SIR ROBERT HADFIELD, BART. 27 



SECTION VL— CRYSTALLOGRAPHY. 



As crystallography is, if not directly then indirectly, related to the 

 work of the microscope, I have asked my friend, Dr. A. E. H. 

 Tutton, F.R.S., the eminent crystallographist. to communicate sugges- 

 tions to this Symposium by way of a Paper or to the Discussion 

 from his point of view. 



During the recent Meeting at Bournemouth of the British Associa- 

 tion, Miss Nina Hosali, B.Sc. of the University of London, exhibited 

 interesting Models of Crystals. This worker has most kindly sub- 

 mitted her collection this evening and I am sure they will be found 

 useful. 



As explained by Miss Hosali, the object of these models is to 

 illustrate : — 



(a) The forms possible to crystals. 



(6) The different kinds of symmetry possessed by these forms. 



(c) How the forms are referred to crystallographic axes. 



Each model illustrates one of the thirty-two classes of symmetry, 

 and represents several crystal forms correctly orientated with regard 

 to crystallographic axes, the latter being shown by black threads. 

 A model consists in the first place of a glass envelope whose shape is 

 hat of some simple crystal form, and within this envelope two or 

 three other forms are represented by means of coloured silk threads 

 stretched over frameworks of thin copper wire. By this means it is 

 ■easy to make the forms intersect if necessary, and they are readily 

 ■distinguished from one another by the use of differently coloured 

 threads. 



The symmetry elements of the class represented by any model are 

 shown as follows : — 



(a) The traces of the Planes of Symmetry on the Glass envelope 

 are shown by steel wires. 



(6) Axes of Symmetry are shown by white threads. 



The set of 24 models exhibited represents 21 out of the 32 classes 

 and over 70 different forms. In many cases different varieties of the 

 forms may be produced by rotating or inverting the models, or by 

 reflecting them in a mirror, and when these modifications are taken 

 account of , the number of the forms shown is brought up to about 140. 



It may be interesting to add that there has been recently developed 

 and described by the Research Committee of the American Society ot 

 Mechanical Engineers an instrument called the Microcharacter (from 

 the Greek — to engrave or scratch on a small scale). This instrument 

 -determines that characteristic of a crystal which is the combination 

 of three of the five fundamental conceptions of hardness, namely, 

 the combined effect of cutting, scratch, and penetration hardness. 

 It can be employed for determining the hardness of the micro-con- 

 stituents of steel and .should be very useful to the Metallographist. 

 This Apparatus should be very useful to the Metallographer, as the 



