1903.] GOODSPEED—FIELD SURROUNDING CROOKES TUBE. 101 
The whole was in a perfectly dark room optically, and the eye was 
placed above the tube looking down. After the eyes had acquired 
a maximum sensitiveness by the total exclusion of light for ten to 
fifteen minutes, the Crookes bulb was set in operation and the space 
within the brass tube critically examined from above. The screen 
was unmistakably luminous to the eye and the windows were clearly 
located. Now the polished zinc was moved about in front of one of 
the openings, in the hope of detecting a variation of luminosity on 
the screen opposite this window. The result was at first disappoint- 
ing ; the position of maximum effect was certainly not that of 45°, as 
employed in the photographic experiments. In fact, very incon- 
sistent positions seemed to give the greater illumination through 
the window under attention. Finally it became quite obvious that 
the zinc had little to do with what was visible. In fact, on laying 
aside the metal I was able to light up brighter than ever the inside 
of the brass box by holding my hand in a suitable position in front 
of the window. 
This experiment made certain by ocular demonstration that the 
human hand has by being placed in the path of the X-rays absorbed 
some sort of energy, by means of which it has acquired the prop- 
erty of emanating something capable of exciting fluorescence 
upon the screen. It remains now to demonstrate what effect 
these emanations will have upon a photographic plate as compared 
with those from the zinc, and Fig. to shows the result of a three- 
minute exposure with my hand only, placed opposite one of the 
windows, the tube resting upon a photographic plate in its usual 
protecting envelopes. A similar experiment was next tried (see 
Fig. 11) by holding a hand in front of each window, one of the 
latter being closed by a thin sheet of plate glass.. It is obvious 
from the results obtained that the physiological rays emitted by the 
hands affect the plate through its protecting covers, but are unable 
easily to penetrate glass. 
It is only a step now to produce a ‘‘ physio-radiogram,’’ and Fig. 
12 is a reproduction of a record made by the secondary activity 
emanating from my own hand stimulated by a stream of Roentgen 
rays with an exposure of three minutes. The shadows are those of 
acent, a gold finger-ring and a piece of aluminum about half a 
millimetre thick, and it is apparent that aluminum is somewhat 
translucent to these rays. 
Although Guilloz‘* had made just such shadow radiographs with 
