D O C U M E N T 9 4 N O V E M B E R 1 9 2 7 1 0 7 94. From Emil Rupp[1] [Göttingen,] 29 November 1927 Dear Professor, I wish to take the liberty of informing you that I have carried out an experiment on the modulation of the frequency of light by the frequency of a Hertzian oscillator.[2] I want to report the results to you here:[3] The intensity of a light beam of frequency is periodically varied by making use of the Kerr effect, controlled by an electrical oscillation n. A schematic of the ex- perimental setup is shown in the drawing: R = Resonance with thallium vapor A = Absorption rate in " " K = Kerr cell between 2 crossed Nicol prisms the frequency of a damped Hert- zian oscillator is applied to the cell. P = photographic plate. The resonance lamp emits a narrow line. The green Tl-line was employed, at temperatures of 280° & 480° C.— With a suitable choice of the length and the va- por density, A absorbs all of the light that enters it when a DC voltage is applied to the Kerr cell.— If an AC voltage is applied to the cell, according to the classical picture,  is split up into ± 2n (not ± n, since the Kerr effect varies quadratically with the field strength). These “sideband frequencies,” however, cannot be com- pletely absorbed by the vapor in A the photoplate P is exposed all the more strongly, the greater the splitting of the original line, hence the higher the frequency n. The brightening of A with increasing frequency of the elect. oscillation is thus to be expected. 1 would mean that the sidebands ± 2n are completely separated from . My ex- perimental result is the following: