400 - Short Method of expreffing 
former. For example, Ap (fig. 3.), being the fame form 
as fig. 1. the decrements which the angle A undergoes, oe- 
cafion fimilar ones on the angle p (fig. 3.). The eafe is 
the fame with the edge A r, in regard to the edge Op; of 
Iz in regard to Es, &c. After this, nothing is neceflary 
but to denote the number of folid angles or edges which 
undergo decrements really diflin&t, becaufe the latter a&tually 
contain thofe which take place on the analogous angles or 
edges. 
5. It, will, however, be fometimes neceflary to point out 
thefe laft angles or edges; and in that cafe we may employ 
the fmall.letters which bear the fame names as the large 
letters employed in figure 1; that is to fay, that p (fig. 3.) be 
denoted by a; sp by c; pu by 4, &c.. But it will not be 
requifite to mark thefe fmaill letters on the figure. It will 
be fufficient to introduce them in the fign of the eryftal ; 
becaufe they may be cafily referred, in idea, each to its 
proper place. 
6. To denote the effeéts of decrements by one, two, three 
er more ranges, in breadth, the figures 1, 2, 3, 4, &e. may 
be employed, as thall be explained hereafter ; and to denote 
the effects of decrements by two, three, &c. ranges int 
height, the fractions !, +, 1, &c. may be ufed. 
7. ‘The three letters P M T will ferve to denote either the 
form of the nucleus, without any modification, when they 
compofe alone the fign of the cryftal, or the faces which 
~ would be parallel to thofe of the nucleus in cafe the decre- 
mients did not extend to their limits; and thefe letters will 
then be combined in the fign of the cry{tal with thofe which 
have relation to the angles or the edges on which the decre- 
ments act, 
~ 8. The principal or moft fimple decrement, in regard to 
any folid angle whatever, fuch as O, may take place either 
on the bafe P or on the plane T, which is to the right of the 
obferver, or on the plane M fituated to the left. But it 
muft be remarked that the obferver is fuppofed to move 
3 round 
