of a Transit Circle. 139 
which hangs from the top plate, and passes by the side of the 
axis; at about 8 degrees, or 1 inch from the centre. For 
this purpose there are dots made on the limb, at a suitable 
distance on each side of the zero, both above and below, 
whether the telescope be horizontal or perpendicular either 
way. These are viewed through two compound microscopes, 
(of inches long, and their object-glass at 3 inches distance 
from the limb) carried by the same frames as the other mi- 
croscopes. 
The cursor, or moveable wire, in tl?e micrometer-micro- 
scopes, is adjusted much in the same way as General Roy's ; 
excepting that the micrometer head is made to turn stiffly on 
the neck of the screw, so as to allow of bringing the point of 
zero to front the eye, without the trouble of re-adjustment, if 
it happened to fall behind. 
It may be asked, since I use a compound microscope for 
viewing the wire, why I choose a plain plumb-line close to 
the limb, in preference to one in the combined focus of the 
glasses l My reason is this ; I use a compound microscope, 
because my eyes do never, with any comfort, adapt them- 
selves to a single magnifier ; and in this way I have more 
light, and can keep my eye at a greater distance. I approve 
much of Mr. Ramsden's ghost, as it is called, where it can be 
used with safety. But in this instrument, I thought I could 
not confide in it, as the microscope must be on a different 
support : whereas the looking at the dot itself, and the wire 
together, through a tube of above 5 inches, and at a distance 
of 10 or 12 from the limb, could admit of little or no parallax. 
I had intended making use of the original dots of the di- 
visions for this purpose ; but they are so minute, that the 
