Appendix

For calculation of the erythemal efficiency, the apparent optical density D of the stratum corneum must be measured under conditions in accordance with the definition given in eqns. (2) and (3a ); the physical meaning of this is that, with an infinitely large field of normally incident light of intensityf 0 the total energy of the forward scattered light crossing unit elementary area in unit time is given byf 0 10-D . The experimental fulfilment of the conditions is qualified by the restriction that only that part of the light falling on the slit of the recording spectrograph, which is contained in a cone whose vertical angle is equal to the angular aperture of the collimating lens, will ultimately be measured. However, the intensity of the components scattered at large angles must be assumed to be small.