224 Mr. Cavendish on the Manner of 
we must depend on the microscope not altering its position in 
respect of the point during the operation ; for which reason I 
should prefer placing the axis of the microscope at exactly 
the same distance from the center of motion d., as the point; 
but removed from it sideways, by nearly the semi-diameter 
of the object glass ; so that having made the division, we may 
move the beam compass till the division comes within the field 
of the microscope, and then see whether it is bisected by the 
wire, and consequently see whether the microscope has altered 
its place, 
In the operation of bisection, as above described, it may be 
observed, that if the two scratches are placed so near toge- 
ther, that in making the second the point of the compass runs 
into the burr raised by the first, there seems to be some 
danger that the point may be a little deflected from its true 
course ; though in Bird's account of his method, I do not find 
that he apprehends any inconvenience from it. One way of 
obviating this inconvenience, if it does exist, would be to set 
the beam compass not so exactly to the true length, as that 
one scratch should run into the burr of the other ; but as this 
would make it more difficult to judge of the true point of bisec- 
tion, perhaps it might be better to make one scratch extend 
from the circle towards the center, and the other from it. 
It is clear, that the entire arc of a circle cannot be divided 
to degrees, without trisection and quinquesection ; and I do 
not know whether our artists have recourse to this operation, 
or whether they avoid it by some contrivance similar to Bird's, 
namely, that of laying down an arch capable of continued 
bisection ; but if the method of quinquesection is preferred, it 
may be performed by either of the three following methods : 
