GROWTH AND NUTRITION 91 



absorbed by the lens, and cause fluorescence. Rays of a lower 

 wave length are absorbed by the cornea and produce a patho- 

 logical condition. 



Because of the complexity of the higher animals such knowl- 

 edge as we have of the action of light has been obtained chiefly 

 by a study of single-cell organisms, and by a study of proteins. 

 Thus it has been shown by Schanz 39 that ultra-violet rays de- 

 crease the solubility of proteins in solution. Bovie and Klein 40 

 exposed paramoecia to ultra-violet rays for a short period, and 

 found that though they were not visibly affected, they were 

 definitely sensitized to heat. If the temperature were raised to 

 the optimum for that organism, the exposed cells were killed. 



The first quantitative studies of this subject were made by 

 Hertel. 41 He investigated the physiological action of light on 

 single-cell organisms and showed that the lethal effect varied 

 with the wave length of the rays. The shorter the wave length 

 the greater its toxicity. A wave length of 232 fxfx was distinctly 

 toxic, while a wave length of 440 [*>(!< was only slightly so. 



We also know that the skin is sensitive to light, for most of 

 us have experienced sunburn. In a practical way it has been 

 noted that eczema and acne may be eradicated by rays of short 

 wave length. Successful treatment of other skin affections has 

 also been reported. 



We have intimated that sunlight seems to play an important 

 role in rickets, and one of the more important of the earlier re- 

 ports is that of Palm, 42 who stated that rickets could be pre- 

 vented or cured by sunlight. The most precise statement of our 

 present knowledge was first given by Huldschinsky, 43 who 

 stated that rickets can be cured by exposure to ultra-violet rays. 

 This effect was not local, for irradiation of one portion of the 

 body led to the deposition of calcium salts in all the bones of 

 the body. So far as investigated, Huldschinsky's observations 

 have been amply verified. It has been stated that X-rays have a 

 similar action, but this has not been confirmed. 



