the in situ approach is capable of yielding useful quantita- tive results in a reasonably uniform radiation environment, and at the very least can be used as a basis for evaluating more direct—but not necessarily more accurate—mothods of estimating mean population exposure-levels. Another conclusion which is suggested by the New England results is that the basic limitation of the pocket ionization-chamber technique in terms of measuring normal human exposure to environmental radiation is now the difficulty in determining mean leakage rates under actual field conditions while being worn and handled. There appears to be no fundamental reason why this difficulty cannot be at least partially overcome by suitably controlled experimentation, and thus the pocket chamber technique can be considered as a potentially practical one for this kind of measurement. It should be remarked that the dosimeters admirably fulfilled their basic purpose in the Harvard investigation, namely, the determination of differences in population exposure-levels between areas. There are, of course, a numberof other possible methods for determining mean population exposure to environmental radiation. For example, photographic film dosimetry techniques have been applied to this goneral problem area with some success. O’Brien et al.!° described a film-scintillator (sodium iodide) system which Roser and Cullen®* have utilized in the measurement of population. exposure in Brazil on a limited scale. The approximately thousand-fold enhancement of the film response produced by the scintillator is almost too great for the high-background areas of Brazil; such a method would almost certainly be feasible in areas of more normal background levels for certain kinds of studies. The basic Hmitation here is the cost of the dosimeters, which precludes their widespread use. The problem of reciprocity law failure must also bo taken into account in the calibration of the dosimeters. A similar kind of dosimeter has been described by Henson", using photographic film anda plastic scintillator (N.#. 102). While less sensitive than the sodium iodide system, it exhibits little energy dependence and good precision (+10 per cent S.D. for two weeks’ exposure at normal background}. Reciprocity failure was observed but has not proved excessive. The main problem seems to be a strong dependence on temperature in its response, which varies with the dose rate. The error present in any particular reading is not known, so that the use of this dosimeter has not been recommended. There has also been recent progress in increasing the sensitivity of normal radiographic film by means of post- exposnre to visiblo light and improved development techniques that may render such film useful for environ- mental radiation studies without the necessity for external enhancement of its response. McLaughlin” has reported Io