cil 
IIl.—ON A NEW FORM OF EQUATORIAL TELESCOPBR. By HOWARD 
GRUBB, M.E., F.R.S. (Prats IT.) 
[Read, January 21st, 1884. ] 
Some years ago I deyised an Equatorial which was intended to afford special facili- 
ties for spectroscopic observations. I here append a woodcut and description from 
the Proceedings of the Royal Dublin Society (Vol. u., N.S., page 362, April, 1880): 
“This instrument, though obviously useful in many ways, is 
principally intended for solar spectroscopic investigation. It 
occurred to me that while in observing faint objects, such as 
nebulz, faint companion stars, &c., observers are always most 
careful to keep their eyes protected from even the faintest light ; 
in observing the faint portion of the extreme ends of the solar 
spectrum, they are under the very worst possible conditions, for 
they must needs be in a room blazing with sunlight if they use 
a spectroscope attached to an ordinary equatorial mounting. I 
leave it to any observer to say what chance he would have of 
seeing faint objects in a telescope for ten minutes or a quarter of 
an hour after leaving a room in which he had been exposed to the 
electric or other bright light. 
“Of course, when it is desired to concentrate on the slit the 
general light of the Sun for examination, a hole in a shutter can 
be used and a siderostat outside. But I refer to those obserya- 
tions where it is necessary to form an image of the Sun on the slit of the spectroscope, and examine 
each part seriatim, and this at present can only be done with an equatorially-mounted telescope and 
spectroscope attached thereto. 
“The instrument consists of a cast-iron frame supporting a 4-inch achromatic telescope pointing 
directly at the south pole of the earth. Below the objective and in line with it is supported a mirror 
polished with the greatest care to an optically-plane surface, and silvered with a coating of chemically 
deposited silver. 
“The telescope revolves on its own axis, either by hand or by clock work, as required, and this motion 
corresponds to the right ascension movement of an Equatorial. The mirror has a motion on its axis, 
and this corresponds to the declination movements of an Equatorial. 
“The whole framing is carried, when not in use, on four’rollers, rolling on a pair of iron rails let into 
the floor. When required for use the instrument is rolled over to a window specially prepared for it. 
The lower part of this window opens, and the instrument is rolled out until the mirror, objective, and a 
certain portion of the tube project outside the building, while the eye-piece remains in a convenient posi- 
tion inside. The wheel is now turned, and the whole frame is lowered down until three levelling screws 
bear on three cushions of iron specially prepared for them, and let into a solid stone pier just projecting 
K 
TRANS, ROY. DUB. SOC., N.S. VOL. III. 
