1903.] GOODSPEED—FIELD SURROUNDING CROOKES TUBE. 99 
behind a thick steel plate was explained variously, the most thought- 
ful suggestion perhaps being by Prof. Elihu Thomson, who pro- 
posed that the screen was rendered luminous by the action of 
X-rays reflected from various objects in the room. 
From all the experiments yet made in the effort to account for what 
at first seemed to be, to say the least, a paradox of science, it looks 
as if the whole space field in the neighborhood of a focus Crookes 
tube in operation is full of some sort of subtle energy, radiant pos- 
sibly, but incapable of affecting the human eye, though leaving its 
mark on a photographic plate. 
It was found by Sagnac* that many bodies in the path of X-rays 
acquire the property of emitting emanations of some sort capable 
of causing fluorescence and photographic action. 
Undoubtedly then the effects above described are due to the 
secondary radio-activity of the air, the table and other bodies favor- 
ably located to be impinged by the X-rays directly. 
In order to gain more knowledge of the possible limitations of 
this ‘‘radious”’ field, metal tubes of various sizes and lengths were 
placed on the plate, and now for convenience the entire local order 
of the articles used was completely réversed. Furthermore, the 
Crookes tube was enclosed in a black wooden light-tight box, and all 
experiments were made in the night, so that every trace of optical 
light might be more easily excluded. We have now the tube in its 
box, so placed that the axis of the ray-cone is directed vertically 
upwards. On the upper surface of the box and over the focus of 
the tube is a bundle of lead plates about one centimetre thick. On 
this, film upward, is the photographic plate. 
This arrangement differs from that of Sagnac in that the fvores- 
cent light from his tube was not filtered out, as it is here, by the 
box enclosing the X-ray bulb. 
In Fig. 8 we have the result of a twenty-three minute exposure, 
when a brass tube five centimetres high, eight centimetres in diam- 
eter and three millimetres thick is placed on the plate, the tube 
being open at the top. This experiment was repeated with a thick 
block of pine wood, placed on top of the brass tube, with no 
change in result. The condition of the enclosed space is inde- 
pendent of the presence of the wood, and the enclosed area of the 
film is much affected. 
When however the tube is covered with a thick block of zinc, 
this seems to protect the sensitive film completely from outside 
