§54« Dr. Wollaston's Description of a 
an inch in breadth may be determined with as much precision 
as those of any larger crystals. 
For this purpose, the ray of light reflected from the sur- 
face is employed as radius, instead of the surface itself, and 
accordingly for a radius of ~ of an inch, we may substitute 
either the distance of the eye from the crystal, which would 
naturally be about twelve or fifteen inches ; or for greater 
accuracy we may, by a second mode, substitute the distance 
of objects seen at a hundred or more yards from us. 
The instrument which I use, consists of a circle graduated 
on its edge, and mounted on a horizontal axle, supported by 
an upright pillar (Plate XI). This axle being perforated, 
admits the passage of a smaller axle through it, to which any 
crystal of moderate size may be attached by a piece of wax, 
with its edge, or intersection of the surfaces, horizontal and 
parallel to the axis of motion. 
This position of the crystal is first adjusted, so that by turn- 
ing the smaller axle, each of the two surfaces, whose incli- 
nation is to be measured, will reflect the same light to the 
eye. 
The circle is then set to zero, or i8o°, by an index attached 
to the pillar that supports it. 
The small axle is then turned till the further surface re- 
flects the light of a candle, or other definite object to the eye; 
and, lastly, (the eye being kept steadily in the same place) 
the circle is turned by its larger axle, till the second surface 
reflects the same light. This second surface is thus ascer- 
tained to be in the same position as the former surface had 
been. The angle through which the circle has moved, is in 
fact the supplement to the inclination of the surfaces ; but as 
