LAW OF DECREMENTS. 321 



RHOMBIC DODECAHEDRON. 



much more than to indicate the results of their appli- 

 cation to new cases. 



This may generally be effected, by giving the gene- 

 ral symbol of each particular class of planes, and the 

 formulae which are required for determining the par- 

 ticular values of the general indices of each class 

 respectively. 



Fig. 340. 



Simple and mixed decrements on the obtuse angles. 



General symbol q 

 ~q q~ 



Let the inclination of P on a plane of class f adja- 

 cent to it, or on the upper of the three planes on 

 which g is placed, be known and called / 2 , and 



pm sin. (/ a A^) 



q n sin. / 2 



If we divide $ by-^, being the particular value of 



qn V3 



in this case, we shall obtain the particular values 

 n 



of p and q, and hence the law of decrement producing 

 the plane we have measured. 



