Note 150 — Chapter XI · GRAVITATIONAL ASTRONOMY IN THE 18TH CENTURY.
Reading edition
∑e2m√a = c, ∑tan2im√a = c′, where m is the mass of any planet, a, e, i are the semi-major axis, eccentricity, and inclination of the orbit. The equation is true as far as squares of small quantities, and therefore it is indifferent whether or not tan i is replaced as in the text by i.