MR. W. CROOKES ON MOLECULAR PHYSICS IN HIGH VACUA. 
655 
is supposed to be coming towards the observer. It can only be seen in section as a 
dot at the base of each arrow. 
615. When the indicator tube is in front of the magnet, so that the molecular 
stream goes to the right or to the left, as shown by the black arrows in fig. 17, the 
deflection at the S end is upwards for left-handed rays, and downwards for right- 
handed rays. The section across a b shows the direction taken by the molecules when 
the magnetism is acting. When not under magnetic influence the stream can only be 
represented in this section by dots at the base of each arrow. 
In the central position c d , no movement is noticed, whatever way the ray is viewed. 
At the N end of the magnet the action is exactly opposite to that at the S end. 
Fig. 17. 
Section across a b. Section across c cl. Section across ef. 
Molecules moving right and left. 
616. When the indicator tube is held so that the molecular rays travel along the 
prolongation of the axis of the magnet, no movement to one side or the other is given 
to them, but the whole bundle twist round the axis to a greater or less extent as they 
are near to or far from the magnetic pole. This twist is well seen in some tubes 
where the green phosphorescence of the glass is strongly excited by rays driven from 
a pole some distance off. The rays appear to be drawn in, vortex-fashion, and the 
green lines on the glass strikingly resemble some of the spiral nebulae. All the results 
above described can be obtained when a wire helix carrying a current is used instead 
of an electro-magnet ; they are however much fainter. 
617. A long tube was made similar to the small indicator shown in fig. 14, but 
having a molecular trajectory six inches long. It was only exhausted to the point at 
which the image of the spot was just seen sharply defined on the screen, as at higher 
exhaustions the action of magnetism is less. The phosphorescent screen was divided 
into squares for convenience of noting the deflection of the spot of light. So sensitive 
was this to magnetic influence, that when the tube was placed parallel to the earth’s 
equator, the earth’s magnetism was sufficient to cause the spot to move 5 millims. away 
from the position it occupied w r hen parallel to the dipping needle (in which position the 
earth’s magnetism did not appear to act). When held equatorially and rotated on its 
axis, the spot of light being always driven in one direction independent of the rota¬ 
tion of the tube, appeared to travel round its normal position in a circle 10 millims. 
diameter. 
MDCCCLXXIX. 
4 p 
