1890.] on Astronomical Telescopes, 167 



one of 6 feet aperture. All these were of speculum metal, and all on 

 the Newtonian form. In 1870, Grubb completed for the Melbourne 

 Observatory a telescope of 4 feet aperture, on the Cassegrain plan, 

 the only large example. The mirror of this is of speculum metal. 

 In 1856 it was proposed by Steinheil, and in 1857 by Foucault, to 

 use glass as the material for the concave mirror, covering the surface 

 with a fine deposit of metallic silver in the manner that had then 

 just heen perfected. In 1858 Draper in America, completed one on 

 this plan of 15 inches aperture, soon after making another of 28 inches. 

 In France several large ones have been made, including one of 4 feet at 

 the Paris Observatory : in England this form of telescope is largely 

 used, and mirrors up to 5 feet in diameter have been made and 

 mounted equatorially. 



Optically the astronomical telescope, particularly the refractor, 

 has arrived at a splendid state of excellence ; the purity of the glass 

 disks and the perfection of the surfaces is proved at once by the 

 performances of the various largo telescopes. No limit has yet been 

 set to the increase of size by the impossibility of getting disks of 

 glass or working them, nor is it probable that the limit will be set 

 by either of these considerations. We must rather look for our 

 limiting conditions to the immense cost of mounting large glasses, 

 and the absorption of light by the glass of which the lenses are made, 

 coming injuriously into play to reduce the light-gathering power, 

 though it will be probably a long time before this latter evil will 

 be much felt. 



With the reflecting telescope the greater attention given to the 

 working and testing of the optical surface has enabled the concave 

 mirror to be made with a certainty that the earlier workers never 

 dreamed of. The examination of the surface can be made optically 

 at the centre of curvature of the mirror in the manner that was used 

 by Hadley in the beginning of the last century, and revived some 

 years ago by Foucault who brought this method of testing specula 

 to a high degree of perfection ; in fact, with the addition of certain 

 methods of measuring the longitudinal aberrations we have now a 

 means of readily testing mirrors with a degree of accuracy that far 

 exceeds the skill of the worker. It enables every change that is 

 made in the surface during the progress of the figuring, as the para- 

 bolisation of the surface is called, to be watched and recorded, and 

 the exact departure of any part from the theoretical form measured 

 and corrected ; mirrors can be made of very much greater ratio of 

 aperture to focal length. I have one here where the focal length is 

 only 2i times the aperture : such a mirror in the days of speculum 

 metal mirrors with the methods then in use would have necessarily 

 had a focal length of about 20 feet. The difference in curvature 

 between the centre and edge of this mirror is so great that it can be 

 easily measured by an ordinary spherometer, amounting as it does 

 with one of 6 inches diameter to 3/10,000 of an inch, an amount 

 Bufficient to make the focus of the outer portion about 1 inch longer 



