large Reflecting Telescopes. 1S9 



the labours of the experimentalist, or of the practical optician. 

 Sir William Herschel also found that he was unable to polish 

 large specula so as to give them as accurate a figure as small 

 ones. His twenty foot telescope, which I believe has been ad- 

 mitted to be the best reflecting telescope ever constructed, was 

 seldom used with a power above 200 ; and I believe the same 

 observation will apply with equal correctness to the forty foot 

 telescope. 



The defects, therefore, common to all very large specula 

 hitherto constructed may be thus stated : a defective metal- 

 lic composition ill suited either to receive or retain a poUsh, 

 .or to show objects of their natural colour and brilliancy; a 

 want of sufficient stiffness in proportion to their weight \o ena^ 

 ble them to retain their figure with that great degree of exact- 

 ness necessary ; and thirdly, a want of as perfect a polish and 

 figure as has been given to small specula. 



My first experiments were undertaken with the view of ob^ 

 viating the two first defects. Having had some experience in 

 the process of painting on glass, in which the glass is made 

 red hot, and subsequently annealed, it occurred to me that the 

 precautions employed in that process might be transferred 

 with advantage to the construction of specula, and that it 

 might thus be practicable to prevent large specula, cast of the 

 highest metal, from cracking before they were finished. It 

 also occurred to me that large specula might possess sufficient 

 stiffness without any additional weight, were they cast thin, 

 but with a deep rim round them connected by ribs of equal 

 depth. A speculum, fifteen inches diameter, was accordingly 

 cast with a rim round the edge two and a half inches deep, 

 and half an inch thick, and with two ribs of the same depth 

 and thickness as the rim, intersecting each other at the cen- 

 tre of the back of the speculum. The composition employed 

 was the best speculum metal. As soon as the metal had 

 become solid, while still red hot, the sand was entirely re- 

 moved from the four cavities at the back between the ribs 

 and the rim, and the metal, still red hot, was placed in a 

 red hot iron vessel upon a bed of wood ashes, and the cover 

 of the vessel, also red hot, was then put on, the whole was im- 

 mediately placed in a red hot oven and shut up there. In 

 about forty-eight hours the metal was perfectly cool. It was 



