150 Captain Kater's description of 
from turning the instrument in azimuth is avoided. The star 
is taken, and the telescope being afterwards directed to the 
reflected image, the very small movement in azimuth re- 
quired to follow the star cannot well be supposed to occasion 
any strain ; but the same observations of the time, and the 
same reductions to the meridian are necessary in this as in 
the preceding method. 
If the instrument is fixed in the meridian, as is the case 
with the mural circle, the observation by reflection cannot 
be made on the same night as that by direct vision ; and it 
may be supposed that unfavourable weather might occur for 
several nights and prevent the completion of the observa- 
tion, during which interval a change in refraction, or in the 
relative positions of the circle and microscopes might take 
place, impossible to be detected, and which would vitiate the 
result. This inconvenience has been felt at Greenwich, and 
a second mural circle is nearly ready at the Royal Observa- 
tory, for the purpose of simultaneously observing the same 
star by reflection and by direct vision. 
From the slight description that has been given of the dif- 
ferent methods of observing, it must be evident, that the 
important desiderata are to keep the circle constantly fixed 
in the meridian, using no other motion than that of the tele- 
scope, and to possess the means of instantly determining and 
verifying at pleasure the place of the horizontal or zenith 
point with a degree of accuracy, limited only by the powers 
of vision assisted by the telescope. 
If a telescope furnished with cross wires be adjusted to 
distinct vision upon a fixed star, the parallel rays proceeding 
from the object are converged to a focus, and an image of 
