1880.] on Electricity in Transitu. 433 



with tlio finger over a finite surface, or by a ring of wire laid close ujion 

 the tube, the pliospborcsccnce takes the form, approximately, of the 

 evoluto of an ellipse. In all these cases tlie illumination is somewhat 

 irregular ; but the geometrical elements of which the phosphorescent 

 figure is composed, and the stripes or striations of more intense light, 

 are always formed at right angles to the longer dimension of the 

 contact piece. This being so, suppose that we place on the tube a 

 strip in such a curve that the normal planes to the curve will pass 

 through the tangent at the corresponding point of the image of the 

 curve, i. e. the curve on tlie opposite side of the tube, each point of 

 which is exactly opposite to a point on the tinfoil. In such a case, 

 all the striations will lie along the curve formed by the locus of 

 the central patches of phosphorescence, and the result will bo a 

 single bright curved line of phosphorescence without any spreading 

 out or striated margin. The curve fulfilling these conditions will be 

 a helix, whose pitch is half a right angle. Experiment confirms the 

 anticipation. 



One more stej^ in the study of these molecular streams is necessary 

 for our present purpose, namely, an application to them of the same 

 method w^hich we have used with the electrical discharges them- 

 selves; viz. we must examine the eftect of an inductive stream pro- 

 duced ah extra upon a direct stream due to the discharge inside the 

 tube. These effects may be described generally as the interference 

 of molecular streams. 



If the finger be placed upon a highly exhausted tube through 

 which a discharge with a positive air-spark is j)assing, the phospho- 

 rescence due to the molecular streams from the negative terminal is 

 seen to fade aw\ay from the place where the finger rests, and from a 

 region lying thence in the direction of the positive terminal. The 

 effect is that of a shadow over that part of the tube ; and as this is 

 produced not by any real intervening object, but by an action from 

 outside, we have termed it a virtual sliadow. The phenomenon is due 

 to a beating down of the streams of molecules coming from the 

 negative terminal, by the transverse streams from the side of the tube 

 immediately within the part touched. 



The interference of two molecular streams may be further illus- 

 trated by a variety of experiments ; and in particular by arranging 

 within the tube a conductor of some recognizable form — say skeleton 

 tetrahedron. If the tube be touched at a place opposite to this 

 object, a shadow of the latter will be formed in the relief phospho- 

 rescence; but if the tube be touched also at a point on which the 

 conductor rests, the shadow will be splayed out in a striking manner. 

 This splaying or bulging of the shadow is due to the interference 

 of the molecular streams issuing from the surface of the conductor, 

 which then acts as a quasi negative terminal, with the original relief 

 streams issuing from the first point of contact. 



A still more striking instance of the interference of the molecular 

 streams will occur if the tube bo furnished with an intermediate 



Vol. IX. (No. 72.) 2 g 



