140 Prof. W. C. Rontgen on the Change in the Double 



The bands were next adjusted first to the left and then to 

 the right of the depression, but remaining close to it ; in these 

 positions also no effect of electrification upon the double re- 

 fraction was observed ; neither the lower nor the upper end 

 of the band altered its position. 



The observation that also the upper end of the band was 

 not displaced is of importance; for since the lines of force 

 which issue from the hemispherical depression run there 

 horizontally, and coincide with the direction of the principal 

 axis to the left and to the right of the upper electrode, it 

 follows that also in the direction of the principal axis of the 

 quartz no perceptible change can be produced in the double 

 refraction by means of electric forces. 



The remark made already in a footnote of course holds 

 here also. The piezoelectric investigation, as remarked, had 

 shown that change of pressure in the direction of the prin- 

 cipal axis did not produce any electricity at the points of 

 pressure. This result seemed worthy of direct proof. For this 

 purpose the plate cut at right angles to the principal axis 

 was provided with a central hemispherical depression just like 

 the plate cut parallel to the axis, and introduced into the 

 flask in the place of the latter; the plates were simply inter- 

 changed. Having by this arrangement obtained an inter- 

 ference-band under the depression (that is, at the point where 

 the lines of force run parallel to the principal axis), I was 

 unable to produce any displacement of the band by increasing 

 or decreasing the difference of potential between the elec- 

 trodes; consequently in this plate also the double refraction 

 was not perceptibly altered by the action of electric forces in 

 the direction of the principal axis. This plate also, upon 

 pressure parallel to the principal axis, gave no piezoelectricity 

 at the points of pressure. If the band were situated at one 

 side of the depression, but very near to it, I observed a pheno- 

 menon upon electrification which furnished a very welcome 

 confirmation of the results obtained with the quartz parallele- 

 piped. The lower end of the vertical band remained fixed ; 

 but the upper end inclined towards the right or towards the 

 left ; and the direction of the motion changed with the sign 

 of the electricity upon the electrodes. Further, I observed 

 that, when the sign of the electricity remained the same, the 

 upper end of a band moved in a different direction according 



and a more intense source of light had been employed. If such a change 

 were to be observed, it would certainly be much smaller than that which 

 takes place in the direction of an axis of maximum piezoelectricity. The 

 above experiments would therefore not lose their significance. 



