Note 24 — Chapter III

We can, in fact, discover the ratio of the velocities from the amount of bending the ray experiences and the angle BAC of the prism, called its refracting angle. It can be shown that if we denote this refracting angle by the letter A, and the deflection or total bending of the ray by the letter D, then the ratio of the velocity of the wave in air to its velocity in carbonic acid gas (called the acoustic refractive index), being denoted by the Greek letter μ; we have⁠—

μ = ^{sin ( A + D/₂ )} / _{sin ( ^A/2 )}