402 An Account of the Measurement 
shall now return to a farther consideration of the construction 
of the axis. 
In Plate XII. there is also a horizontal view of the upper part 
of the axis ; A being the head of the microscope, and B the little 
diagonal speculum, for throwing the light on the diaphragm. C 
is the opening in the axis above the object-glass ; and D a brass 
slider, for covering the opening at C. E and F are two pulleys, 
attached to the side of the axis, over which pass two strings, 
having their ends united in opposite points of the shutter D; 
the other two ends of the string being within reach of the ob- 
server, who by this means easily opens or shuts the slider. 
In Plate XIV, ABCD is the moveable frame, fastened to the 
top of the external stand, and having an octagonal opening at E, 
for receiving the brass work connected with the four horizontal 
pipes carrying the Ys. The touching points between the octa- 
gon and cylinder, are g, h, n , at which parts of the frame hard 
pieces of metal are inserted. To prevent all possibility of shake 
in the cylinder, which would render an adjustment of the 
instrument troublesome, if not impossible, there are two strong 
screws at m and n. One is a helix, which acts against m, and 
against the end of the sliding piece n ; so that, by a condensa- 
tion of the helix by the screw m, the piece n acts against the 
head of the cylinder inserted in E. 
Plate XI. represents an elevation of the instrument seen side- 
ways, and is that part to which the long microscope is attached: 
it serves to show the formation of the interior stand carrying 
the sectorial tube. A B C D E F are mahogany uprights, firmly 
united at the bottom and sides by proper cross pieces, and at top 
by the plate of metal abed, through the ends of which pass 
those of four horizontal pipes, the plate abed being one which 
