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VIII. Note on the Regularity of Structure of Actual Crystals. 

 By Lord Raylbigh, O.M., F.R.S.* 



THE question must often have presented itself as to how 

 far the mathematical regularities dealt with by the 

 crystallographer are realized in actual crystals. That the 

 natural faces of crystals tend to be plane is fundamental ; 

 on the other hand, it is well known that in practice it is 

 difficult to get any but very small faces to stand the roughest 

 optical test. Explanations of the discrepancy may readily be 

 suggested. The ideal conditions under which alone the 

 tendency to flatness could fully assert itself may be scarcely 

 attainable in practice. 



The case of surfaces obtained by cleavage would seem to 

 offer a better chance. To test this one naturally refers to 

 mica. Mr. Boys, I think, has somewhere remarked upon the 

 fact that a piece of mica held in front of the object-glass of a 

 telescope does not disturb the definition in the way that a 

 piece of glass does, unless the latter be carefully worked. 

 Mica thin enough to be convenient for such tests is of course 

 too flexible for an examination of flatness. And it is easy to 

 recognize that flexibility is not the only cause of deviation. 

 There are also local irregularities, due possibly to particles 

 of foreign matter or to strains which have exceeded the elastic 

 limit. But these irregularities do not seem seriously to 

 disturb the thickness of a thin plate ; and the inquiry sug- 

 gests itself as to how far the thickness of well split mica is 

 really uniform. 



A very delicate test of such uniformity is afforded by 

 reflexion in the mica of a soda-flame. For a preliminary 

 •examination, at any rate, it is best to dispense with all 

 optical accessories, simply holding the mica in the hand and 

 observing the reflexion of the flame at various parts of the 

 surface and at various moderate angles of incidence. The 

 interposition of a piece of card to shield the eye from the 

 -direct light of the flame is convenient. In this way I have 

 examined a number of sheets of superior mica, obtained many 

 years ago from a photographic warehouse. Most of these 

 exhibit serious irregularities in the splitting. The surface 

 is divided into patches where the reflexion varies, the patches 

 themselves appearing uniform and the boundaries sharp to an 

 eye focussed upon the plate. In some cases the boundary- 

 lines cross one another — a feature difficult to understand 

 until it is remembered that the irregularities may be upon 



* Communicated by the Author. 



