April, 1930 
The Queensland Naturalist 
65 
100.000. For X-Ray Therapy, and for the examinations 
of metals in industry, the voltage frequency used is up to 
250.000. The current is measured in milli-amperes, and 
proportional as to time, that is to say, one exposure of 
ten milli-amperes for 10 seconds, and another exposure of 
100 milli-amperes for 1 second are of equal value. 
There is no fluctuating gas pressure in the electronic 
type of tube ; it is constant under all 
conditions of voltage and current, and can be 
run for hours at a time. The only limit is the limiting tem- 
perature that the target and the focal spot thereof can 
stand. Tungsten metal is generally employed as target 
material on account of its high melting point. Great 
heat :is developed at the target, and for diagnostic work 
the construction of the Cathode is such as to focus the 
electrons on to a small spot on the target; this produces 
better definition in the radiograph. Care has to be taken 
so as not to overload the focal spot. 
This is a brief outline of the production of the Roent- 
gen Ray. The presence cf the rays must be determined 
by the effect they produce when acting on material bodies. 
We shall now consider the properties of the rays: 
Their penetration of substances opaque to light ; their 
effect on the photographic film ; their ability to cause 
fluorescence ; and their ability to affect the living cell. 
Roentgen Rays are not visible. The wave length is 
very much shorter than the range of the visible spectrum. 
The wave length is about 1/10,000 part of that of light, 
but the velocity is the same as light. 
X-Rays cannot be reflected or focussed; they travel 
into and through, and out of material bodies, be such 
bodies opaque to- light or not. This is the penetration 
effect, and is analagous to light passing through turbid 
water. The rays gradually get absorbed in the material 
body with increasing depth. 
Penetration and absorption in material bodies is in 
the same order as the density number of elements. Alu- 
minium has a density of 2.7 ; copper, 8.9; lead, 11.3; glass, 
2.5; pinewood, .5; hardwood, 1.2; water (which is a com- 
pound), 1; bone, up to 2; and body tissues, about 1. It 
is the absorption effect, differing with dissimilar elements, 
that is of great value in diagnostic work. 
The density of bone, which is about twice as high as 
the body tissues, is the effect of varying density of shadows, 
