20 Mr. L. Wright on Spiral Figures observable in Crystals, 



another, as is the case, for instance, between the sun and the 

 earth. The heavenly bodies are in this way in a sphere of mutual 

 action, not only by universal gravitation and by the radiation 

 of light and heat, but also by the electric force. If vacuum 

 is a good conductor of electricity, an electric motion arising 

 or disappearing on one heavenly body, will of necessity excite, 

 by induction in the vacuum, an electric motion which can be 

 propagated to any distance whatever in that good conductor, 

 and in its turn call forth electrical phenomena upon another 

 heavenly body. The inconsistency with the electrical nature 

 of the aurora borealis which has been supposed to be found in 

 its altitude, sometimes very great, above the surface of the 

 earth, loses thereby all its importance. In the last place, the 

 assumption that the presence of ordinary matter is necessary 

 for the propagation of an electric current from one place to 

 another to be possible will have to be abandoned. If it be 

 agreed that it is possible to an electric motion to propagate 

 itself with the greatest facility in vacuo, the notion of conduc- 

 tivity will be deprived of all physical meaning. The various 

 material bodies produce only a greater or less resistance to the 

 propagation of electricity ; their effect in this respect is not 

 active, but passive. 



II. Some Spiral Figures observable in Crystals, illustrating the 

 Relation of their Optic Axes. By Lewis Weight*. 

 [Plate I.] 

 PI^HE true relation of the optic axes in uniaxial and biaxial 

 JL crystals has always been an interesting subject. We 

 know that if the crystals be both polarized and analyzed cir- 

 cularly, and we disregard any dispersion of the axes for various 

 colours, the axis of a uniaxial and one of the two axes of 

 a biaxial present ultimately similar phenomena. Here, for 

 example, is the well known system of rings and brushes pre- 

 sented by a plate of calcite (fig. 1 ). As is well known, if 

 we interpose between the polarizer and the crystal a quarter- 

 wave plate, the black cross disappears, to be replaced by grey 

 nebulous lines (fig. 2), on alternate sides of which the quadrants 

 are dislocated] and if now we interpose a second quarter-wave 

 plate between the crystal and the analyzer, when the latter is 

 either ci'ossed or parallel even these lines disappear, and we 

 get simply a series of circular rings with no break whatever 

 (fig. 3). Let us now take a plate of sugar cut across one of 



* Communicated by the Physical Society, having been read at the 

 Meeting on November 12, 188] . 



