GruBB—On a New Form of Equatorial Telescope. 67 
“The observing astronomer, whose attention has been divided and engrossed 
by a series of operations in reality foreign to these studies, may soon feel the 
approach of lassitude ; his wearied eye no longer distinguishes with the same clear- 
ness stars of faint lustre; and from this result perceptible accidental errors and a 
notable loss of time. These inconveniences are so serious, that for’certain studies, 
as the search for comets, when it is necessary to range over a great extent of space, 
one is obliged to renounce the use of Equatorials of large dimensions; one is re- 
duced then to instruments smaller and less suitable for the exploration of the 
heavens.” 
From this I imagined that the new telescope was to be of great size; but it 
appears to be only about 103-inch diameter, and I have never heard of any great 
difficulty in working such a comparatively small instrument as this. 
It is, however, as I said, evidently in the case of very large instruments that 
this form would be most useful. Now, according to M. Loewy’s own description, 
the French form is quite unsuited for such, for he insists that the mirrors must 
have a thickness of, at least, one-fifth of the diameter. This would make the 
outside mirror alone for such an imstrument as I have sketched about half a ton 
weight, besides that of the objective, which would be about 200 Ibs., and the 
mounting of the mirror ! 
Even if any glass manufacturer could be found to undertake the manufacture 
of such a mirror (which I believe would be impossible), the absurdity of having 
this three-quarter ton at the end of the cross tube, and yet calling the instrument 
one of precision, is too apparent to need demonstration. 
In the case of the instrument described above, the total weight to be supported 
on the head of the tube need not be more than 80 lIbs., or about 3, of that of the 
other form. 
Again, it will be observed that, although in the French form the observer is as 
comfortably situated as in mine, still he has himself to exert all the energy ueces- 
sary for working the instrument, while in my form this is applied by hydraulic 
means. 
Finally, as to cost. It would appear that the French instrument has cost rather 
more than the same sized Equatorial of the usual form, with dome and equatorial 
room. I estimate that the form described above can be made for about one- 
fourth the cost of the usual form of Equatorial and equatorial room, dome, &e. 
The necessity has lately become very fully recognized of having some form of 
Equatorial constructed, in observing with which the astronomer can be placed under 
the favourable conditions rendered possible by the above described instrument. 
Professor Pickering, U.S., in a late address on astronomical subjects, makes 
special mention of a simple form of instrument with a mirror in front of the objec. 
tive, as possessing great advantages, particularly for mountain observations and in 
exposed situations, where it would be practically impossible to work in the open 
