of the Equatorial Instrument. 81 
either for adjusting the instrument, or observing a meridian 
altitude, y and % are thin perforated brass plates, attached to 
the cover that goes on the object glass, and, by occasionally 
turning them over it, change the aperture to or ^ The 
cross wires, of which there are 3 vertical, and 1 horizontal, 
within the eye tube S, have all the requisite adjustments by 
screws, &c. as in a common transit instrument, and are en- 
lightened by night, by a lamp fixed near to one end of the de- 
clination axis U, viz. that opposite the end V; but this part of 
the apparatus is hid behind the axis and the telescope, except 
the weight i, which is a counterpoise. This lamp throws a 
light through the conical axis, which is perforated at that end 
on purpose, on a speculum in the centre of the telescope, 
placed at \ right angles to the axis of the object glass, and 
from thence reflected to the cross wires. This speculum, 
which is an elliptical diaphragm, is perforated to permit all the 
rays from the object glass to pass unobstructed to the eye. 
This contrivance has been mentioned by Mr. Vince. (Practi- 
cal Astronomy, p. 80.) From what has been already de- 
scribed, it must now be evident that if the principal, or po- 
lar axis, as it has been called, LN, be elevated to the lati- 
tude, and adjusted to the meridian of the place, if the line of 
sight of the telescope be at right angles to the axis V U, and 
this latter at right angles to the polar axis LN, the brass 
circle 14 and 15 will correspond with the equator in the 
heavens, and the circle QR will become an horary circle ; 
viz. that if the centre of the wires, in the field of the tele- 
scope, be directed to any celestial object, on QR will be had 
its declination, and on 14 and 15 its distance from the meri- 
dian, from whence, by knowing the hour, the right ascension 
M 
MDCCXCIII. 
