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ECHXICAL improvements developed during the past T few years have greatly reduced the uncertainties attending quantitative spectrum analysis and have resulted in a n increased application of emission spectrography to studies of trace metals in biological PhotometryThe earlier method of photometry (2, S), which gave sufficiently accurate results (5) but was applicable only to relatively low concentrations of metals, has been replaced after a study of other technics (IO, 11, IS, 15, 28, SO) by the niethod of Preuss (25). This method. which employs the Hansen gage (17) to incorporate the blackening mark in the analytical spectrum, extends the analytical range, thereby reducing the dilutions otherv-iqe required to handle relatively high con-centrations of metals. However, a modification by means of which determinations of opacity could be substituted for determinations of density was found to be advantageous in the evaluation of very weak lines. This modification inyolves adherence to the authors' earlier method (2, 9) of dealing directly with faint lines instead of attempting to isolate and concentrate the test metal ( 2 ) .In applying the modification, the quantity of internal standard used must be such as to produce a standard line the density of which is below 0.30 (opacity of 2) in a t least two steps of the spectrogram produced tiy means of the step sector. The measured opacities (galvanometer reading of emulsion/ galvanometer reading of line) for the two weak steps of the standard line are plotted as a straight line against the relative exposures of the steps. The opacities for the test line are also plotted and the distance between the two lines at a base opacity (1.30) can then be correlated n i t h the concentrations of the test metal (Figure 4). Figure 1 is a photograph of duplicate spectia with faint test lines and Figure 2 illustrates the niethod of ohtaining the separatiorii Ph X'7833.07 0 b e * 0

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