700 Dr. A. E. H. Tutton [March 14, 



of r' is only a feature of this particular salt of a very large isomorphous 

 series, and the form c{00l} is in general the plane clearly and promi- 

 nently announcing itself as the basal plane. The slide (Fig. 22) of a 

 crystal of another member of the series, potassium nickel sulphate, 

 will show this. Hence, Prof, von Fedorow naturally does not rely on 

 the fortuitous relative development of faces, but calculates the relative 

 reticular density of all the principal faces present, in order to discover 

 the true primary faces by their superior reticular density. 



Having thus determined the correct setting, and measured the 

 principal angles, including the five fundamental ones, the results are 

 recorded in the index-table in an abbreviated symbolic form if the 

 substance be a new one, or, if it has previously been measured and 

 therefore appears in his index-table, he discovers the fact at once by 

 the identity of the elements found with those of a substance given 

 in the table. The average time occupied in all this by Prof, von 

 Fedorow or one of his skilled assistants is about two hours. Mr. T. V. 

 Barker, who studied with Prof, von Fedorow before acting as 

 Demonstrator of Mineralogy at Oxford, has been of considerable help 

 in submitting the new method to a very severe test, from which it 

 has emerged with flying colours. He collected, at Prof, von 

 Fedorow's suggestion, fifty specimens of substances which had been 

 crystallographically examined in this country and described in the 

 recognized publications. Five of these were furnished l)y the 

 lecturer, six others by Prof. Armstrong with the aid of Messrs. Colgate 

 and Rodd, others by Drs. Chattaway and Drugman, and Mr. Marsh 

 at Oxford, and the remainder by Mr. Barker himself. Each specimen 

 was only marked by a number, no name or formula being given, on 

 its dispatch to St. Petersburg. The result was that Prof, von 

 Fedorow identified without any difficulty 48 of the 50 substances. 

 The crystals of one of the two others were too imperfectly developed 

 to be of use, and the fiftieth specimen was that of a substance which 

 it was afterwards discovered had never hitherto been measured, a 

 fact which was first indicated by its elements not tallying with those 

 of any substance mentioned in the table. This latter occurrence 

 confers even greater confidence in accepting the new method. 



It thus appears that in Prof, von Fedorow's hands, or those of 

 his pupils, the method is practically infallible, provided the crystals 

 are well developed and not of cubic symmetry. If the latter perfect 

 symmetry be developed, reference must be made to the optical pro- 

 perties, which the lecturer has always insisted have been far too much 

 neglected, and are here seen to be indispensable. The optical methods 

 themselves, moreover, as regards their use with small crystals on the 

 polarizing microscope, have been further perfected by von Fedorow, 

 his Universal Stage placing the rapid methods of two- and three- 

 circle goniometry at the disposal of the microscopist. It must also 

 be remembered that Prof, von Fedorow's method does not discrimin- 

 ate between the members of isomorphous series, as the crystals usually 



