Now if this plane disturbance is in nature accompanied by its real image, why should not a twist such as takes place in the electric current also be accompanied by its image twist when it impinges on a medium which it cannot twist as a whole—that is, when it comes to an insulator in its path?
The reason, obviously, is that we cannot conceive such an image produced mechanically. And the reason of this can be exhibited thus.
When we had a plane disturbance like L M N O we only used up two dimensions of space, and we have a third coming up from the plane; and this direction enables us to imagine a turning which will alter A B into its image.
But when we have a twist proceeding along an axis, as in the case of electricity, we have no direction left over in space whereby we may conceive the twist turned round.
Now when the displacement itself involves all these directions how will our rule hold?
How shall we get the image displacement? We can find what it is by using a looking-glass; but the same rule which served in previous cases ought to work here also.
We want a direction which is neither up and down, right and left, towards and away.
Now let us adopt a mathematical device, and suppose there is such a direction, and let us call it the X direction, the unknown direction.
Then if we turn the twist round, using this X direction, we shall get the image if our rule is correct. And as a matter of fact, by twisting a figure round in this way, using a direction different from any of the three mentioned above, we do get its image.
Hence the rule we have formed works consistently.