§3] EFFECT UPON GENERAL FUNCTIONS 171 



some time, slowly diminishing, however, in intensity. Certain 

 chemical changes in the living retina may, indeed, be studied 

 optically. These especially concern the visual purple. This 

 is a substance lying in the outer ends of the rods of the retina, 

 which, under the action of light, becomes bleached, but regains 

 its color in the dark (Helmholtz, Handb., pp. 265-273). 

 These facts serve to indicate that light may influence metabo- 

 lism even in organisms destitute of chlorophyll. 



2. Vital Limits of Light Action on Protoplasm. — We have 

 seen above (p. 167) that the rate of assimilation diminishes in 

 chlorophyllaceous plants with a diminution in the intensity of 

 the light. At last a point is reached where the intensity is so 

 low that no further assimilation can occur, and after the con- 

 sumption of the stored-up food-stuffs, starvation and death must 

 eventually ensue. For non-chlorophyllaceous organisms, how- 

 ever, no such lower limit exists. Many, as parasites or cave 

 dwellers, live in complete darkness, even through many genera- 

 tions. A lower vital limit to the action of light exists only in 

 the case of chlorophyllaceous plants. 



With the upper vital limit, it is, however, quite different. 

 This is found in the most diverse groups. Its occurrence in 

 bacteria being of especial hygienic importance, these organisms 

 have been made the object of exhaustive studies. Monte- 

 GAZZA (see NiCKLES, '65) was perhaps the first to discover that 

 strong light kills bacteria, but Downes and Blunt ('78 and 

 '79) were the first to study the matter thoroughly. Since their 

 time, numerous experiments have been made upon bacteria, as 

 well as the higher fungi. For literature, see Feankland 

 and Ward ('92), and Ward ('93, p. 309). Even the earliest 

 observers found that, while cultures of bacteria reared in the 

 dark rapidly flourished, they not merely did not thrive when 

 subjected to sunlight, but actually became sterilized. That the 

 sterilization was complete was shown by the fact that when the 

 culture was placed again in the dark, no bacteria developed in 

 it. This result is most striking when certain bacteria, say of 

 the species Bacillus anthraeis, are mixed with gelatine or agar- 

 agar, poured uniformly over a glass plate. If the glass plate 

 is then covered by a black paper stencil containing some 

 character, e.g. the letter H, and exposed to a November sun- 



