198 REPORT—1857. 
of the smaller reflecting instruments, and, assuming that either the Cassegrain 
or Gregorian form is selected, a beautiful principle of correction is indicated, 
viz.: let both large and small mirrors be made each of a single piece of glass, 
let the onter surface of the larger lens (that which when silvered becomes 
the larger speculum of the telescope) differ in its radius of curvature from 
that of the silvered surface by the least quantity which will sufficiently dis- 
sipate its reflected image in the field, and let the outer surface of the smaller 
lens (that which when silvered becomes the small speculum) differ from the sil- 
vered surface of the same in an opposite manner, é.e.(allowing for the distance 
between the two lenses) so that the colour produced by the refraction of the 
larger lens shall be balanced by the colour of an opposing refraction in the 
smaller. This done, the combination as a whole will be achromatized, and 
the secondary images (or “ ghosts”) so far dilated as to be insensible in the 
field. 
For the great speculum of instruments of the largest class we probably 
must retain the speculum metal; there is, however, a construction which is 
possibly practical up to a considerable size, viz. that of a comparatively thin 
lens, silvered at the back, and supported throughout its back (or nearly so) by 
a thick or ribbed disc or casting of glass or metal, ground to fit with ade- 
quate accuracy. 
It may be useful, in concluding this section of the subject, to make a rough 
comparison of the achromatic and reflector. A 15-inch reflector, in which the 
suggested improvements were carried out so far only as the small metal is 
concerned, would equal a 12-inch achromatic in light, and a reflector of 36 
inches in diameter, similarly cireumstanced, would be more than equivalent 
to an achromatic of the size of the 29-inch discs already spoken of, while, 
the length of the telescope being in each instance, for the achromatic, more 
than double that of the reflector, the expense of the mounting may be 
estimated as fourfold. 
In passing to the second division of my subject, viz. the improvement of 
the equatorial mounting of large telescopes, I would first briefly advert to 
the several constructions in use, and which may be classed under three 
varieties, two of which are of English, the third of German origin. We have, 
then, the long-polar axis variety, which has the great disadvantage of the un- 
steadiness resulting from the telescope being attached to nearly the weakest 
part of an axis longer than itself. Secondly, we have the overhanging con- 
struction, consisting of a cone of great comparative weight and dimensions, 
and prolonged beyond its upper bearing in a biforked manner, thereby ad- 
mitting of the telescope turning on bearings within the projecting fork. This 
construction requires for steadiness an unwieldy mass of moving matter in 
proportion to the optical power it supports, four tons being used in the case 
of a telescope of 8 inches aperture, a mass tenfold that required with a 
better construction. 
The third variety, or German form of equatorial, has the advantage of the 
telescope being supported as close as possible to the strongest part of its 
polar axis; and the efficiency of such mounting is placed beyond doubt by 
the well known Dorpat instrument, and subsequently by the working of 
still larger instruments, for example, that erected many years since in this 
country for E. J. Cooper, Esq., where the telescope is 1353; inches aperture 
and 243 feet focus, and which has remained in effective use, with scarcely any 
repair, from the time of its erection, although unprovided with a dome or 
other roof,—a point of no slight importance when we consider the expense of - 
such for so large an instrument, not to speak of the labour and time con- 
sumed in moving it during obseryation. 
a 
