In a paper "Über das Doppler'sche Princip", published in 1887 (Gött. Nachrichten, p. 41) and which to my regret has escaped my notice all these years, Voigt has applied to equations of the form (7) (§ 3 of this book) [namely Δ Δ Ψ Ψ − − 1 c 2 ∂ ∂ 2 Ψ Ψ ∂ ∂ t 2 = 0 ] a transformation equivalent to the formulae (287) and (288) [namely x ′ ′ = γ γ l ( x − − v t ) , y ′ ′ = l y , z ′ ′ = l z , t ′ ′ = γ γ l ( t − − v c 2 x ) ]. The idea of the transformations used above (and in § 44) might therefore have been borrowed from Voigt and the proof that it does not alter the form of the equations for the free ether is contained in his paper.