180 | ANNUAL REPORT SMITHSONIAN INSTITUTION, 1946 
or aluminum. A spectrometer can be employed, but is too time- 
consuming and troublesome for routine use. The quality is expressed 
as the thickness of an absorber which reduces the intensity of a given 
beam to one-half its initial value, and the resulting figure is known as 
the half-value layer for that beam. Tor low-voltage rays (up to 20 | 
kilovolt) cellophane may be used as the absorber ; between 20 and 120 
kilovolt, aluminum; between 120 and 400 kilovolt, copper; and for more 
than 400 kilovolt, lead or tin. A refinement of the method is to record 
a second half-value layer, the ratio of the second to the first being used 
as an index of homogeneity. It is to be noted that the expression 
half-value layer is only part of the description of the quality of a given 
beam; for scientific purposes one also must specify the nature of the 
target material, the tube wall, and the generator wave form. 
The quantity of X-rays in a given beam at a given point may be 
measured by recording the ionization in a fixed volume of air or 
gasat that point. The “roentgen” is the unit of X-ray quantity and is 
defined as “that quantity of X or gamma radiation such that the 
associated corpuscular emission per 0.001293 gram of air produces, 
in air, ions carrying one electrostatic unit of electricity of either sign.” 
The mass of 1 cubic centimeter of air at 0° centigrade and 760 millime- 
ters of mercury is 0.001293 gram; in biclogical work, 1 roentgen equals 
83 ergs per gram of tissue. The rays usually are measured by an 
electroscope attached to a small thimble chamber composed of a special 
light plastic material. It is of interest to note that the almost uni- 
versally used instrument at the present time is a “condenser Roentgen 
meter” designed and manufactured in Cleveland, Ohio. 
X-RAYS IN SCIENCE 
The importance of X-rays to science in general is so great that a 
volume would be necessary to describe it. However, a few of the more 
interesting applications of X-rays will be mentioned in this article 
and an attempt made to give a clue to their true value. For the sake 
of brevity the sciences will be grouped into a few general categories. 
Anatomy and physiology.—The importance of X-rays to anatomy, 
comparative anatomy, paleontology, and associated sciences is now 
well appreciated. However, it is not realized generally that the modern 
method of teaching anatomy to medical and other students involves the 
use of roentgenological demonstration of the skeleton and its various 
associated soft parts, both in the cadaver and in the living subject. In 
this manner the appearance and behavior of bony and other structures 
in the living is demonstrated in a way never before possible. The 
study of the skeletal development in vertebrate embryos is facilitated 
enormously by Roentgen methods. The anatomy of small and large 
animals is revealed in zoological work. The status and often the 
