Simpler Methods in Crystallography. 417 



applied. Given a correct determination of a few of the 

 leading forms of any one of a large number of crystals, the 

 identification of most of the remainder follows easily enough, 

 when once the knowledge of the maps and their uses has 

 been gained. 



In the course of an extensive experience in the determina- 

 tion of the crystalline forms in the large collection of Scottish 

 Minerals under my charge here, I have but rarely had need 

 to make use of the fine reflecting goniometer which belongs 

 to the Edinburgh Museum. True, there does now and then 

 arise a case in which I have to deal with crystals of a species 

 which has not been determined. Then, certainly, accurate 

 determinations of augles have had to be made, and they 

 could be made only by using a proper instrument like this. 

 But in the great majority of cases, where the species of the 

 mineral under consideration is already known, a much 

 simpler method suffices for the identification of any doubtful 

 forms. First of all, the most essential requisite is a good 

 stereogram of the species in question. (I have now made 

 such of all the crystallised species occurring in Scotland.) 

 This serves, as will be presently shown, as an excellent 

 substitute for the divided circle of the goniometer ; and as 

 most of my stereograms are made to five inches radius, it is 

 possible to measure with them to a considerable degree of 

 accuracy. Next, for use with this, I have constructed an 

 arrangement of three stout brass wires, each bent at two right 

 angles, and fitted into hard corks, in such a manner as, when 

 combined together, to form an efficient universal joint, by 

 means of which the crystal to be measured can have any 

 part brought into exact line with the chief axis of rotation, 

 which runs through a hard cork mounted on a heavy stand, 

 and is prolonged some distance beyond. On this prolonga- 

 tion another cork is fixed, which carries a long steel needle, 

 used as a pointer. For use with this is a low-power micro- 

 scope on a stand, and provided with a considerable range of 

 movement. The whole is placed in front of a mirror which 

 reflects the light of a gas flame about nine feet away. There 

 is no difficulty whatever in adjusting a crystal in such a 

 manner as to bring the edge to be measured over into the 



