DISCOVERY OF RADIO-ACTIVE ELEMENTS. 



91 



Fig. 23. Photograph by ordinary daylight of a section of pitchblende, by 

 Sir William Crookes. 



Fig. 24. Photographic imprint of the same section of pitchblende as 

 it imprinted itself in the dark when laid on a plate enveloped in black 

 paper. The white portions of the print are due to the rays. 



