C>U Prof. J. Joly and Mr. A. L. Fletcher : 



pupil by intermediate shading less intense than that which 

 makes the ring visible. This ring is very probably repre- 

 sentative of the effects of RaA. The following are some 

 measurements : — 



r'. 



r. 



RhA. 



0-0228 



R'. 



0-0295 



R. 



0-0333 



00103 



0169 





0-0187 



0-0227 





00327 





0186 



0-0229 



0284 



00339 





0-0192 



00233 



00280 



00338 





00201 



0'0219 



00283 



00323 



In the case of the first halo the inner radius, r\ of a dark 

 band edging the pupil is recorded. 



It will be seen that there is a very close agreement among 

 the measured radii of the ring ascribed to RaA. The radii 

 are measured to the outside of the ring. Allowing for some 

 effects due to the central nucleus the coincidence between 

 the measured and calculated ranges (the calculated maximum 

 is 0*0201 and limit 0*0229) is sufficient to leave hardly any 

 doubt as to the influence of RaA in forming this ring. In 

 PL IX. fig. 2 we reproduce photographically a field of halos 

 in various stages of development, one showing the pupil 

 well developed and three others in various stages of incom- 

 pleteness. In the more rudimentary ones the appearance of 

 the pupil will serve to convey the form of the simple halos 

 which precede the advent of the corona. In PL IX. fig. 3, a 

 very much enlarged halo is shown bearing the ring due to 

 the rays of RaA. This halo is 0'033 mm. in diameter, and 

 as it scales here about 3 cm. the linear magnification is 

 about 450 times. 



It is difficult to offer any explanation of the relatively 

 backward effects of RaA. If there was "initial recombi- 

 nation " progressing and such recombination was stimulated 

 by the shock attending the passage of the fast rays of RaC, 

 such an effect might be more marked here then elsewhere, 

 and so the absence of the effects of RaA be explained upon 

 their elimination. It must be borne in mind that the changes 

 are progressing in a crystalline medium, and there may be 

 forces tending to restore the original molecular grouping 

 which may be stimulated into action where they are not 



