142 Proceedings of lloycd Society of Edinburgh . [sess. 
shadow will become less distinct, but if the pencil be held in that 
position, and the source of light removed still further from the 
pencil, the shadow becomes again distinct. So it is with the 
x-rays, and we can formulate a rule that there is a definite relation- 
ship between the position of the source of light, the object to be 
photographed, and the screen or sensitive plate upon which the 
shadow is to be thrown. The further the distance of the object 
to be photographed from the sensitive plate, the greater must be 
the distance between the object and the source of x-rays. In 
attempting, therefore, to photograph the deeper structures of the 
body or the tissues of the neck, it is evident we cannot get the object 
close to the sensitive plate, so the Crooke’s tube must be removed a 
distance from it. ISTo doubt this increases the time of exposure, 
but that will shortly be overcome with further improvements 
in the tubes. Where it is possible it is useful to test the re- 
sult first on a fluorescent screen before photographing, just as a 
photographer would focus on a piece of ground glass before 
exposing. 
The third point, viz. : How are we to photograph particular 
objects and omit others that may be lying in their vicinity? We 
can photograph straight through the human skull and omit one 
side of the head in the picture, and photograph the other although 
both are in the course of the x-rays, and between them and the 
sensitive plate. This is one of the many advantages of the tube 
above described. In this particular apparatus the cathodal torrent 
is led to the aluminium disc at one end of the tube, and is focussed 
on a small square of platinum directly in its course. The x-rays 
spring from this point, and radiate in every direction ; in other 
words, form a cone, the apex being at the platinum plate. It 
naturally follows that if the x-rays be not proceeding on parallel 
lines, but diverging from a point, an object placed very near to 
the source of the x-rays will cast an indistinct image on the sensi- 
tive plate, but the structures which are near the plate will be 
distinctly brought out. Of course, were the bones of the head 
capable of arresting all the x-rays, nothing would be got, but it 
is only a matter of absorption in degree ; so that, when we are 
photographing through the skull, sufficient rays pass through the 
one side to photograph the bone on the other, but the image on 
