482 
AMERICAN JOURNAL OF BOTANY 
[Vol. 9. 
the film at various definite angles while the larger part passes through the 
powder and is absorbed by the lead shield, L, just before it would otherwise 
reach the film. Concerning the details of construction only enough will be 
described to supplement figure 3 in making the experimental work clear. 
Several detailed descriptions, which vary only slightly, are in the literature 
(42,43,44,46). 
A Coolidge tube with a broad-focus rhodium anticathode was used as 
the source of the X-rays. The bulb of the tube is 18 cm. in diameter. 
The box which incloses the tube is approximately 30 cm. on each side and 
is covered with sheet lead 2 mm. thick. The tube is supported by sheets 
of asbestos, T, T. The lead slides, S, S, are about 2 cm. high and adjustable 
to form a slit of any width. 
The film-holder, H, is a hollow, semicylindrical box 5 cm. deep with a 
radius of 14 cm. The block, M, which fits around it is of solid wood and 
also 5 cm. deep. The film lies between these two parts. The film-holder, 
H, is made from three semicircular pieces of sheet lead. One forms the 
top, one the bottom, and the third divides the box into halves, an upper 
and a lower. They are held in place on the straight side by two pieces of 
wood and on the curved side by the lead shield, L, which is about 5 mm. 
thick. Without this absorbing shield the film would become heavily 
fogged at this place. The whole of the curved side of the holder is covered 
with black paper. 
The container for the powder is merely a piece of sheet lead, about 3 
