Nov.. 1922] SPONSLER — STRUCTURE OF STARCH GRAIN 
477 
unit (i A. (Angstrom) = mm.)- Every metal produces more 
10,000,000 
or less of all wave-lengths. These can form a spectrum comparable to that 
of white light. Further, each metal produces a few wave-lengths which 
are characteristic of itself and which have an intensity far in excess of the 
others; just as in light, each element has its characteristic lines. The 
characteristic wave-lengths produced by a rhodium anticathode, such as 
was used in the experimental work which is to be described below, are 
.617 A. and .533 A. in length. The energy conveyed by these waves is not 
carried in equal amounts by all wave-lengths, usually one or two carrying 
almost all of it. Of those from rhodium the .617 A. wave-lengths carry 
perhaps 70 percent of the total characteristic ray energy, while the .533 A. 
wave-lengths carry perhaps more than half of the remainder. 
A beam of such waves, when reflected from a crystal, will be sorted out 
into a spectrum of several lines which may be caught on a photographic 
plate. The Hne due to the .617 A. wave-lengths will be much darker than 
the others, and is called the a line. The .533 A. line, called the /3 line, 
will be much less intense, and a third line, the 7 line, will be still fainter. 
The first two are all that are of importance here. In fact, the lines obtained 
in the present set of experiments are all a lines, with the exception of a 
few faint jS lines. The ''white light" or general radiation, as it is some- 
times called, merely causes a general darkening of the negative. It follows 
that the beam, for purposes of explanation and for coarse work, may be 
considered as consisting of only the .617 A. wave-length. 
The statement that the waves are "reflected" is not quite true, but the 
end results are comparable to those of light reflection so that the idea is 
conveyed by the word. A light wave, which may be 10,000 times the 
length of an X-ray wave, will be wholly reflected from a layer of atoms 
such as the surface layer of a crystal, while the short X-ray wave will be 
only partly reflected; in fact, only a very small part is reflected from a 
single plane. By far the larger part passes through to the next plane, where 
a minute portion is again taken from it and reflected, and the remainder 
passes on to the next plane beneath, continuing in this way for perhaps a 
million planes each reflecting only a minute portion. 
The so-called reflection is brought about in this way: when a wave 
hits an atom it sets the atom to vibrating, and that atom in turn produces 
a secondary set of waves of the same kind. These secondary waves from 
a plane of atoms will form a wave front which will leave the plane at the 
same angle at which the primary beam strikes it; that is, the angle of 
incidence is equal to the angle of reflection. It is readily demonstrated 
geometrically that these secondary waves, produced by planes of atoms, 
will reinforce each other, resulting in a strong wave front when a certain 
relation exists between the wave-length, the distance between the planes of 
atoms, and the glancing angle of the primary beam. This demonstration 
