390 
An Account of the Measurement 
position by construction close to the dot ; so that, by looking 
through the axis in this way, the plumb-line appears like a 
small black line on the face of the mother-of-pearl. 
Now it is evident that, to an eye thus placed, when the in® 
strument is adjusted for observation, the plumb-line should 
appear as if accurately bisecting the dot. To give, therefore, the 
observer the means of moving it to the right or left, when 
standing on the ground, (avoiding thereby the inconvenient 
necessity of elevating himself on steps as high as the axis,) Mr* 
Ramsden placed a microscope, about 5 feet in length, parallel 
to the telescope, on the outside of the interior mahogany frame. 
This microscope, bent as it were at right angles at both ends* 
has one of them open, and placed close to the pivot of the axis 
carrying the small lens. In the upper part of the microscope, 
and just under its roof, is placed a speculum, inclined, at an 
angle of 45°, to the line passing through the centre of the 
sector's axis, and close to its end. This reflector receives the 
converged images of the dot and wire on the illuminated spec- 
trum, and transmits them down the tube of the long microscope ; 
the rays, falling on a plano-convex glass, at no great distance 
from the bottom, are finally sent out to the eye by a prismatic' 
glass at the end of the tube. Thus viewed, that which to the 
naked eye above appeared a small dot on the illuminated lamina, 
when magnified, as delivered below, is seen to be a small and 
well defined circle with a luminous area* admitting of the most 
accurate means of deciding on the right position of the plumb- 
line, by exhibiting small portions of light between it and the 
periphery of the little circle. 
The mode of illuminating the hollow axis is likewise inge- 
nious. On the side of the interior mahogany frame, and opposite 
