412 SEC. 11. ASTKONOMY. 



7 feet focus (6 in. aperture). Smaller machines for smaller specula 

 were made of this construction, and are in part preserved. The 

 polishing machines used to figure and polish large specula (18 in. 

 and 49 in. diameter) for the 20 foot and 40 foot telescopes used at 

 Slough and at the Cape of Good Hope appear to have been 

 of the same construction as this instrument and of proportionally 

 larger size. P ro f- A. S. Herschel. 



183Gb. Brass foot used in place of the lead block to carry 

 the furrowed pitch rubber or polisher revolving with the bed plate 

 of the machine, to whose centre it is fastened down by screws. 

 For the larger sized mirrors these brass plates were used as well 

 as for smaller sizes to replace the lead foundation or bearer of the 

 pitch, and some 18 in. ones besides this small one are preserved. 



Prof. A. S. Herschel. 



1836. Compound Speculum, of 2 feet aperture. 



EarlofRosse, 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 qualitj' 

 of speculum metal. 



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

 speculum metal was sawed into laminse, and these were laid side by side on a 

 ribbed backing of a zinc-copper alloy of the same co-efficient 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.* 



1836aa. Speculum (Experimental) , with Axmulus se- 

 parate from Central Portion. Earl of Rosse, F.R.S. 



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). 



1835. Discs of Optical Glass for Refracting Equatorial : 



1 Hard crown. 



1 Dense flint. Chance Brothers fy Co. 



1835a. Series of seven Glass Parabolic Mirrors, 



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

 vered on the surfaces by Liebig's process. John Browning. 

 1836c. Achromatic Telescope with Rochon's Prys- 

 matic Micrometer (in its main tube) with vernier reading to 

 1-", formerly belonging to the Rev. Dr. Peacock, F.R.S., Treasurer 

 to the Royal Astronomical Society, and probably the identical 

 instrument used by the late Mr. Francis Baily, F.R.S., &c., in his 

 observations of the Solar Eclipse of September 20, 1820. 



William Lawton* 



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

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



