Nov., 1922] SPONSLER — STRUCTURE OF STARCH GRAIN 
mm. thick, with two windows cut through it, one above the other, into 
which the powder is pressed just firmly enough to support itself so that no 
covers are needed. When in position (at P) each window opens into one 
compartment of the holder and so provides for making photographs of two 
powders at the same time. 
When the apparatus is ready for exposure, the two adjustable slits are 
opened only enough to allow a narrow, ribbon-like beam of X-rays to pass 
through. This beam is about i to 2 mm. wide and 20 mm. high. When 
it strikes the powder container, the upper half passes through the powder 
in the upper window and the lower half through the lower window. The 
part of the beam which goes straight through strikes the lead shield, L, 
and is absorbed; while the part which is reflected by the tiny crystals 
meets but little resistance in the black paper and so reaches the film. The 
rays which react on the film fall into two categories: the first contains the 
reflected characteristic rays which form the lines; the second contains those 
rays which produce the general darkening of the film between the lines. 
The rays of the second category are in part reflected "white light," or 
general radiation, and in part from any amorphous material present; both 
will cause a diffuse scattering like that produced on light by smoke or fog. 
Rays of the first category only can produce the lines and so indicate the 
presence of crystalline matter. The lines will be approximately the width 
of the lead shts, 5, S. 
In this type of film-holder the primary beam cuts the film into halves 
which, so far as the lines are concerned, are mirror images. There are at 
least two advantages arising from this: (i) since the lines must appear in 
each half, the possibihty is eliminated of mistaking a developer flow line; 
and (2) the error in measuring the arc of the glancing angle is reduced, 
since the distance between pairs of similar lines is four times the length of 
the arc. 
The electrical part of the apparatus was calibrated so that the voltage 
could be kept at approximately 40,000 volts and the current at from 3 to 
5 milliamperes. 
The powder for examination was ground fine enough to pass through a 
sieve having 200 meshes per inch. It was found that if the particles were 
too large to go through such a sieve, the lines produced on the photographic 
film were composed of large spots; but if the particles are small enough to 
pass through, the spots are invisible and the lines appear solid. 
Each sample was pressed into the lead slide with just enough pressure 
to make a tablet-like mass which would be solid enough to support its own 
weight. The thickness of this tablet would vary with the substance used, 
for there is a fairly definite relation between the atomic weights of the 
elements in a substance and the absorption of X-rays by it. For example, 
the elements in salt are almost twice as heavy as those in starch, so the 
tablet of starch which would allow the maximum amount of reflected 
