Astronomical Circles and other Instruments, 
42J 
support. ABCDE, is a thick plate of brass, having a tube sol- 
dered in the opening C, to receive the microscope. The pro- 
jecting square pieces B, D, receive the conical points of the 
screws F, F, of the cutting frame, the general construction , of 
which will be readily understood from the engraving. The 
cutting point which is elliptical, as recommended by Mr. 
Troughton, passes obliquely at an angle of about fifty de- 
grees through the flat piece of brass G. The pieces pro- 
ject half an inch or more beyond the piece G, and to their 
under surfaces a thin slip of silver or gold is soldered, or 
otherwise securely fastened, on which a well defined circular 
dot is made, as represented in the plate. The sides HI, HI, 
of the cutting frame, must be of a sufficient length for the ends 
I, I, to reach nearly to the surface of the instrument to be 
divided when the frame hangs perpendicularly from B, D ; by 
this construction of the cutting frame, it is evident that the cir- 
cular dot may be adjusted so that its image may pass through 
the point of intersection of the wires of the microscope when 
carried through its field of view, and that the cutting point 
may be seen through the same microscope when employed in 
tracing the divisions on the limb of the instrument. A profile 
of the cutting point and frame is given at fig. 8. 
As every thing depends on the rectilineal motion of the 
cutting point when tracing a division, too much care cannot be 
employed in its use, and it is greatly to be wished some better 
mode of giving it motion could be adopted in place of the in- 
flexible projecting handles at present employed, as a lateral 
deviation of only --eioo inch, in a circle of two feet dia- 
meter, would occasion an error in the division amounting to 
about a whole second. Perhaps an universal joint applied to 
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