514 
LORD OXMANTOWN ON THE REFLECTING TELESCOPE. 
failure, a single casting of very great size, or at once to have recourse to the expedient 
of combining together a number of small castings, a process perhaps more tedious, 
possibly less perfect, but more certain. However, a further comparison of the two 
specula as to defining power, under all circumstances, and with the utmost care, will 
determine the future course of these experiments ; at present there is no appreciable 
difference referable to their very different principles of construction ; they both are 
free from flexure in the different positions of the instrument, and have defined equally 
well when polished with equal success. With a single lens of one quarter of an inch 
focus, giving a power of about thirteen hundred, they have both shown satisfactorily 
the dots on the dial-plate of a watch more distinctly than a very good refractor with 
a much lower power. 
Hitherto the processes I have described were so effectual in producing the desired 
result, that there seems to be but little room for improvement, except in the discovery 
of new and better materials, an event by no means probable ; but the case is far other- 
wise in the remaining operation, that of polishing the speculum ; there, though the ex- 
periments have been even more numerous, much still remains to be accomplished. 
Before the speculum is polished, it is worked to a spherical figure by a process 
technically called grinding, where the mutual attrition of the speculum, and a mass 
of nearly equal size of some hard substance, eventually produces a figure nearly 
spherical ; and that, notwithstanding the irregularities, however great, of the sur- 
faces of either, or both, at the commencement of the operation. Several ingeni- 
ous devices have been, indeed, from time to time, suggested, more or less inde- 
pendent of the process of grinding, among which, perhaps, the most remarkable is 
that of Mr. Barton, who proposed to communicate the figure and the polish 
at once, by turning the speculum with a diamond, constrained, by very delicate 
machinery, to move in the proper path, and with a motion so slow that the 
resulting grooves should act on light as a polished surface ; but when we recollect 
the extreme accuracy required, that an error of figure amounting to but a small 
fraction of a hair’s breadth would destroy the action of a speculum, it is scarcely 
to be expected that any process can succeed in practice, which has not, like that of 
grinding, a decided tendency to correct its own defects, and to produce a result in 
which the errors may be said to be infinitely small in comparison with the errors in 
any of the previous steps from which it was derived. I need, perhaps, therefore hardly 
say that all my experiments have been directed to the one object, that of endeavour- 
ing to improve the common published process of grinding and polishing, particularly 
in its application to large surfaces ; for although the accuracy usually attained is so 
great that we fail in detecting by mechanical means, among a variety of specula made 
at different times and by different persons, any deviation from the proper figure, still 
by optical means, in fact, by trial in a telescope, the defects are at once apparent ; and 
we shall probably find among them examples of every grade of defining power, from 
the speculum which is almost perfect, to that which does not define at all. This dif- 
