48o 
AMERICAN JOURNAL OF BOTANY 
[Vol. 9, 
The accent marks indicate different values of the angles which produce 
first-order reflections. 
Fig. 2. X-ray tube with crystal in position. 
A diagram of an X-ray tube with a crystal in position (fig. 2) may 
make this somewhat clearer. A represents the anticathode which is the 
source of the X-rays. 5 is a lead screen which absorbs all the rays except 
a beam which a slit allows to pass through. This beam is incident on the 
crystal C from which the reinforced waves are reflected to the photographic 
plate P, where they produce an effect resulting in a black line when the 
plate is developed. In the position shown, the rays are incident on the 100 
planes. If, without disturbing the tube and the plate, the crystal was 
moved so that the glancing angle 6 would be increased the proper amount, 
then a line of the second order would be produced from the same planes. 
If still further changes were made, without disturbing the tube and the 
photographic plate, the two orders of reflection from each of the two other 
planes, no and in, would be thrown on the plate, one at a time. Since 
each reflection has a definite glancing angle which is different from the 
others, there would be produced on the plate six lines. 
It seems perfectly plausible that six small crystals could be put in place 
of the one large crystal, C, and be so arranged that one would produce the 
first-order reflection from the 100 planes; another one so that it would 
produce the second-order reflection from the same planes; a third so that 
it would produce the no first order, and a fourth the second order; the 
remaining two could produce the first and second orders from the in 
planes. With this arrangement all six lines could be photographed at the 
same time. 
