These values may be collected in the relation
To the second approximation we must then put
where the
are to be regarded as small of the first order.
Both terms of our equation of motion are then small of the first order. If we neglect terms which, relatively to these, are small of the first order, we have to put
We shall now introduce an approximation of a second kind. Let the velocity of the material particles be very small compared to that of light. Then