In **octonions** the analogue of Hamilton's vector is localized to the extent of being confined to an indefinitely long axis parallel to itself, and is called a rotor; if p is a rotor then wp is parallel and equal to p, and, like Hamilton's vector, wp is not localized; wp is therefore called a vector, though it differs from Hamilton's vector in that the product of any two such vectors wp and coo- is zero because w 2 =o.

(Note that the z here occurring is only required to ensure harmony with tri-quaternions of which our present biquaternions, as also **octonions**, are particular cases.) The point whose position vector is Vrq i is on the axis and may be called the centre of the bi-quaternion; it is the centre of a sphere of radius Srq i with reference to which the point and plane are in the proper quaternion sense polar reciprocals, that is, the position vector of the point relative to the centre is Srg i.

Thus, in place of his general tri-quaternion we might deal with products of an odd number of point-plane-scalars (of form, uq+wr) which are themselves point-plane-scalars; and products of an even number which are **octonions**; the quotient of two point-plane-scalars would be an octonion, of two **octonions** an octonion, of an octonion by a point-plane-scalar or the inverse a point-plane-scalar.

McAulay, **Octonions**, a development of Clifford's Bi-quaternions (Cambridge, 1898); G.