Royal Irish Academy. 585 



mirable quality up to this size, or even fifteen inches ; but with all 

 his distinguished mechanical talent, he is believed to be doubtful of 

 the possibility of more than doubling this last magnitude in perfect 

 speculum metal. 



Under these circumstances, too much praise cannot be given to 

 Lord Oxmantown, who, in the midst of other pursuits, has found 

 leisure for such researches ; and by a rare combination of optical 

 science, chemical knowledge, "and practical mechanics, has given us 

 the power of overcoming the difficulties which arrested our pre- 

 decessors, and of carrying to an extent which even Herschel himself 

 did not venture to contemplate, the illuminating power of this tele- 

 scope, along with a sharpness of definition scarcely inferior to that of 

 the achromatic. 



The chief difficulties which are to be overcome in the construction 

 of reflectors, arise from the excessive brittleness of the composition 

 of which specula are made, and from the necessity of giving them 

 figures which shall be free from aberration. The great mirror in the 

 Newtonian form is (if the eyepiece and plane mirror be correct) the 

 conical paraboloid. 



It is necessary that speculum metal should possess, in the highest 

 attainable degree, the qualities of whiteness, brilliancy, and resistance 

 to tarnish. Lord Oxmantown has found that these conditions are 

 best satisfied in the definite combinations of four equivalents of cop- 

 per to one of tin; or by weight, 32 and 14*7 nearly. Metals differ- 

 ing from this by a slight excess of either component, are, when first 

 polished, scarcely less brilliant, but are dimmed so rapidly that the 

 lapse of a few days produces a sensible difference. On the other 

 hand, some large specula of the atomic compound have been lying 

 uncovered for years, without material injury to their polish. 



But this compound is brittle almost beyond belief ; a slight blow, 

 or even the application of partial warmth, will shiver a large mass of 

 it ; though harder than steel, its surface is broken up with the ut- 

 most facility, and it has a most energetic tendency to crystallize. 

 The common process of the founder fails with it, except for masses 

 of very limited magnitude, as the cast cracks in the mould ; and the 

 subsequent difficulties of the annealing are such, that it has been a 

 very general practice to use an alloy lower (containing more copper) 

 than the atomic standard. Even Sir William Herschel was obliged 

 to yield to this necessity. It appears from a letter of Smeaton, (Rees's 

 Cyclopaedia, Art. Telescope,) that for his 20-foot mirror of 19 inches 

 aperture, the composition was 32 copper to 12-4 tin, and that for 

 the 40-foot it was even lower; yet two out of three attempts to 

 cast this huge speculum failed. 



Lord Oxmantown at first endeavoured to evade the difficulty, by 

 constructing a speculum in pieces, soldering plates of fine metal to a 

 back of a peculiar brass, ascertained to have the same expansion ; 

 and has completed one of thirty-six inches aperture and twenty-seven 

 feet focal length, which performs very well on stars below the fifth 

 magnitude, but above that exhibits' a cross formed by the diffraction 

 at the joints ; and in unsteady states of the air exhibits the sixteen 



Phil. Mag. S. 3. Vol. 19. No. 127. Suppl. Jan. 1842. 2 Q 



