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Hawke's Bay Times, Volume IX, Issue 486, 20 June 1867, Page 3

Earthquake. —A slight shock of earthquake was felt in Napier at about 5 - 30 this morning. English Mail. —The English mail is due in Napier on Sunday next, the 23rd instant. The Nathan Troupe. —There are few of our readers who have not heard or read of the above favourite troupe. A dav or two ago, we were informed that, on leaving Auckland, it was very likely they would pay us a visit. Nelson files, however, received since the above reached us, inform us that they arrived in that town on the oth instant. However, it is not impossible that the good folk of Napier will vet have an opportunity of witnessing this'talented company.

A MONSTER TELESCOPE. In the ensuing sprint? there will be ready for trial one of the most important telescopes which have been the offspring of the scientific and constructive skill of this country. The Legislature of Victoria bavins voted the sum oi ct-oOOU for tns construction of a large reflecting telescope, to be erected at Melbourne, for the purpose of effectpurvey of the nebuloe ctuu muiupiu Shiii.s Oi tiic buuiUciii hemisphere, the President and Council

joi ciits i-\o\ai oociyiy (vyuoss sclvict? and co-operation had been requested) ■ selected Mr Grubb, of Dublin, the i eminent optician, to construct this important instrument. This gentleman, , we need not remind our readers, is as i well known for his various contribu- , tions to the optical department of scion- ■ title literature as for the photographic ■ lenses manufactured by him, and with which his name is always associated ; ■ and photographers will naturally feel 1 a 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 i impossibility of obtaining discs of glass . of the requisite purity has rendered it f imperative to employ catoprics instead ; of dioptrics —reflection rather than re- [ fraction—when a telescope of large size is to be constructed. The famous telescope of Lord Russe is one of this i description. An image is formed in . the focus of the mirror, and is exanined i by suitable eye-pieces, a number of these being supplied of different degrees ul 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 id the object under examination upon a small convave speculum placed in the tube of the instrument and in the axis oi the largo speculum, iu the centre of which there is a hole through which the reflected image is transmit ted to be examined by the eye-piece. Iu tne Newtonian telescope the image is received upon a small plane mirror j placed at an angle of 45 degrees, and a little distance within the focus, bv which means the image is bent at a right angle to tire axis of the instrument, and is examined by an eye-piece through an aperture in the side of the , telescope. The form which has been selected by Mr Grubb and the savants appointed to aid him by their suggestions (the Earl of Rosse, Dr Robinson, and Mr \\ arren De La Rue) is known among cognoscenti as the Cassegrainian telescope. It differs from the Gregorian form in apparently a very slight degree, but to an extent which confers uu it some striking advantages over the latter. In the Gregorian the reflected image is received upon a small concave speculum ; in the Cassegrain - iau the survace of the small mirror is 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 now under notice is of the enormous dia- ■ meter of 4J feet, and of proportional length. Tne diameter of the speculum is but 6 inches less than that of the tube, or 4 feet, being 4*- inches in thickness, and weighing about 27 cwt. We learn from the address of Lmut.-

General Sabiue, the president uf the Royal Society, delivered at the annis r ersary meeting of that body, that although the first speculum which was cast was so nearly perfect as would have made it not many years ago almost inestimable, Mr Gmbb, in consequence ut two small blemishes uu its surface, which could have been ground out in time, broke it up without hesitation. The second casting amply rewarded him, for ic 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 w T hicb will accompany it to Melbourne. The weight of the telescope when com pleted will be about 10 tons, and it will be moved by clockwork.—British [Journal of Photography.