12 



Mr. J. Joly. 



cially observable in the positive figure. The straggling lines of the 

 tufts on this pattern are distorted considerably into the direction of 

 cleavage of the crystal. The outlying " crow's feet," common on 

 these figures, are symmetrically oriented with reference to the 

 cleavage. The entire figure, indeed, evinces unequal development in 

 directions coinciding with the cleavage of the crystal. Wiedemann 

 describes experiments on the conductivity of crystals by discharge 

 from a Leyden jar over dusted surfaces. The dust was thrown into a 

 ring more or less elliptical, according to the degree in which the con- 

 ductivity of the crystal differed in different directions. (' Poggendorff's 

 Annalen,' vol. 76 (1849), p. 406). 



It would be interesting to try the effects on stressed dielectrics. 

 An attempt of my own to deal with stressed glass failed ultimately 

 from want of adequate means of putting a uniform and sufficient 

 stress on the material. 



The distortion- of the figure produced by an unequally conducting 

 plate shows that the plate, as might be expected, shares in the con- 

 duction of the current, which it will do more or less, of course, 

 according to the degree of conductivity it possesses compared with 

 that of the gaseous dielectric. The characters of the figures are, 

 however, probably conferred by the gas, the parts of which being 

 isolated and mobile will tend to favour want of uniformity in the 

 discharge, losing equilibrium under small electrostatic stresses. 



(7.) Figures may be formed by inductive action exerted through 

 the dielectric plate. 



A glass plate, dusted on both sides, is supported about 5 mm. above 

 a smooth metallic surface which is placed in contact with one pole of 

 the coil. The other pole touches the upper surface of the plate cen- 

 trally. On the current passing a figure is formed on the upper 

 surface corresponding to the pole in contact with it, and on the lower 

 surface a pattern of the opposite kind, but less distinct. Looking 

 through the plate, it is seen that these figures are fairly superim- 

 posed, i.e., the outline of the + form corresponds with the outline o 

 the — form. If two glass plates be laid one above another separated 

 by 3 or 4 mm., the lower resting on metal, the figure on the upper 

 surface of the lower plate corresponds in a feeble way to the figure on 

 the upper surface of the upper plate. Similar inductive effects were 

 observed by Mr. Brown in the case of his photographic figures 

 (loc. cit.). 



(8.) If a plate be dusted on both sides and touched at each side 

 centrally by one of the poles, figures may be formed by direct action 

 on both sides of the plate. The plate may be held in a vertical 

 position. When the dielectric plate is thin the coincidence of the 

 figures so formed is very remarkable. Thus, taken in a plate a couple 

 of millimetres in thickness, every tuft of the pattern on the + side 



