162 



Dr. Silvanus P. Thompson on 



XIV. Some Experiments with Rontgen's Rays. 

 By Silvanus P. Thompson, D.Sc, F.R.S* 



THE following is a brief narration of points observed by 

 me during the past three months, and which are now 

 brought before the Physical Society. 



1. Many experiments have been made to observe polariza- 

 tion of #-rays, but no trace has been found. Tourmalines of 

 several colours, and thicknesses varying from 0*1 millim. to 

 6 millim., have been used. Andalusite, mica, calc-spar cut 

 in slices parallel to the axis, epidote, and ripidolite have been 

 tried without result. The method pursued has chiefly been 

 to cut the slice of crystal into three parts, laying two of them 

 upon the third, one of the upper parts having its axis parallel 

 to the axis of the under part, while the other had its axis laid 

 at right angles. In this way equal thicknesses of crystal were 

 compared side by side. Thinking that results might be 

 obtained from dichroic crystals containing a metal of consi- 

 derable atomic weight, slices were tried of crystals of sulphate 

 of nickel, sulphate of nickel and potassium, sulphate of cobalt 

 and potassium, sulphate of cobalt and ammonium, fluosilicate 

 of cobalt, and fluosilicate of nickel, but no trace of polarization 

 was seen. 



Another method consisted in comparing the opacity of 

 tourmaline in a direction parallel to the axis with that of an 

 equal thickness in a direction at right angles to the axis. 

 Not the slightest difference was observed either in the photo- 

 graphic shadows or with the use of a luminescent screen of 

 barium platinocyanide. 



2. For several weeks in the months of February and March 

 experiments were made with many different forms of bulb to 

 determine the source of the ^-rays and the form of tube most 

 favourable to their production. In common with the con- 

 clusions of so many other observers, it was found that the 

 effective source was in every case a surface against which the 

 kathodic discharge was directed. A form of tube which gave 



Fig. 1. 



results superior to those of any Crookes tube at the time in 

 the market is shown in fig. 1, in which the kathode consisted 

 * Communicated by the Physical Society : read June 12, 1696, 



