460 Description of the Great Southern Telescope. [ Oct., 
speculum, all the curves given by Lord Rosse’s machine, and any 
of those given by other machines which have been found practically 
useful, are obtained. Some consider that the motion of the grinder 
or polisher should be itself controlled and capable of bemg regulated 
at pleasure, but it has been found that the free motion of the 
polisher forms a most accurate criterion of the action, pro tem. If 
that action be good, the polisher revolves just as it is desirable ; but 
if anything occurs to disturb that good action, immediately it com- 
mences to revolve fitfully, either too quickly or too slowly, and thus 
gives notice of something requiring attention. 
For the purpose of trial, as it would have been very troublesome 
to be obliged to remove the speculum to the telescope on every 
occasion of trial, the machine 1s so constructed that by turning a 
handle (e), the whole frame carrying the speculum is tilted into a 
vertical position by the endless screw and worm f, and a dial-plate 
and artificial star being placed at 800 feet distance, the mirror, used 
as a front-view telescope, is ready for trial in five minutes, which 
saves some two days’ work on every occasion, besides avoiding risk. 
So efficiently and with such certainty did this machine do its work, 
that on no occasion, having once ascertained what was required, did 
it fail in providing the requisite alteration of figure, while as com- 
pared with other machines the ease and steadiness of its working 
was remarkable. 
The photographic apparatus embraces every improvement pro- 
posed by Mr. W. De la Rue, and more especially those suggested 
by some experiments made with a rough trial apparatus fitted to 
the great telescope, with which photographs of the moon have been 
taken, exceeding in beauty and sharpness any hitherto accomplished 
in this or any other country. 
The spectroscope has been made with a totally different style of 
mounting to those in general use, with a view of making it a more 
practically useful tool, whilst the prisms which are adapted to it are 
made of one very dense flint of about 90° or 100°, with prisms of light 
crown cemented on each side of angles of about 20° to 25°, Fig. 14 ; 
this gives a very large amount of dispersion with 
Fic. 14. very little deviation, whilst the crown prisms pro- 
tect the surfaces of the flint from tarnish, which 
UN has proved such a drawback to the use of heavy 
flint, for although it can be removed by friction and 
nitric acid, yet every time this operation is resorted 
to it leads to a depreciation of surface. 
; The contract signed in the beginning of the year 1866, included 
all the above-mentioned parts, with the exception of the photo- 
graphic apparatus and the spectroscope, and amounted to 4,6602. ; 
the instrument to be completed in twenty months, and although 
that time was slightly exceeded, yet, as stated by the committee, 
