504 
LORD OXMANTOWN ON THE REFLECTING TELESCOPE. 
which, in increasing’ the size, much of the perfection of smaller instruments had inva- 
riably been sacrificed. For instance, to avoid the brittleness of the best speculum 
metal, it had been found necessary, as the dimensions were increased, to use an in- 
creased proportion of copper, so that the alloy was inferior in brilliancy, yellower, and 
much more liable to tarnish*. In polishing large surfaces also there were great and 
peculiar difficulties, all the defects having a tendency to augment rapidly with the 
size, and proportionately to impair the defining power. I thought, therefore, that were 
it possible to discover means of securing, in the construction of large instruments, the 
same excellence of material and accuracy of surface, which on a small scale was at- 
tainable by the processes already published, much would be gained ; but even in the 
construction of small instruments there was no reason to suppose that the utmost 
limits of perfection had already been attained. On the contrary, the unequal per- 
formance of specula wrought with the utmost skill by the same hand, and with the 
same materials, was a fact which clearly proved that certainty of effect, the usual cha- 
racteristic of a perfect mechanical operation, was still to be sought for. It was evi- 
dent, therefore, that there was ample room for experiment with a prospect of some 
success. With these views, the first experiments were undertaken : they were on a 
small scale, the earliest as far back as 1827, and some account of them appeared at 
the time in Sir David Brewster’s Journal for July 1828. Since that time, the scale 
was gradually increased, till an aperture of three feet was attained, beyond which as 
yet I have not proceeded. 
I should, perhaps, however, remark here, that although these experiments were ex- 
ceedingly numerous, they were not near so tedious as might be supposed from the 
length of time that has elapsed since their commencement ; for in the midst of other 
avocations, long interruptions occurred, so that probably more might have been ac- 
complished in one-third of the time employed continuously : there is nothing, there- 
fore, really calculated to deter others, who have the means, from devoting their time 
to the improvement of the reflector : the joint labour of many may effect much, and 
the object is of no less importance than the advancement of those inquiries in prac- 
tical astronomy, in which for further progress we must now look solely to an increase 
in the optical power of instruments. 
In describing these experiments, it will be necessary to enter somewhat into detail, 
so as to enable others to repeat them without much difficulty ; and as success often 
depends upon attention to minute particulars which have only been observed after re- 
peated failures, it will be impossible to be as concise as it would otherwise be desi- 
rable ; nothing, however, shall be inserted which has not been repeatedly tried with 
care, and is not calculated, either by its success or failure, to be practically useful: 
and first as to the best materials for constructing the reflecting surface. On this sub- 
ject many experiments have been tried, but upon the whole I have little to add to 
what is generally known : tin and copper, the materials employed by Newton in the 
* Pearson’s Astronomy, p. 74; and article Telescope, Rees’s Cyclopedia, Smeaton’s Letter. 
