1 2 FBEBE. 



Paramecia, when exposed, showed an instantaneous state of unrest, with a 

 tendency to escape from the illuminated field, longer illumination finally brought 

 about their complete decomposition. Nematodes were more resistant, death fol- 

 lowing only after two or three minutes. Larvae of amphibia showed a decided 

 wandering of pigment toward the spot in contact with the light. All of the 

 organisms investigated had a marked reaction toward light of 280 nix, first in 

 increased motility or contraction of contractile tissues, followed by lassitude, 

 slowness of motion and final death. Toxins and ferments were also tested, so, 

 for example, the toxin of diphtheria was rendered entirely inert after five min- 

 utes; trypsin, diastase, and the ferment of rennet were also profoundh' affected, 

 but not in as great a degree as the toxins. 



The experiments next were extended to lights of various wave lengths, the total 

 energy of each region of the spectrum being measured thermo-eleetrically, it 

 developing that "the action of one and the same spectral line is directly propor- 

 tional to its total intensity, as measured thermo-eleetrically." 



The physiological action of the rays diminishes with increasing wave length, 

 a difference of 50 /tu. showing a marked effect. Light of 280 u./x kills organisms 

 almost at once, whereas that of 440 /xfx, of equal intensity, does so only after a 

 number of hours of action. 



Perhaps the most important result of Hertel's work, for our present purposes, 

 is his proof that not only the total energy and the wave length of the incident 

 light are of importance, but also the relative proportion of the rays absorbed by 

 the organisms, for he developed the fact that the absorption of radiant energy 

 by the tissues of the organisms investigated is diminished the longer the wave 

 length of the light which is employed. 



The action, of a specific kind of light on organisms is therefore not only 

 dependent on its total intensity, but is also in the greatest degree related 

 to the power of absorption for this light possessed by the tissues. By 

 means of erythrosin Hertel was able so to stain the cells of living organ- 

 isms that their power of absorption for waves of greater length than 280 

 fi.fi was markedly increased. The introduction of this experimental modi- 

 fication enabled Hertel to demonstrate that even the visible light rays 

 could bring about destruction of tissue in the same time as the ultra-violet. 



The physiological action of light rays is therefore not dependent upon 

 any specific region of the sun's spectrum, the wave length is only of impor- 

 tance in such degree as the total energy, and the power of absorption is 

 determined therein'. A plant, for instance, for the existence of which 

 light is absolutely necessaiy, takes on a color which is complementary 

 to the incident rays, for example, Oscillaria sancta colors red in green 

 light and green in red. Light also has an unfavorable effect on the 

 phenomenon of cell division, but this ouly takes place with higher intensi- 

 ties. Different pigment cells have a different absorptive power for inci- 

 dent light, according to its wave length, but the ultra-violet rays are 

 equally absorbed by all. Therefore, the latter differ markedly from visible 

 light, in which the absorption maxima, according to the color, lie at 

 diverse and far-removed parts of the spectrum. 



The character of the pigment, therefore, is of fundamental importance 

 in determining the effect of insolation for all save the ultra-violet rays : 

 but the experiments of Baly on chemical substances, in distinction to 



