66 
The Queensland Naturalist 
April, 1930 
as it is noted on the fluorescence screen and also the radio- 
graph. The Kay absorption is greater at the denser 
elements, and a smaller amount of penetrating ray passes 
through to expose the film. Glass and aluminium have 
practically the same density. Glass, although very trans- 
parent to light, is just as dense to X-Ray as aluminium. 
When a beam of Roentgen Rays falls on an object, 
part of it is absorbed, part of it is transmitted, and part 
of it is scattered. The percentage absorbed depends on 
the wave length or quality of radiation, the thickness of 
the media, its density and atomic weight. Radiographs 
are in reality only shadow pictures made possible by the 
different, absorbing effect of the media. 
We have seen that part of the ray is transmitted. 
This transmitted ray becomes further absorbed by the 
photo film and produces a chemical change. Scattered 
rays or secondary rays are produced in all absorbing 
media by internal refraction. As they are radiated in 
all directions it can readily be seen that a general fog 
on a radiograph is set up and interferes much with fine 
detail. 
Certain chemicals have a property of absorbing X- 
Rays and converting their energy into a longer wave 
length so as to produce ordinary visible light. This pro- 
perty is called fluorescence. Quite a number of substances 
will produce this effect. Barium Platino-cyanide and 
Calcium Tungstate (known as white salt) is generally 
used in the manufacture of X-Ray fluorescence screens. 
The fine powder of the salt is evenly spread on a piece of 
cardboard and protected with a lead glass cover, which 
also- protects the operator from the rays when examining 
an object. Without this protection he would run a risk 
of an X-Ray burn in the face as he is in direct line of the 
emerging ray. 
X-Ray fluorescence is a greenish light and not too 
brilliant, consequently screen examination must be car- 
ried out in absolute darkness. The property of fluores- 
cence plays another important part, namely : — by reduc- 
ing the time of X-Ray exposures when taking radio- 
graphs. One of such prepared screens is placed on each 
side of the X-Ray film and directly in contact with it. 
When an exposure is made, not only the ray itself is act- 
ing on the film, but the fluorescence or the light of the 
screen also acts. By such means the exposure only needs 
to be 1/5 as long as would otherwise be the case Not 
only is there a great saving in apparatus, but less chance 
