THE GEEAT MELBOUENE TELESCOPE. 
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III. The Committee recommended that the tube of the telescope should be of metal 
lattice-work, with a view to lessen the disturbing influence of currents in its interior. 
The speculum after sunset is in general warmer than the air which is in contact with it, 
and therefore heats it. It of course rises, and in an ordinary tube escapes along its 
upper side, while a cold stream descends along the lower side to replace the ascending 
one. The two form eddies at their common surface ; and the result is a medium of 
irregular density very unfavourable to good vision. As the night advances the external 
air cools faster than the speculum, and the evil increases so as in extreme cases to de- 
stroy all sharp definition. In the proposed construction the warm air escapes through 
the openings almost as soon as it leaves the speculum, and the counter-current enters 
similarly, so that the disturbance is very much diminished. This plan was proposed by 
Sir J. Herschel in the Preface to his Cape Observations, but was neglected till revived 
in 1852. Lord Posse then applied it to his 3-feet, and Mr. Lassell and Mr. DeLaPue 
to their telescopes — in every case with marked advantage. It is also strong, light, gives 
very little hold to the wind, and from the arrangement of its framing is unfavourable to 
the propagation of tremors. 
IV. It Avas decided that the great speculum should be of metal, not silvered glass. 
The Committee were unwilling to risk the success of the noble work entrusted to them 
by venturing on an experiment whose success on so large a scale Avas very uncertain. 
All the telescopes Avhich have been made on this latter plan are, they believe, with one 
exception, of which little is known, not larger than 12 or 15 inches. When it is a 
question of 48 inches, several difficulties present themselves which at present cannot be 
solved. Such a speculum must be of considerable thickness to keep its shape ; it Avould 
be no easy matter to manufacture so large a block of glass, which must be homogeneous 
in structure and well annealed ; the latter to prevent its breaking in Avorking ; the first 
because otherwise it will expand unequally and change its figure. Nor is it known 
Avhether a silver film of uniform thickness (which according to Foucault is essential) 
can be deposited over so large a surface. And there are two still more decided objec- 
tions. Glass, though rather less than half the specific gravity of speculum metal, 
somehow seems more liable to abnormal flexure. One of these silvered specula, appa- 
rently of uniform thickness and consistence, has been found to give a good image with 
one diameter vertical and not with another ; and they all require the utmost precaution 
in supporting them. This doubt has recently been confirmed ; for one of the small spe- 
cula of the Melbourne Telescope, Avhich Avill be described hereafter, is glass 8 inches in 
diameter ; yet even though so small it gave bad images while it was mounted as the metal 
ones were which acted perfectly, until its edge pressure Avas uniformly distributed. 
Secondly, there is not the great increase of light Avhich was expected from the high 
reflective power of polished silver. By Jamin’s experiments, solid silver reflects at per- 
pendicular incidence 0 - 93 of red light and 087 of indigo, Avhile speculum metal gives 
0’69 and 0-60. But the silver film which is deposited on the glass is much inferior. 
Lord Bosse, who expected considerable gain from using it as the small speculum of his 
