PQ

This spot of light will have the same outlines as the luminous body PQ and being formed as just described will be inverted.

Now imagine a small cylinder of section ω described round PQ as an axis.

The right-hand side of this equation is the sum of terms of the form pQ—a price multiplied by a quantity bought.

The right-hand side of this equation is the sum of terms of the form pQ—a price multiplied by the quantity bought.

Consider any path joining PQ, and moving with the fluid, so that the line contains always the same fluid particles.