214 On the Differences in Lichtenberg's Dust-Figures. [June 13, 



corresponds very closely with the results of excitation of the fibres in 

 the living animal of the same species (Beevor and Horsley). In all 

 the cases in which there was coarse degeneration in the internal 

 capsule it was, with two exceptions (both hallux cases) grouped on 

 the outer edge of the capsule. Attention is called to the fact that a 

 large proportion of the coarser fibres passing down through the 

 capsule enter the substantia nigra, and these experiments show this 

 tract to be nearly or quite as large as that passing down into the 

 pyramid. These are apparently fibres which have been looked upon as 

 pyramidal, and, as the " pyramidal tract " has been shown to be even 

 more extensive in the medulla and below the decussation than in the 

 internal capsule, it follows that the fibres passing to the substantia 

 nigra are probably replaced by others arising at lower levels. These 

 degenerations show that in the monkey the facial fibres are situated 

 in the middle third of the eras, in which they are mingled with the 

 fibres of the pyramid, and that they do not occupy a space by them- 

 selves mesial to the pyramid. 



VI. " On the Cause of the Differences in Lichtenberg's Dust- 

 Figures : Preliminary Note." By SlLVANUS P. THOMPSON, 

 D.Sc., F.R.S. Keceived May 9, 1895. 



As ordinarily produced by dusting a mixture of red-lead and lyco- 

 podinm upon a surface which has been charged by contact with the 

 knob of a Ley den jar, the dust-figures present a remarkable and 

 hitherto unexplained difference of form. The positive figures consist 

 of white lines branching in stellate or dendritic patterns, whilst the 

 negative figures exhibit red patches of circular or ovate outline. The 

 differences, save in the matter of colour, are not due to the powders used 

 nor to the nature of the dielectric surface chosen for the experiment. 

 They vary only slightly with the nature of the gas ; but are more 

 considerably altered by the rarefaction of the air. The author found 

 that the dendritic patterns of the positive figures are correlated to 

 the brush form of discharge, whilst the rounded patches of the 

 negative figures are due to the silent discharge of electrified winds. 

 When polished metal surfaces are used in air for producing the 

 discharges (as in the usual case when the knob of a Ley den jar is 

 employed), negative electrification more readily discharges itself in a 

 wind, positive electrification less readily, disruptively, as a brush. 

 But where a smooth surface of a peroxide, such as the peroxide of 

 lead, is substituted for a metal knob, positive electrification will dis- 

 charge itself as a wind, giving rise to white positive figures of 

 rounded outline ; while negative electrification will under certain 

 conditions produce a brush discharge from the peroxide surface, 



