48 ©. BULLETIN 737, U. S. DEPARTMENT OF AGRICULTURE. 
side of leaf tobacco, (c) eggs under glass cover. <A check was kept for 
each experiment. It was found that with eggs exposed directly to the 
rays, without covering as under arrangement (a), at a distance of 
about 8 inches from the quartz mercury arc, an exposure even as short 
as 380 seconds was effective in sterilizing eggs under two days old. As 
has been found with Réntgen-ray radiation, the further embryonic 
development has advanced, the more resistant the eggs become, and 
longer exposures are required for their successful sterilization. Eggs 
exposed on the underside of the leaf, arrangement (0), and those 
under a cover of ordinary glass, arrangement (c), hatched normally. 
Ordinary glass has the property of transmitting ultra-violet rays only 
to a slight degree, its transmission being approximately only one one- 
thousandth that of quartz glass. It was found that a screen over the 
eggs, even of the thinnest glass such as is used for cover glasses on 
microscope slides, was sufficient to protect them completely from the 
effect of ultra-violet rays. These exposures served as an additional 
check on the results obtained from direct exposures to the rays. The 
experiments with the eggs on the underside of the leaf showed that. 
the ultra-violet rays would not penetrate even the thinnest leaf of 
cured tobacco. This fact makes it impracticable to apply the method 
in treating infested tobacco. 
Although eggs of the tobacco beetle when exposed directly to the 
ultra-violet rays can be quickly and effectively sterilized, experiments 
made so far do not indicate that other stages of the beetle are de- 
stroyed by the same exposure. Treated larve and pupze completed 
transformations to the adult stage, and treated adults laid fertile 
egos and died off at a normal rate. The effect of more intense radia- 
tion or of prolonged exposure of adults, pups, and larve has not 
been determined. 
In order successfully to treat leaf tobacco containing eggs, it has 
been suggested that in preparing the leaf for use it might be pos- 
sible to devise means for smoothing out the leaves and thus make 
more powerful exposures of ultra-violet rays from all sides while 
the tobacco passes through an exposure chamber containing the 
lamps. As it would be extremely difficult, or perhaps impossible, by 
any mechanical means, to smooth out completely all the creases or 
wrinkles in the leaf tobacco, and as the rays do not penetrate opaque 
substances like tobacco. the method, in the opinion of the writer, 1 
is not likely to prove entirely effective or practicable, as the eggs of 
the tobacco beetle ordinarily are deposited in wrinkles of the leaf. 
The results obtained from several of the experiments with ultra- 
violet rays are briefly summarized in Table VII. 
