142 
MR. W. CROOKES OK THE ILLUMINATION OF LINES OF 
to be well supported by experiment, and justifies the theory that the induction spark 
actually illuminates the lines of molecular pressure caused by the electrical excitement 
of the negative pole. The thickness of the dark space is the measure of the length 
of the path between successive collisions of the molecules. The extra velocity with 
which the molecules rebound from the excited negative pole keeps back the more 
slowly-moving molecules which are advancing towards the pole. The conflict occurs 
at the boundary of the dark space, where the luminous margin bears witness to the 
energy of the discharge. When the exhaustion is sufficiently high for the length of 
path between successive collisions to be greater than the distance between the fly and 
the glass, the swiftly-moving rebounding molecules spend their energy, in part or in 
whole, on the sides of the vessel, and a production of light accompanies this sudden 
arrest of velocity. 
CONVERGENCE OF MOLECULAR RAYS TO A FOCUS. 
508. In the sixth part of my researches on “ Repulsion resulting from Radiation,” 
which was presented to the Royal Society in June last, I gave at par. 415 a theoretical 
explanation of the movement of radiometer flies with curved surfaces, on the sup¬ 
position that the lines of molecular pressure acted in a direction normal to the surface. 
In the figures illustrating this theory (fig. 14, A and B, pars. 415 and 416) I drew 
the lines of pressure radiating outwards from the convex surface, and predicted their 
converging to a focus and thence diverging at the concave surface. These figures 
were drawn long before the experiments just described were commenced, and it is a 
corroboration almost amounting to absolute proof that the theory was correct, to see 
how. well those old drawings represent the actual lines of pressure as illuminated by 
the induction spark. 
508. The convergence of the lines of force to a focus, demanded closer investigation 
than was possible when the cup was in rapid rotation. A bulb was therefore made 
similar in general character to the electrical radiometers just described, but having the 
cup-shaped negative pole fixed instead of movable. Exhaustion was proceeded with, 
and the successive appearances noted were like those shown in fig. 11. On further 
continuing the exhaustion, the dark space spreads out still further, and the focus of 
light converging from the concave side falls on the luminous boundary. Inside this 
focus the rays can be seen converging, and are of a violet colour. Outside the line 
bounding the dark space the rays diverge from the focus, and are of a lighter colour 
than those inside ; the whole appearance being strikingly similar to the rays of the 
sun reflected from a concave mirror through a foggy atmosphere. 
GREEN PHOSPHORESCENT LIGHT OF MOLECULAR IMPACT. 
510. When the exhaustion approaches 30 M,* a new phenomenon makes its ap¬ 
pearance. The dark space has spread out so much that it nearly fills the bulb ; the 
* M signifies the millionth of an atmosphere. 
