I. The measured absorption is the difference between absorption and forced emission. This has to be taken into account in the interpretation of measurements in the infrared and the absorption of hot gases, particularly in the sun. II. According to Maxwell's theory, ${n}^{2}=\ensuremath{\epsilon}\ensuremath{\mu}$. The influence of the magnetic susceptibility on the dispersion is discussed for a gas. The paramagnetic part depends on the frequency formally in the same way as the electric part, but is determined by the magnetic absorption lines. The diamagnetic part is practically independent of the frequency up to x-ray frequencies.