mirror placed at an angle of 45 degrees, auu a little distance within the focus, by which "means the image is bent at a right angle to the axis of the instrument,, and is examined by an ey r e-piecv through an aperture iu the side of the telescope. The form which has beer, selected by Mr Grubb and the savants appointed to aid him by their suggestions (the Earl of Rosse, Dr Robinson, and Mr Warren De La Rue) is known among cognoscenti as tho Cassegrainian telescope. It differs from the Gregorian form in apparently a very slight degree, but to an extent which confers on it some striking advantages over

the latter. Iu the Gregorian theTreflected image is received upon a small concave speculum; in the Cassegrainian the survace of the small mirror in convex. By this apparently slight difference is secured greater shortness, by more than twice the length of the focal of the small speculum, more light, and a better defined image in consequence of the small convex speculum correcting the aberration necessarily present in the large concave mirror. The.tube of the telescope nowunder qotice is of the enormous diameter of 4| feet, and of proportional length. Tue diameter of the speculum is but 6 inches less thau that of the tube, or 4 feet, being inches in tnickuess, and weighing about 27 cwt. We learn from the address of Lieut.General Sabine, the president of the Royal Society, delivered at the anniversary meeting of that body, that although the first speculum which was oast was so nearly perfect as would have made it not many years ago almost inestimable, Mr Grubb, in conseq'ienca of two small blemishes on its surface, which, could have been ground out in time, broke it up without hesitation. Tiie second casting amply rewarded aim, for it turned out to be faultless. A duplicate speculum has since been successfully cast. The grinding was performed by a polishing machine and steam engine constructed for and belonging to the telescope, and which will accompany it to Melbourne. The weight of the telescope when -completed will be about 10 tons, and It will be moved by clockwork. Y7e need scarcely add that it is mounted aquatorially.—British Journal of Photography.