In order to follow his argumentation and to evaluate Bohr's response, it is convenient to refer to the experimental apparatus illustrated in figure A. A beam of light perpendicular to the X axis (here aligned vertically) propagates in the direction z and encounters a screen S 1 with a narrow (relative to the wavelength of the ray) slit. After having passed through the slit, the wave function diffracts with an angular opening that causes it to encounter a second screen S 2 with two slits. The successive propagation of the wave results in the formation of the interference figure on the final screen F.