8 "N RAYS 



indeed what occurs. The interposition of a 

 sheet of mica, set so that its axis makes an 

 angle of 45 with the plane of action of the 

 radiations emitted by the tube, destroys their 

 rectilinear polarization, for their action on a 

 small spark remains sensibly the same, whatever 

 be the direction of the spark-gap. If a second 

 sheet of mica is interposed, identical with the 

 first, so that the axes of the two sheets are 

 perpendicular to each other, rectilinear polariza- 

 tion is re-established. This result can also be 

 obtained by the use of a Babinet's compen- 

 sator. Consequently, we are dealing with elliptic 

 polarization. 



Now, if the sheet of mica changes rectilinear 

 into elliptic polarization, such a sheet must be 

 doubly refractive for the radiations thus trans- 

 formed. But if double refraction exists, a 

 fortiori simple refraction must exist ; and I was 

 thus led to examine whether, in spite of the 

 fruitless attempts to discover the refraction of 

 " X " rays, I could not obtain a deviation by a 

 prism. I then arranged the following experi- 

 ment : a focus tube sends through an aluminium 

 screen a pencil of rays, limited by two vertical 



