142 



ilfr. WilUa7n Croohes 



[April 4, 



dark space. A stream of water falls from a small jet on to a hori- 

 zontal plate of glass. The water spreads over the plate and forms a 

 thin film. The jet of water in the centre, from the velocity of its 

 fall, drives the film of water before it on all sides, raising it into a 

 ring-shaped heap. As I diminish the force of the jet the ring con- 

 tracts: this is equivalent to the exhaustion getting less. When I 

 increase the force of water the ring exj)ands in size, the effect being 

 analogous to an increase of exhaustion in my tubes. The extra 

 velocity of the falling particles of water drives the in-coming particles 

 of water before them, and raises a ridge round the side which exactly 



Fig. 2. 



represents the luminous halo to the dark space to be seen in this 

 tube. 



If, instead of a flat disk, a metal cup is used for the negative pole, 

 the successive aj)pearances on exhausting the tube are somewhat 

 different. The velvety violet halo forms over each side of the cup. 

 On increasing the exhaustion the dark space widens out, retaining 

 almost exactly the shape of the cup. The bright margin of the dark 

 space becomes concentrated at the concave side of the cup to a 

 luminous focus, and widens out at the convex side. When the dark 

 space is very much larger than the cup, its outline forms an irregular 

 ellipsoid drawn in towards the focal point. Inside the luminous 

 boundary a dark violet light can be seen converging to a focus, and, 

 as the rays diverge on the other side of the focus, spreading beyond 

 the margin of the dark space ; the whole appearance being strikingly 

 similar to the rays of the sun reflected from a concave mirror through 

 a foggy atmosphere. This proves a somewhat imjiortant point ; it 

 shows that the molecules tbro^vn off the excited negative jjole leave it 

 in a direction almost normal to the surface. 



I can illustrate this property of the molecular rays by an experi- 

 ment. This diagram (Fig. 3) is a representation of the tube which is 

 before you. It contains, as a negative pole, a hemi-cylindcr (a) of 



