Abstract
The effect of radiant energy has long been known to be the result of a certain intensity working in a given period of time, during which period a definite amount of energy is being transferred to the object irradiated. Bunsen and Roscoe 1 have correlated these factors and state that identical effects are obtained if the product of the intensity and time is kept constant. This, however, is merely a modification of the Schwartzschild law which states that intensity multiplied by the time to the exponent K must equal a constant. The factor K is a number slightly less than one, whereas the Bunsen-Roscoe law is a special case where the exponent is equal to one, and holds true for a certain limited range of the factors (intensity and time), whereas the Schwartzschild law must be applied for excessive variations in these factors. Therefore, with marked decrease in the intensity the time must be increased more than the proportional decrease in intensity.
In this experiment we have attempted to ascertain whether the Schwartzschild or Bunsen-Roscoe law is applicable to the destructive effect of ultraviolet light on bacteria. The method consists of spreading in a unicellular layer B. prodigiosus on an agar plate. By means of a metal shield, the area of exposure to a water cooled Kromayer lamp can be restricted to a certain field on the plate and at the same time protect the remaining portion. Throughout the experiment the energy applied was constant, but the intensity decreased cal from 7.5 to 0.12 cal/cm2 min 10-4, whereas the time was increased in an inverse proportion to the intensity, i. e., from 2 to 120 minutes. The result as indicated in Fig. 1 shows a greater biological effect at short exposures to high intensities and suggests that increased rather than identical energy applications are needed in prolonged exposures to obtain similar biological activity.
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