1879*] Molecular Physics in High Vacua. 41 7 
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 
Fig. 3. 
to the rays of the sun reflected from a concave mirror 
through a foggy atmosphere. This proves a somewhat im- 
portant point ; it shows that the molecules thrown off the 
excited negative pole leave it in a direction almost normal 
to the surface. 
I can illustrate this property of the molecular rays 
by an experiment. This diagram (Fig. 3) is a repre- 
sentation of the tube which is before you. It con- 
tains, as a negative pole, a hemi-cylinder (a) of polished 
aluminium. This is connected with a fine copper wire, 
h , ending at the platinum terminal, c. At the upper 
