D O C U M E N T 2 9 O P T I C A L E X P E R I M E N T 5 1 qualities that, in principle, permit one to establish whether the emission comes from a resting molecule or a moving one. We now inquire about an experimental criterion for the trait that according to the undulatory theory is granted to light originating from a moving particle. Accord- ingly, the distance between two surfaces of the same phase and therefore the radi- ation’s frequency is a function of position. If we let such “fanned” radiation pass through a dispersive medium, then at frequency ν the standard propagation velocity of the equally phased surfaces is also a function of position. From this then follows that during propagation the surfaces of equal phase must be subject to a rotation, i.e., the light rays wave normals are subject to a continued change of direction, which must be detectable as a deflection of the light after the wave train leaves the dispersive medium. Since this derivation has been rightfully criticized by Mr. Laue as not rigorous enough, I offer another in appendix to the theoretical part, whose conclusiveness no one should be able to doubt.[9] We observe the equally phased surface of waves, which is emitted by the molecule moving perpendicularly to the optical axis within the focal plane of the lens L, as it passes the optical axis. This is a spherical surface of growing radius up to lens L after passing lens L it is a plane that remains per- pendicular to the optical axis until its entry into the dispersive medium. Let V be the propagation velocity on the abscissa y.[10] is then the deflection angle of the wavelength normal upward per unit path in the dispersive medium. The deflection along the entire path in the dispersive medium is l times larger. Through refraction upon exiting the dispersive medium, this deflection multiplies itself n- fold ,[11] so one obtains for the total deflection A (of the wavelength nor- mal) . On the other hand, however, ,[12] where Δ signifies the lens’s focal distance.[13] One thus obtains for the deflection angle the expression already indicated in the first communication:[14] [p. 5] W1 W2 L q ϑ l y 1 V ∂V ∂y –--------- n c V --- = A l----- dn dy - = dn dy ----- - dndν dνdy ----- ------ - dnνq dν ----- ------ c ---------------------- sin dϑ′ dy dn dν ν ------ ----------- ------ q cΔ - l = = =