{ 128 ) 



u])on a powder sucli as gypsum. As long as it is weak the electro- 

 static field leads to well detined continuous lines of force, but as 

 tlie lield becomes stronger the directive forces become too powerful 

 and the powder particles are heaped together and the lines of force 

 arc l)rokeii. In th[? present instance the eifect is probably the same. 

 In a weak field the particles are well disposed for conduction, but 

 as the field becomes stronger this disposition of the particles is 

 altered in many ])laces. The inversion of the effect of the waves 

 should thus take |)lace at shorter and shorter distances the lirmer 

 tlic binding of the platinuui to the glass. This, too, wiis observed 

 with these mirrors. 



With tube N". 7 not the slightest change was observed even when 

 it was brought to within 1 cm. of the spark. 



These phenomena, therefore, support in a different way Lodge's 

 assumption as to the mechanical influences of electric waves. 



T take this opportunity of thanking the Director and Professors 

 of the Delft Physical Laboratory foi- their kindness in gi'anting me 

 facilities for experimental work. 



Crystallography. — "O» the or'nmtdilon of crj/slaJ sirtions irith 

 the help of f he t races of two [jhmes and the optic extinction". 

 By J. ScHMUTZKR. (Communicated by Prof. C. E. A. Wicumann). 



(Communicated in the meeting of May 27, 1911). 



In the problem of the orientation of crystal-sections the given 

 direction of the trace of a plane can be replaced by the direction ^) 

 of the optic extinction. 



With optically uni-axial crystals the extinction, with regard to the 

 trace of a plane supposed ± on the optic axis, is always straight, 

 so that the problem is reduced here entirely to that of the orientation 

 with the help of the traces of three planes. 



With optically bi-axial crystals the solution of the problem becomes 

 less simple. Be in fig. 1 the projection plane T applied ± on the 

 bisectrix O of the optic axes A and B, be further the crystal-plane 

 Fi given liy the azimuth fij = CEP and the height r^ = Pv, of 

 its pole i\, r, by the coordinates ft, = CRQ, v., = Qv. of its pole 

 v.„ the secant-plane S by the coordinates ^= CR, o= Rs of the 

 pole s, then the angles h,=:^FOG and h, = ^ DOG, which are 



1) Proc. Royal Acad. Amst. 1911, 720. 



