[89] A distinction of this sort does not apply, of course, to the equality of two invariants; for, as we have seen, a change of mesh-system can produce no effect on the value of an invariant, seeing that an invariant has no components.
[90] The tensor
being twice covariant, and the vector
repeated twice in the formula, being contravariant.
[91] It is customary to represent scalars by ordinary letters, and tensors of the first, second and third orders, and so on, by letters followed by indices equal in number to the order of the tensor. Thus,
is a tensor of the second order,
