360 Prof. Owens on Thorium Radiation. 



placing this pair with their principal axes at 45° to the 

 polarizer and rotating the analyser, the field appeared white 

 and of uniform intensity, the variation being very much less 

 than with a true 1/4X, plate of mica or of selenite, but not so 

 perfect as was obtained with the Fresnel rhomb. Fresnel s 

 formula gives for crown-glass a difference of phase of 1/10A, 

 between the extreme red and violet rays when the relative 

 retardation is one X. 



When these combinations were examined in divergent light 

 with the polarizer and analyser either crossed or parallel, the 

 isochromatic lines in the centre of the field were nearly 

 colourless, and the neutral hyperbolic line became pink when 

 the line of the optic axes was not parallel or perpendicular to 

 the principal plane of the polarizer ; both sets of fringes, how- 

 ever, had the same general form as when mica was used 

 alone. 



Similar results were obtained with mica and quartz, this 

 latter combination not giving quite as good compensation as 

 the former. 



Achromatic wedges might also be obtained by a similar 

 combination, the proper ratio of the orders being maintained 

 over the entire length of the wave. Combinations of such 

 systems might also be used as an achromatic compensator in 

 a similar way to that of Babinet's compensator, the distance 

 apart of the bands within the field being given in the equations 

 already discussed. 



With systems of more than two crystals better achromatism 

 would be possible, but the mechanical difficulties would be 

 serious. Over parts of the spectrum very close achromatism 

 may be obtained with the above crystals. 



Further observations on other crystals may reveal better 

 combinations than already examined. A careful study of the 

 distribution of the bands of different crystals in a normal 

 spectrum would furnish more definite data than the observa- 

 tions here given with a prismatic spectrum. 



Physical Laboratory, University of Nebraska, 

 Lincoln, July 22, 1899. 



XXXIX. Thorium Radiation. By B. B. Owens, E.E., 

 Tyndall Fellow, Columbia University, Neio York ; Mac- 

 donald Professor of Electrical Engineering, McGill Uni- 

 versity, Montreal*. 

 THE nature of the radiations emitted by uranium and its 

 compounds was studied in 1896-97 by its discoverer 

 * Communicated by Prof. J. J. Thomson, F.R.S. 



