Refraction of Quartz produced by Electric Forces. 135 



angles to the plane containing the principal axis and a 

 secondary axis of one of the crystals, and consequently at 

 right angles to two lateral surfaces. 



When placed between crossed Nicols, the centre of the 

 field of view (that is, the place between the perforations) was 

 nearly uniformly dark (employing sodium-light, the intensity 

 of which was quite sufficient for these experiments), not re- 

 garding certain small irregularities, probably resulting from 

 pressure during boring. Brass wires of about 0*15 centim. 

 thickness, with well-rounded ends, were placed in the two 

 perforations of the crystal under examination, which were so 

 connected with the electrodes of a Holtz machine as to admit 

 of rapid reversal of the connexions. The difference of po- 

 tential between the two electrodes could be varied at pleasure 

 and continuously, whilst the machine was rotated with uni- 

 form velocity by means of a secondary connexion containing 

 a variable air-resistance. This method, which I have em- 

 ployed for some time, consists in connecting one electrode 

 with an insulated sharp point, the other with an insulated 

 metal plate. The point and plate are opposed to each other ; 

 and their distance apart can be altered at pleasure ; the 

 further they are apart the greater is the resistance of air for 

 the dark discharge, and the greater therefore is the difference 

 of potential. 



In order to avoid the undesirable passage of sparks between 

 the wires within the crystal, which is liable to occur when 

 the difference of potential becomes too great, the crystal 

 was placed in a small flask filled with sulphide of carbon, and 

 in the later experiments with benzol ; the polarized light 

 entered one side at right angles where the flask was perfo- 

 rated, and the opening closed by a piece of plate-glass, and 

 passed out at the opposite side, which was perforated in a 

 similar manner. 



For the purpose of control I have also examined the pieces 

 of quartz in air, and have observed in all essential points the 

 same behaviour as when they were immersed in sulphide of 

 carbon or in benzol. 



The direction of the secondary axis, the axis of length of 

 the crystal examined for electro-optical effect, was placed 

 vertical ; the direction of the lines of force in the middle of 

 the crystal was therefore vertical ; and the principal plane of 

 the Mcol consequently made angles of 45° with these lines 

 of force (the arrangement previously designated " Position I." 

 of theNicol*). 



* Compare Ber. d. Ob. Ges. vol. xix. p. 1 (1880); Wied. Annul 

 vol. x. p. 77 (1880). 



