LIGHT THEEAPY MAYEE 393 



The simple conclusion that the shorter the wave length of ultra- 

 violet the greater the bactericidal action is in error, as it has been 

 shown that there is a characteristic curve of bactericidal effectiveness 

 for different bacteria, where the striking maximum is between 260 

 and 270 millimicrons.'^ The longer wave lengths limit of direct bac- 

 tericidal action on Staphylococcus aureus was found to be between 

 303 and 313 millimicrons. Bactericidal action has been observed at 

 225 millimicrons. Polarizing of light has no demonstrable effect on 

 this action. Temperature elevation usually increases the action. The 

 hydrogen-ion concentration of the environment has no appreciable 

 effect on the bactericidal reaction between limits of pn 4.5 and 7.5. 



The use of monochromatic radiation in experiments on paramecia 

 and on certain bacteria has shown that the bactericidal action is 

 probably due to a destruction of the protein molecules within the 

 cell as well as to a lipoid destruction of the surface membrane 

 (fig. 2). Ked blood cells in vitro are hemolysed by ultra-violet 

 energy, possibly because of increased permeability of the cellular 

 membrane or of the destruction of the cell stroma or of both.^" 

 Despite the statements of previous workers that the susceptibility of 

 protoplasm to ultra-violet light is conditioned by the absorption of 

 the toxic rays by the aromatic amino-acid radicals of the protems, 

 still the close reciprocal correspondence between the curve of bac- 

 tericidal action and the curve of absorption of ultra-violet by the 

 nuclein derivatives (cystine, thymine, and uracil) promotes the prob- 

 ability that a single reaction is involved in the lethal action of 



ultra-violet. 



Protoplasm may be so affected by ultra-violet rays as to become 

 especially sensitive to heat radiations.^* The visible manifestations 

 of tissue hyperemia due to ultra-violet radiation occur only after the 

 lapse of a certain latent period, two hours or more. There probably 

 takes place an absorption into the blood of products of tissue injury 

 or an enhanced absorption of normal tissue products produced by 

 such injury, with consequent erythema and edema. On repeated 

 exposures, a pigment, melanin, is generally formed in the basal cells 

 of the epidermis; this may be due to the action of certain skm 

 oxidases on a breakdown product of tyrosine."" 



Unicellular organisms are stimulated to certain physiologic changes 

 «uch as fission, or they are destroyed by ultra-violet radiation, de- 

 pending on such factors as the intensity of the irradiation, its penetra- 

 tive power and the size of the o rganism. Thus, ultra-violet rays may 



23 Harris and Hoy t, Science, Tol. 46, p. 318, 1917. 



-Bovie W. T.. and Klein, A.. Journ. Gen. Physiol., vol. 1, p. 331, January. 1919- 

 -Hinri^hr Marie A.. Proc Soc. Exper. Biol, and Med., vol. 26. p. 175 November 1928. 

 Bovie W T and Hughes, O. M.. Journ. Med. Research, vol. 39. p. 223, November, 1918. 



