Note 23 — Chapter III
Reading edition
This follows from the ordinary formula for the focal length f of a biconvex lens, each surface having a radius of curvature equal to r. For then it can be shown that
f = r/2 · 1/μ – 1
where μ is the index of refracture of the lens material. As shown later on, the acoustic index of refraction of carbonic acid, when that of air is taken as unity, is 1·273. Hence, μ – 1 = 0·273, and 1/μ – 1 = 3₂/³. Hence, f = 2r ₁₁/¹², or f is slightly less than twice the radius of curvature of the spherical segment forming the sound-lens.