182 | ANNUAL REPORT SMITHSONIAN INSTITUTION, 1946 
may be used, provided it is opaque to the rays. On an early morning 
walk near San Francisco several years ago, I came across a plump 
horned toad. Having a little time to spare that day, I brought the 
toad into my fluoroscopic room and placed it on the table. I then cap- 
tured a common house fly and dusted it with barium sulphate. After 
setting the tasty meal beside the toad I turned on the X-rays and 
watched the inevitable and fascinating sequence of events. The ba- 
rium cast just as good a shadow in the toad’s stomach as it does in the 
human, and the progress of the meal could be studied easily. 
Physics and chemistry.—The use of X-rays in the sciences of physics 
and chemistry and all their innumerable ramifications is an ever- 
expanding chapter in X-ray history. The gross analysis of many 
materials may be performed in part by roentgenoscopy or roentgenog- 
raphy, and the detailed analysis by methods such as Roentgen diffrac- 
’ tion, crystallography, or electron microscopy. The diffraction method 
of measuring small objects is well established. The principle depends 
on the fact that a beam of light or X-rays which has traversed a col- 
lection of small objects is seen surrounded by a series of rings or dif- 
fraction spectra, from the diameter or pattern of which the size of 
the object can be calculated. A familiar example is the ring visible 
around lights on a misty night, the diameter of which is determined 
by the mean diameter of the mist particles. The interpretation of 
X-ray diffraction patterns in terms of the ultimate structure of crys- 
tals and solids has its foundations in crystalography. By such meth- 
ods it is found that rubbers, plastics, and fibers, although superficially 
different, are intrinsically similar materials. X-ray studies also have 
assisted greatly in analyzing the structure of the higher polymers. 
The essential features of such diffraction apparatus are: 
1. A source of X-rays. 
2. A device to limit the rays to a beam of minimum divergence. 
3. A holder to support the test specimen in the beam. 
4, A means for recording the X-rays diffracted from the sample of critical angles 
(determined by the crystal structure of the samples). 
A recent issue of Electrical Engineering included a description of 
a method of X-ray analysis of unknown chemical substances by em- 
ploying a new photoelectric roentgen intensimeter (1). The meter is 
said to be so delicate that if the X-rays are passed through a pile of 
100 sheets of paper, the difference in absorption caused by adding or 
subtracting a single sheet can be recorded. 
As a direct result of investigations in physics and chemistry there 
are numerous industrial and commercial X-ray developments which 
include the following: 
1. The examination of various complicated appliances, such as radio tubes, 
without the necessity of breaking open the tubes. 
