of the Equatorial Instrument. 103 
den, has executed them, I feel myself bound to bear this tes- 
timony to his merit. 
(22.) It remains that I now say something of the power of 
the telescope ; for it is to little purpose that the divisions be 
accurate, or the levels sensible, unless the force of the tele- 
scope be such as to correspond with the sensibility of the 
one, and the accuracy of the other. The object glass is a 
well corrected double achromatic, whose joint focus is 65 
inches, with an aperture of 4,2 inches. The telescope is fur- 
nished with two sets of eye glasses, one single, the other 
double ; of these latter there are 6, of different magnifying 
powers, from 60 to 360 times ; of the former there are 5, 
with powers from 150 to 550. To these may be added a 
prism eye tube, with a power of about 100, for objects near 
the zenith, or the pole, and similar to the one described by 
General Roy ; (see Philosophical Transactions, Vol. LXXX. 
p. 155) also a tube with a divided eye glass micrometer ; (see 
Philosophical Transactions, Vol. LXIX.) it has a power of 
80, but the images are not distinct, or equally bright, and 
the extent of the scale is so small, not more than jo', that it 
is, in truth, but of little use. The double eye tubes are com- 
posed of two eye glasses, to enlarge the field and render it 
more agreeable, both placed on the hither side of the cross 
wires, so that they may at any time be changed, without de- 
ranging the wires. The lowest of the compound eye tubes, 
with a power of about 6 o, is what is generally used for 
transits and polar distances.* For telescopical observations 
* If, as has been generally imagined, an angle of i' is about the smallest that is vi- 
sible to the naked eye, (Smith’s Optics, § 97) with a power of 60 times 1" will be- 
come visible; and, in that case, the power of this telescope will correspond with the le- 
vels, and the divisions, as was required above. 
