Pleocliroic Halos. 



643 



Two shells of maximum ionization succeed closely tho^e 

 due to the radium and uranium rays; that due to RaF at 

 0*0150 mm., that due to emanation at 0*0170 and extending 

 to 0*0200 mm., about. Just beyond this limit should lie the 

 maximum effect of RaA. The following are typical measure- 

 ments of halos whose limitations may be ascribed to RaF or 

 to emanation :— 0-0158, 0*0167, 0*0165, 0*0170, 0*0181, 

 0-0182, 0*0184, 0*0188. We might quote very large 

 numbers of such measurements. Such halos are very often, 

 if not most generally, still without a corona. These simple 

 halos may even be quite blackened up and yet show no signs 

 of the effects of RaA or RaC. 



The dimensions of the pupil at which RaC first makes its 

 appearance are, typically, somewhat greater than the above, 

 but not such as can well be ascribed to RaA. This is one of 

 the most remarkable points about the development of these 

 uranium-radium halos. RaC seems to create an effect in 

 advance of RaA, although the density of the former is less 

 owing to the greater radius, and although RaA is somewhat 

 strengthened by a coincident actinium ray. The radius 

 of the faint, eai'ly, corona is never less than can be ascribed to 

 RaC, as will be seen from the list of dimensions which follows. 

 R is measured to external radius of corona, R x to its inner 

 radius. 



r. 



R\ 



R. 



I 

 Nuclear 

 radius. 



00165 





00297 





0-0169 





00301 > 





0-0170 





0-0300 





00171 





00H18 





00174 





00328 



00020 



00176 





00314 



0029 



00174 





00319 



00019 



0-0177 





00306 





00179 





00323 





00180 





0-0328 





0O183 



00290 



00323 



00025 



00184 



0-02B7 



00323 



00029 



00185 



00286 



00323 





00185 



0284 



00317 





Such halos as the above are not, of course, fully developed. 

 The corona changes indeed but little in its subsequent dimen- 

 sions, but the pupil expands considerably. The next feature 

 observed on close examination is sometimes the appearance 

 of a very fine and faint ring lying outside and close to the 

 pupil. It is. indeed, probablv in all cases connected to the 



2'T 2 



