THE EXHIBITION OF 1862. 73 
clockwork, which, as he is also an exhibitor in the department 
of Horology, may be expected to be perfect. This apparatus, 
whilst in motion, keeps the telescope always pointed to the 
same celestial object. The other telescopes which he exhibits 
are smaller, but the fittings and brass-work are in all cases 
very fine. Dallmeyer exhibits some astronomical telescopes in 
which the stand and slow-motion apparatus are very commodious. 
Wandering about the cases in the gallery, we perceive many 
fine telescopes of from one to five foot focus, and fixed on 
stands of all descriptions — nothing novel in design, although 
perfect in manufacture. Of the object-glasses (which are not 
exposed to view) we can form no opinion; but, considering 
their small size and the consequent ease with which they are 
made, it is almost certain that they are in all respects equal 
to the instruments of which they constitute so important a 
part. 
The gem of the Exhibition, in this department, is the great 
28-foot telescope belonging to J. Buckingham, Esq., C.E., who 
has fitted it up for his own private use, and has himself devised 
and manufactured all its various and novel machinery. The 
object-glass of twenty inches aperture is the largest successful 
attempt that has hitherto been effected. It is the work of 
Wray, and has been ground and polished to the true curves by 
his improved methods ; speaking admirably for the perfection 
of the system, as the surface does not show the least grain 
when examined with a microscope, and the chromatic and 
spherical aberrations are quite corrected. Whilst amateurs 
and observers have worked their own mirrors with one surface, 
it has been reserved for opticians proper to produce the perfect 
object-glass of four or even six surfaces. Few can conceive 
the difficulty of producing an achromatic lens of twenty inches 
diameter — the slightest deflection from the true figure, pro- 
duced by an extra stroke of the polisher, renders the glass 
quite worthless. It may be stated that the most powerful 
refractor hitherto in use (if we except that made, during the 
present year, by Alvan Clark, of 18^ inches) is the 16-inch, by 
Merz, at Pulkowa ; but the light-grasping power of this latter 
(varying as the squares of the diameter) is only as 256 to 400, 
in comparison with that here exhibited ; so that when the 
present instrument is brought into operation, we may expect 
many discoveries to be made in the constitution of those faint 
neb uke which Lord Ilosse has rendered celebrated. This 
telescope has as yet been comparatively untried, but it is to be 
hoped that the vast amount of skill, labour, and capital em- 
ployed in its construction will lead to many important results. 
The method of mounting is very commodious — all the clamping 
and slow motions can be effected from the eye end, and the 
