352 .bulletin: museum of comparative zoology. 



In Table II, containing exposures at distances up to five centi- 

 meters from the radium, it is shown that at most only small 

 retardations have occurred and that sometimes there was an actual 

 acceleration. The two tables, then, show a proportion between the 

 intensity of exposure and the effect. A treatment with an intensity 

 of twenty-five is evidently about midway between the greater intensi- 

 ties, which retard, and the smaller, which accelerate. 



TABLE III. 



Treatment. Effect. 



Secondary beta rays emerging from a 

 1 mm. -lead screen which received pri- 

 mary rays of an intensity of 



15 9% retardation 



18 39% retardation 



Averages 16^ 24% retardation 



Table III gives the retardations brought about by secondary beta 

 radiations (consisting of especially slow beta electrons). The con- 

 ditions were such as would have produced- exposures to beta radia- 

 tions of fifteen and eighteen strength had not a millimeter of lead 

 been placed between the eggs and the radium. As a result, the beta 

 radiations coming to the eggs were reduced in^ strength, i. e. there 

 were produced secondary beta radiations, consisting of slower elec- 

 trons, whose energy value was small, not only in relation to the 

 unscreened beta radiations, but even in comparison with the smaller 

 amount of radiations let through the lead. There was an average 

 retardation of twenty-four per cent. The exposures of Table II, 

 where the average intensity was greater than in these experiments, 

 gave only slight accelerations and retardations, which balanced each 

 other. The marked intensification of the effect by the use of the lead 

 screen must therefore be due to the small quantity of secondary rays 

 coming from it. The experiments therefore indicate that the second- 

 ary radiations are much more effective in proportion to their energy 

 value than the more penetrating direct radiations. 



It has been the experience of dermatologists that an unscreened 

 X-ray tube brings about, beside the deep seated effect, a superficial 

 burn, evidently from secondary beta radiations produced in the glass 

 by the X-rays. A thin screen of a substance which cannot itself give 

 rise to such secondary radiation prevents the burn by cutting off the 

 secondary radiations arising from the glass. It is clear from these 



