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



To distinguish the pieces of quartz from each other they 

 may be called " Crystal I." and " Crystal IT." ; one end face 

 of each is marked, and in what follows is called " the marked 

 end/' 



The action which the electricity exerts upon the light 

 passing through the crystal was compared with the action 

 which was caused by the compression, in a vertical or hori- 

 zontal direction, of a piece of glass inserted between the 

 analyzer and crystal. If we have noted, for example, as 

 follows, 



" Below + , above — ; the same action as vertical com- 

 pression/' 



it is to be understood that charging the ends of the secondary 

 axis below with positive and above with negative electricity 

 produced the same optical change in the centre of the field 

 of view as a compression of the plate of glass in a vertical 

 direction. 



Experiment I. Crystal I., marked end at the bottom. 



Below + , above — ; same action as vertical compression. 

 Below — , above + ; ,, horizontal „ 



Experiment II. Crystal I., marked end at the top. 



Below +, above — ; same action as horizontal compression. 

 Below — , above + ; „ vertical „ 



Experiment III. Crystal I., marked end above. 



(a) The centre of the field of view was made somewhat 

 darker by vertical compression of the glass plate, then, while 

 the glass plate remained compressed, " Below + , above — " 

 caused an increase of brightness. This action could be com- 

 pensated by stronger compression in a vertical direction. 



(b) By vertical compression of the glass plate the centre 

 of the field of view was again rendered darker; "Below — , 

 above + " again rendered the field of view brighter; but this 

 brightening was not now compensated, but rather increased 

 by stronger compression in the vertical direction. 



Experiment IV. (after the crystals had been cemented 

 together in the opposite way). Crystal II., marked end at 

 the top. 



Below + , above — ; same action as horizontal compression. 



Below — , above + ; „ vertical „ 



Experiment V. — In the above experiments the quartz was 

 surrounded by sulphide of carbon ; in the following ones it 

 was surrounded by air. 



