96  GOODSPEED—FIELD SURROUNDING CROOKES TUBE. [May 15, 
ON THE PROPERTIES OF THE FIELD SURROUNDING 
A CROOKES TUBE. 
BY ARTHUR W. GOODSPEED. 
(Plates II and III.) 
(Read May 15, 1903.) 
The investigation of the subject implied by the title of this arti- 
cle was suggested by the unexpected presence on some radiographic 
records of peculiar markings outlining certain bodies de/ow the 
plate, in addition to the expected shadows of the objects above the 
plate—z.e., between the sensitive film and the vacuum tube. 
While using an iron tripod stand with a ring-shaped top as a 
support for a radiographic plate, it was noticed that the plate when 
exposed to X-rays seemed to be influenced locally by the presence 
of the iron ring below. For after exposing a circular piece of 
bronze placed on the upper side of the plate which had rested on 
the stand during exposure, the development showed that just above 
the metal of the stand the plate was appreciably less affected 
through the bronze than under that portion of the latter which 
had not been over the metal support. ‘This startling observation 
suggested at once more careful investigation, especially since on 
first thought it would seem that if the metal below the plate could 
have any effect, the result should be quite the contrary to what was 
observed—z.e., if the metal below sends off ‘‘emanations’’ of 
some sort which might produce an effect on the sensitive film, the 
latter would be expected to show an increased density where influ- 
enced both by the rays from above and by the emanations from 
beneath. 
Apparent anomalies have on several occasions been noticed on 
radiographic plates, some similar to that just mentioned, but these 
have never been definite enough to invite special investigation. 
A large number of experiments were made at once in rapid 
succession with strips and plates of various substances both below 
and above. the sensitive film, with results ‘always the same in char- 
acter though differing in intensity of effect in different experi- 
ments and with different materials. As examples of the character 
of some of the tests, sheets of paraffin, mica, and of aluminum were 
successively placed between the under metals and the film, with the 
result that the effect in every case was similar, only a little less intense 
