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Messrs. Robinson and Grubb on the [June 11^ 



IV. ^' Description of tbe great Melbourne Telescope.-" By T. R. Ro- 

 binson^ D.D._, F.R.S., and Thomas Grubb, Esq., F.R.S. Re- 

 ceived June 11, 1868. 



(Abstract.) 



The authorities of the colony of Victoria formed (in 1862) the wish to 

 establish, in connexion with their new observatory at Melbourne, a power- 

 ful telescope for observing the Southern Nebulae, and applied, through 

 the Duke of Newcastle, to the President and Council of the Royal Society 

 for encouragement and advice. That body had in former years given 

 much attention to this subject, and had received a report from a Com- 

 mittee on it, in the hope of inducing the British Government to take such 

 a step. The same Committee was consulted, and made another report 

 almost identical with the first. In consequence of it, the Legislature of 

 Victoria ordered the construction of the telescope in 1865, which was 

 undertaken early in the following year by Mr. Grubb, under the direction 

 of a Committee consisting of the late Earl of Rosse, Dr. Robinson, and 

 Mr. W. De La Rue. After Lord Rosse' s death, his son, the present Earl, 

 was appointed in his stead by the President. It has been very success- 

 fully completed ; and it is thought that some account of it, and the 

 notable things that occurred in its progress, may deserve a record. As 

 an introduction to this account, a notice of the reasons which guided the 

 Committee in some of their decisions may be useful. 



1. They chose a four-feet Reflector, because, from the experience 

 gained with Lord Rosse's and Mr, Lassell's telescope, they were satisfied 

 that this aperture was sufficient for the work proposed, and also because it 

 had not been proved that a larger one could be equatorially mounted with 

 the requisite firmness and ease of motion. This last, however, has been 

 convincingly disposed of by the present experiment. 



2. They chose the Reflector in preference to the Achromatic, because 

 it is not probable that one of the latter will ever be made which shall 

 equal a four-feet in light, and because, if it were, the cost of it would be 

 tremendous. One of the Committee got from a great continental optician 

 an estimate for one of 30 inches, which would have amounted, complete, 

 to ^20,000, and been far inferior in power. Absurd exaggeration is not 

 uncommon in comparing these two kinds of telescopes, but is easily refuted. 

 A speculum reflects, after years of use, 64 of the incident light, which is 

 the intensity of Herschel's front view. In the Newtonian the coefficient 

 is 0*401 ; in the Achromatic light is lost from two causes : — first, by reflec- 

 tion at each of the four surfaces of its lenses. This can be calculated from 

 Presnel's formula, and gives the coefficient of intensity 0'81, whence the 

 equivalent to a four-feet Newtonian is not less than 33 inches. Secondly, 

 it must be greater than this ; for no glass is perfectly transparent, and 

 therefore absorbs light according to a law depending on its thickness and 

 a certain constant. Dr. Robinson discusses this, and comes to the con- 



