1903.] GOODSPEED—-FIELD SURROUNDING CROOKES TUBE. 97 
than when no screen was interposed. The original records of all 
these experiments and the particular conditions in each case have 
been carefully preserved. A single figure will be enough to illus- 
trate this effect. 
Fig. 1 shows the result when two zinc blocks, one of them pol- 
ished, were placed below the photographic plate upon which the 
latter rested. On this were a strip of copper, one of lead, a tri- 
angular and thicker piece of uranium, and a piece of metallic 
indium about one millimetre thick and three centimetres square. 
Fifteen centimetres above this combination the discharge tube was 
operated for twenty-five minutes, the rays being directed down- 
ward. ‘The zinc blocks were below the lateral edges of the plate 
and covered each about a third of its area. There certainly is 
nothing ambiguous about the result, and the degree of polish seems 
to have nothing to do with the effect. The middle third is dis- 
tinctly darker than the rest in those parts just under the metal 
pieces. 
The transverse strip in the middle was lead and is distinctly less 
pervious than the copper on the left. 
In looking up some of the early work of Roentgen, I found that 
one of his experiments was almost identical in character with those 
just described, but less strenuous and designed for quite a different 
purpose.’ He arranged star-shaped pieces of four metals, platinum, 
lead, zinc and aluminum, covered by a light-protected photographic 
plate, film towards the stars and glass towards the tube. On devel- 
opment after exposure to the rays from a focus tube identical in 
principle with that universally used at present, the metal stars 
showed darker than the rest of the ground. The purpose of his 
experiment was to demonstrate a possible reflection from the metal 
stars, and the result obtained was interpreted as conclusive evidence 
at the time that such was the case. 
For obvious reasons it seemed desirable to repeat Roentgen’s 
experiment as nearly as possible as he made it. This was done with 
some difficulty, on- account of the fact that the apparatus in use 
developed rays of such penetrating power that the glass backing of 
the sensitive film offered little obstruction, and even with a very 
short exposure the whole film was so dense as to show nothing of 
the metal pieces. 
Increasing the thickness of the glass made it possible, after several 
trials and by using a contrast-developer especially prepared for 
PROC, AMER. PHILOS. soc. XLII. 172. G. PRINTED MAY 28, 1903. 
