970 Scientific Proceedings, Royal Dublin Society. 
Another advantage which this arrangement has is, that it will 
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enable reflecting telescopes to be sloped downwards, and used in ~ 
such a siderostatic instrument as that which Sir Howard Grubb 
made for Queen’s College, Cork. To slope the telescope down- 
wards it is only necessary to place the airtight compartments A 
and Cin communication with a partially exhausted gasometer and 
the open air, through the passages which before connected it with 
the open air and a gasometer of compressed air. If an instrument 
of this kind is to be used for photography, the telescope is to be 
directed towards the South pole and fixed immovable; while the 
only parts to be carried round by the polar 
axis are the siderostatic flat mirror, mounted 
on a cell of a new pattern, and the photo- 
graphic plate-holder, or these with an eye- 
piece to enlarge the image. The weight to 
be carried round may be still further reduced, 
and at the same time some light saved, since 
the siderostatic mirror, M, and its supporting 
cell, ©’, may be perforated in the middle,” 
and the photographic apparatus, placed below 
them, on the polar axis. 
The mounting of such an instrument would be so much less 
expensive than that of an equivalent equatorial that the whole 
cost, including that of the siderostatic mirror, would probably be 
less, while the instrument itself would be very much more easily 
manipulated than the equatorial. Such an instrument should 
produce good photographic work, now that large flat mirrors of 
great excellence can be produced. For accurate spectroscopic 
work the arrangement seems to offer even greater advantages, for 
the spectroscope, as well as the telescope, may be absolutely fixed, 
and the only thing that needs to be moved is the siderostatic 
mirror. 
There remains a point to beconsidered. A large mirror cannot 
be removed and replaced upon its bed of levers. It must be 
ground and polished upon it, and allowed to continue afterwards 
upon it in the telescope. Can the corresponding arrangement 
be made with the new cell while manufacturing the mirror? I 
think so. What is required will be to place the mirror in a cell 
(not necessarily the same cell) filled with compressed air or with 
Fic. 4. 
