98 GOODSPEED—FIELD SURROUNDING CROOKES TUBE. [May 15, 
over-exposures, to produce a fairly definite result, as shown in Fig. 
2. It is to be noted, however, that the parts of the film just next 
the pieces are Zess dense than the rest—7z.e., the shadows are light 
on a darker ground. 
Fig. 3 shows the result when to the glass of ordinary thickness 
was added thick blocks of zinc. The characteristics of these two 
plates are identical, except that the latter is more dense and shows 
greater contrast. 
In Fig 4 we have reproduced a plate made just as was that of 
Fig. 3, except that the exposure was thirty minutes instead of 
fifteen. The appearance is certainly remarkable, for though the 
direct X-rays had been entirely cut off by the zinc blocks the 
shadows are exactly as would have been produced by reversing the 
process and exposing directly to the Roentgen rays, though for a 
-much briefer time. 
The influence on the side of the plate remote from the tube 
seems to have more than neutralized the Roentgen reflection effect, 
-and the more so the greater the exposure. 
From these three experiments it seems probable that with a much 
less powerful X-ray generator, a suitable exposure would show the 
result noted by Roentgen. As is seen below, this was probably not 
due to reflection. 
The next plate (Fig. 5) shows the impression of the ring stand, 
above spoken of, when the former was covered with a sheet of 
copper about a millimetre thick and exposed twenty-four minutes. 
In Fig. 6 the stand is replaced by a brass ring supported by a block 
of wood. The penumbral effect around the inside edge is to be 
noted. 
As an interesting modification of this experiment I asked one of 
my associates, Dr. Richards, to hold his hand beneath the plate, 
protected above with thick metal blocks, and exposed the combina- 
tion five minutes. The result (see Fig. 7), though lacking in defini- 
tion, is quite like the first radiographs made without a focus tube. 
We seem now to be led up to a satisfactory explanation of what 
we have observed so far—z.e., of this apparent ‘‘ nether effect ’’ 
reaching completely around into the shadow of an obstruction 
totally impervious to X-rays proper, and acting in a direction just 
opposite to that of the rays from the tube. 
It must be noted here that so-called ‘‘ X-ray diffusion’’ has 
long been recognized, and an early experiment” with a fluoroscope 
