In this statement of the law of motion I have neglected the existence of other forces.

But that would lead me too far afield.
The electromagnetic theory has to be modified to allow for the presence of a gravitational field.

Thus Einstein's investigations lead to the first discovery of any relation between gravity and other physical phenomena.

In the form in which I have put this modification, we deduce Einstein's fundamental principle, as to the motion of light along its rays, as a first approximation which is absolutely true for infinitely short waves.

Einstein's principle, thus partially verified, stated in my language is that a ray of light always follows a path such that the integral impetus along it is zero.