ON THE APPLICATION OF THE 



the measurements, describe the plane as Modification 

 c, plane 1, and it may be marked in the figure of the 

 crystal as a 1. 



Let us now suppose we find on another crystal, 

 another plane modifying the same solid angle, and 

 inclining on P, M, and T, at j?', 9, and r% and a 

 third plane, also modifying the same solid angle, and 

 inclining on P, M, and T, at s, t* 9 v, we should de- 

 scribe these planes as we did the first, by 

 Modification a, p on P 



plane 2. 



p on Jf ^ 

 '-M \\ 



r' T J 





Modification c, s* P 



plane 3. 



r r > 

 r-M I 



tf T J 



And having thus recorded the character of the planes, 

 we may in future describe them as 

 Modification a 2, 

 Modification a 3. 



This method of description may be applied, whe- 

 ther the three planes have occurred on the same 

 crystal, or on different crystals. 



The inclination of the modifying plane on too of 

 the primary planes, is generally sufficient, when a 

 solid angle is modified, for determining ttye law of 

 decrement; but the third inclination serves as a 

 check upon the accuracy of the other two. 



If the edge of any prism be modified by one or 

 more planes, it will be sufficient to give the incli- 

 nation of each plane, on either of the primary planes, 

 where the inclination of the primary planes to each 

 other is known, as the inclination on the other pri- 

 mary plane may be readily ascertained. But when 

 an edge is modified, of any crystal whose adjacent 

 primary planes do not meet at a right angle, and 

 when their mutual inclination is unknown, the in- 



