Ocular lesions have been experimentally produced in rabbit by a pulsed optical maser (laser). The high-energy density delivered in a single 0.5 msec pulse was sufficient to cause instantaneous thermal injury to the pigmented retina and iris of the brown rabbit. Ophthalmoscopically, the retinal lesions resembled flash burns from an atomic fireball.

All four causes of indeterminacy, individually and combined, reduce the precision of prediction.One may raise the question at this point whether predictability in classical mechanics and unpredictability in biology are due to a difference of degree or of kind. There is much to suggest that the difference is, in considerable part, merely a matter of degree. Classical mechanics is, so to speak, at one end of a continuous spectrum, and biology is at the other. Let us take the classical example of the gas laws. Essentially they are only statistically true, but the population of molecules in a gas obeying the gas laws is so enormous that the actions of individual molecules become integrated into a predictable-one might say "absolute"-result. Samples of five or 20 molecules would show definite individuality. The difference in the size of the studied "populations" certainly contributes to the difference between the physical sciences and biology. Conclusions

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Recent interest in the dose to man from natural radioactivity has been stimulated by the assumption by many geneticists of a linear relationship between radiation dose and the incidence of genetic mutations. Although this has not been demonstrated at the low dose rates prevailing in nature, the likelihood of such a relationship has led to the suggestion that geographical variations in the frequency of spontaneous mutations may be correlated ultimately with differences in the radiation dose to populations (1). This question has recently been reviewed by Gopal-Ayengar (2).The studies of the dose received by man from naturally occurring ionizing radiations can be divided into that received from external and internal sources. The dose to the germ plasm is primarily due to the external radiation, although one internal source, potassium-40, does deliver a dose to the reproductive organs amounting to about 15 mr/ year (3, 4). Studies of the radiation dose from external natural sources have been reviewed by Sievert (3), Libby (4), and Lowder (5), and extensive sets of measurements with particular emphasis on dwellings have been reported by Hultqvist (6) in Sweden. Although measurements have been made in this country byHess--(7) and Neher (8), no systematic study of the environmental radiation dose rate over an extensive area of the United States has.been reported previously.During the summer of 1957 our laboratory made measurements in the United States to establish the approximate range of population exposures to cosmic and terrestrial gamma radiation. An effort was made to obtain results which would be representative of the unperturbed natural background, affected as little as possible by the occasional substantial variations in the observed natural radiation levels produced by localized sources (for example, the proximity of granite buildings, brick paving, and fallout).Measurements were made with a 20lit., air-filled, polyethylene-walled ioniza-versity of Wisconsin.

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