IOWA ACADEMY OF SCIENCES. 
131 
No. 1 exposed for 5 min. — A was kept as check. 
No. 2 exposed for 15 min.— B was kept as check. 
No. 3 exposed for 30 min.— C was kept as check. 
At the end of forty-eight hours number 1 seemed slightly 
more vigorous than A. At the end of the same time num- 
ber 2 had a much more vigorous growth than B, but at the 
end of three days was much less vigorous than the check. 
Number 3 at the end of forty-eight hours had no growth 
at all while C corresponded with the other checks A and B. 
It seems from this experiment that a short exposure to 
sunlight does not affect the growth. That a fifteen minute 
exposure of a culture stimulates the growth temporarily. 
Gotschlich has remarked that brief exposure of a culture 
to injurious influences may react beneficially to a culture 
as a whole by cutting out the weaker organisms and leav- 
ing only the virulent ones; that is, that there may be a 
selective death-rate. Basing our explanation on the 
above principle, the first effect of the sunlight was the 
destruction of a number of the less resistant violet produc- 
ing cells. This accounts for the fact that the five minute 
exposure was slightly more vigorous than the check, A. 
While in number 2 we must assume that after the weak 
organisms had been killed by the sunlight the actinic effect 
of a fifteen minute exposure was stimulating and promoted 
cell division. To get the benefit from such influence the 
organisms must be returned to normal conditions. The 
accelerating effect of such exposure does not seem to be an 
enduring one, because on examination at the end of three 
days number 2 seemed to be much retarded and develop- 
ment was permanently hindered. A longer exposure to 
sunlight killed the cells outright. It is in this way that 
the sun’s rays prove to be such a good disinfectant for 
bacterial life. 
With these facts gathered no further attempt was made 
to study the biology of the organism. 
