III. HISTORY. 357 



1834. A 10-ft. Newtonian Reflecting Telescope by Sir 

 William Herschel, with 8J inch large mirror, small plane re- 

 flecting mirror, and several eye-pieces of various powers. 



Rev. Robert Main, Director of the Radcliffe Observatory, 



Oxford. 



This telescope^was made by Sir William Herschel for the Radcliffe Observa- 

 tory in the year 1812, and was received at the Observatory in April 1813 ; Sir 

 William himself having come to Oxford to superintend the mounting and the 

 adjustments of the mirror. 



The correspondence with Dr. Robertson, who was then Radcliffe Observer, 

 is preserved at^the Observatory. 



1834a. Eight plans of the Telescope, made in London at 

 the end of the last century, under the direction of Sir William 

 Herschell, for the Royal Observatory at Madrid. 



Astronomical Observatory, Madrid. 



These plans give an exact idea of all the details of the instrument and 

 mounting. 



The speculum was of 2 feet aperture and 25 feet focal length. 



This instrument was sent from London in 1801, and set up at Madrid in 

 1804. Four years afterwards the French converted the observatory into a fort, 

 the telescope was destroyed, the only part remaining being the speculum. 



1835. Discs of Optical Glass for Refracting Equatorial : 



1 Hard crown. 



1 Dense flint. Chance Brothers $ Co. 



1835a. A Series of seven Glass Parabolic Mirrors, 



from 3^ in. to 15 in. in diameter, from 2 ft. to 10 ft. focus, sil- 

 vered on the surfaces by Liebeg's process. 



John Browning, 63, Strand. 



1836. Compound Speculum, of 2 feet aperture. 



Earl ofRosse, F.R.S. 



This is one of the earlier attempts of the late Earl of Rosse to construct 

 specula of considerable dimensions of the hardest and most reflective quality 

 of speculum metal. 



To avoid the difficulty of casting the mirror in one, a cubical block of 

 speculum was sawed into laminae, and these were laid side by side on a ribbed 

 backing of a zinc-copper alloy of the same coefficient of expansion, whose 

 surface had been previously tinned ; the whole was carefully brought up to 

 the melting point of tin, and melted tin applied to unite the whole. Though 

 superior in rigidity to the solid metal speculum afterwards successfully con- 

 structed, it was discarded in favour of the latter, owing to the injury to 

 definition through diffraction at the junctions of the laminae.* 



Speculum (experimental), with annulus separate from central portion, 

 constructed for the purpose of attempting to correct spherical aberration by 

 advancing the annulus before it had been shown to be possible to produce a 

 paraboloid figure. Given up in favour of the solid speculum for same reason 

 as the last (diffraction). 



* N,B. Another compound speculum of 3-foot aperture is still preserved, 

 but the smaller one is sent, as the weight of the other is considerable. 



