mae 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 
NS es 5 ER oo Se 9% retardation 
[JOG 6 G5 SSP er ae 39% retardation 
A Verares bgt vee eke 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, 1. 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 
