416 Proceedings of the Royal Physical Society. 



that a number of the traces intersect at one and the same 

 point, and on studying this relationship by reference to the 

 corresponding stereogram, the reason will be sufficiently 

 evident. 



Now a line joining the intersection of any two traces and 

 the centre gives the direction of intersection of the two faces 

 represented ; and it is mainly by taking advantage of this 

 principle that the problem of determining the intersection 

 line of any two faces to each other is graphically determined. 

 Indeed, a knowledge of the principle in question is indis- 

 pensable to any one who wishes to make a correct clino- 

 graphic (or other) drawing of a crystal. 



In making linear projections of crystals belonging to the 

 Monosymmetric and the Anorthic systems, due allowance 

 must be made for the foreshortening of the a and h axes. 

 But as the method to be adopted has already been described 

 in both the former part and in this, there is no need to 

 repeat the instructions here. 



The application of this projection to the clinographic 

 delineation of crystals will be considered at greater length 

 further on. 



The Determination of Crystalline Forms. — It is assumed 

 in these papers that the reader, knowing something of 

 Mineralogy, desires to know more, and does not aim, at 

 present, at doing much original work. There are large 

 numbers of people who have to do with minerals who 

 perhaps may never have occasion to do much more in 

 crystallography than to determine accurately some of the 

 commoner crystalline forms on minerals whose species are 

 already known. It is to such as these that the maps 

 previously described are likely to be most useful. 



One thing more than another that a student who makes 

 his own maps soon learns, is a knowledge of the zonal rela- 

 tionship of the different forms of each species. Eegarding 

 this, I do not for a moment hesitate to state my belief that a 

 thorough knowledge of these maps and their uses is of at 

 least of equal importance to a thorough knowledge of the 

 reflecting goniometer and the uses to which this may be 



