KNOWLEDGE & SCIENTIFIC NEWS. 



[January, 1907. 



considered highly satisfac- 

 tory. Great difliculties were 

 experienced in the casting in 

 the !-and, and extreme care 

 in the cooling period of 6 

 days was retjuired. .\s it 

 was, several large castings 

 were cracked. However, by 

 using hoop iron at the bottom 

 of the mould he was enabled 

 by this to allow the gas 

 developed to escape, thereby 

 freeing the <;peculum from 

 pores and air-bubbles. After 

 the 6 days' cooling the disc 

 was ready for further develop- 

 ment. It was about 3^; inches 

 thick, and weighed 13 cwts. 

 The metal for it was placed 

 in 2 cast-iron crucibles and 

 melted by turf fires, as these 

 were considered steadier than 

 those produced by coke. 

 Before the speculum was 

 polished it was worked to a 

 spherical figure by the grind- 

 ing procefs. A steam engine 

 of 3-horse power was the one 

 employed for this purpose. 

 The grinder was made of 

 cast-iron, with grooves cut 

 lengthwise, across, and circu- 

 larly on its face. These 

 grooves were one-quarter of 

 an inch wide, and half-an-inch 

 deep. The polisher and the 

 speculum have a mutual action 

 on each other. The polisher 

 was further coated with pitch, 

 and on this was spread 

 either oxide of tin, as used 

 by Newton, or oxide of 

 iron (" crocus " as it was 

 then called), as used 

 by Lord Rosse. It 

 was necessary that 

 this pitch coating, 

 as well as the emery. 









The Machinery of the Telescope with the 3 ft. Speculum as it appeared in 1839. 



which was also employed, should be evenly distri- 

 buted, and that its lateral expansion in the grinding process 

 should be the same. At length the telescope was com- 

 pleted in September, 1839. Lord Rosse, in a paper read 

 by him before the Royal Society in London, in June, 1840, 

 describing the construction and building of the telescope, 

 said that from " September to Christmas, 1839, all op- 

 portunities for observing were taken advantage of. A 

 considerable number of Sir John Herschel's test objects 

 were examined, and as to double stars, perhaps the most 

 striking contrast between its action and that of other 

 instruments was the extreme brilliancy of the minute 

 companions of large stars — e.g, the companion of Polaris, 

 which, with a power of 600, was very like Polaris 

 itself in a 44-inch achromatic with a 2^-inch objective. 

 The companion of Alpha Lyrae and Rigel were brilliant 

 objects." He was very confident that as to the Nebula^ 

 ' the present instrument will add something to our present 



knowledge." " I 

 think I may say," he 

 continued, "that the 

 Nebulffi 27 Messier, 

 the Annular in Lyra, 

 and, what, perhaps, 

 is more curious, the 

 edge of the great 

 Andromeda, have 

 shown evident 

 symptoms of resolv- 

 ability. Clusters 

 were examined and 

 drawings were made. 

 My friends were de- 

 lighted when they 

 examined the Crab 

 Nebula." The 

 powers used were 

 600, 800, and 1,000. 

 The instrument 



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