LORD OXMANTOWN'ON THE REFLECTING TELESCOPE. 
527 
retained in their positions by an alloy of zinc and copper, as easily wrought as common 
brass, and that they can be executed in this manner of any required size ; that cast- 
ings of the finest speculum metal can be executed of large dimensions, perfect, and 
not very liable to break ; that machinery can be employed with the greatest advan- 
tage in grinding and polishing specula; that, to obtain the finest polish, it is not ne- 
cessary that the speculum should become warm, but that any temperature may be 
fixed upon and preserved uniform during the whole process ; and that large specula 
can be polished as accurately as small ones, and be supported so as to be secured 
from flexure. ■* 
To form any other than a very vague estimate of the dimensions which the reflecting 
telescope may yet attain, would be impossible. Without allowing for further improve- 
ments in the process of polishing, which certainly may be confidently anticipated, 
I think that a speculum of six feet aperture could be made to bear a magnifying 
power more than sufficient to render the whole pencil of light available, and that in 
favourable states of the atmosphere it would act efficiently, without having recourse 
to the expedient which Newton pointed out as the last resource, that of observing 
from the summit of a high mountain. The construction, however, of such an instru- 
ment would be a serious task, and I should be sorry to attempt it, till, after addi- 
tional experience in observing, and further opportunities of comparing the two three- 
feet specula already finished, I felt more competent to do justice to the undertaking. 
In the mean time, I hope to receive from scientific men suggestions, which would be 
most valuable ; to continue the experiments already in progress, and to arrange the 
details of the mechanism necessary to render so large a tube conveniently manage- 
able. Everything, then, having been previously determined with care, subsequent al- 
terations would not be required ; tedious experiments would not now be necessary, 
either in constructing the speculum, or in the less interesting but necessary task of 
acquiring a practical knowledge of the mechanic arts ; and an instrument even of the 
gigantic dimensions I have proposed might, I think, be commenced and completed 
within one year. 
