Dr. Wollaston’s Description of a 
turning the small axle may be brought to correspond appa- 
rently with the bottom of the house (or with some other dis- 
tant horizontal line.) In this position the surface accurately 
bisects the angle, which the height of that house subtends at 
the eye (or rather at the reflecting surface); then, by turn- 
ing the whole circle and crystal together, the other surface, 
however small, may be brought exactly into the same posi- 
tion ; and the angle of the surfaces may thus be measured, 
with a degree of precision which has not hitherto been ex- 
pected in goniometry. 
The accuracy, indeed, of this instrument is such, that a 
circle of moderate dimensions, with a vernier adapted to it, 
will probably afford corrections to many former observa- 
tions. I have already remarked one instance of a mis- 
take that prevails respecting the common carbonate of lime, 
and I am induced to mention it, because this substance is 
very likely to be employed as a test of the correctness of 
such a goniometer, by any one who is not convinced of its 
accuracy from a distinct conception of the principles of its 
construction. 
The inclination of the surfaces of a primitive crystal of car- 
bonate of lime is stated, with great appearance of precision, to 
be 104° 28' 40" : a result deduced from the supposed position 
of its axis at an angle of 45 0 with each of the surfaces, and 
from other seducing circumstances of apparent harmony by 
simple ratios. But however strong the presumption might 
be that this angle, which by measurement approaches to 45 0 , is 
actually so, it must nevertheless be in fact about 45 0 so' ; for 
I find the inclination of the surfaces to each other is very 
nearly, if not accurately 105 0 , as it was formerly determined 
