IN making some determinations of the capacity necessary just to produce secondary spectra, I have found that this critical capacity increases very rapidly with decreasing wave-length. The primary spectrum does not go over suddenly as the critical capacity is reached, but the red and yellow portions go over first, then finally, at a much greater capacity, the violet. Critical capacity as a function of wave-length is well represented by the exponential 6/eλ-λ approaching a constant value in the infra-red and the value infinity in the ultra-violet, perhaps not farther out than 250μμ.