Now, without altering the position of the gap, turn it so that it comes parallel to OZ, i.e. normal to the cathode rays. The influence of the "X" rays on the spark is then seen to disappear, and the interposition of a lead or glass plate causes no change in its brightness.
"X" rays have therefore a plane of action, which is the one passing through each "X" ray and the cathode ray which gives rise to it. If the direction given to the spark-gap is intermediate between the two above mentioned, the action is seen to diminish from the horizontal position to the vertical.
The following is another experiment, still more striking: if the spark is made to turn about OX, parallel to plane YOZ, the spark is seen to pass from a maximum brightness when horizontal to a minimum when vertical. These variations of brightness are similar to those observed when a pencil of polarized rays traverses a rotating Nicol's prism, the small spark playing the part of analyzer. The pencil of "X" rays presents the same asymmetry as a pencil of polarized light. According to Newton's definition, it has sides differing from
