Dr. Wollaston on the cutting diamond. 267 
they meet is straight. But in the natural diamond there is this 
peculiarity in those modifications of its crystals that are chosen 
for this purpose, that the surfaces are in general all curved, 
and consequently the meeting of any two of them presents a 
curvilinear edge. If the diamond be so placed, that the line 
of the intended cut is a tangent to this edge near to its extre- 
mity, and if the two surfaces of the diamond laterally adja- 
cent be equally inclined to the surface of the glass, then 
the conditions necessary for effecting the cut are complied 
with* The curvature, however, of the edge is not consi- 
derable, and consequently the limits of inclination are very 
confined ; for if the handle be either too much or too little 
elevated, then one or other extremity of the curve will be 
made to bear angularly upon the glass, and will plough a 
ragged groove by pressure of its point. But on the contrary, 
when the contact is duly formed, a simple fissure is effected 
as if by lateral pressure of the adjacent surfaces of the dia- 
mond directed equally to each side. By that means, adjacent 
portions at the surface of the glass are forced asunder farther 
than the mere elasticity of the parts beneath will allow, and 
a partial separation or superficial crack is produced. 
The effects of inequality in the lateral inclination of the 
faces of the diamond to the surface of the glass, are different 
according to the degree of inequality. If the difference be 
very small the cut may still be clean ; but as the fissure is 
then not at right angles to the surface, the subsequent frac- 
ture is found inclined accordingly. But when an attempt is 
made to cut with an inclination that deviates still more from 
the perpendicular, the glass is found superficially flawed out 
mdoccxvi. N n 
