138 BIOPHYSICALLY ACTIVE LIGHT 



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the erythema threshold ; if irradiated with 2967 A, it took 34 X 10 ergs/ 

 cm 2 , or 5.1 X 10 16 photons/cm 2 . Apparently the erythema accom- 

 panying gradual tanning by long-wave ultraviolet light is a negligible 



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factor if no 2967 A radiation is present. 



If the intensities of these two radiations are so chosen that the same 



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final degree of erythema is developed, that at 3850 A reaches its maxi- 

 mum in 2 to 3 hours, but that at 2967 A is just becoming visible at the 

 end of this period of time. Twelve hours after irradiation each shows its 

 maximal reddening; the former has developed into a brown red, the 

 latter into a pronounced carmine red color. After 48 hours the former 

 is still maximal red and the latter is strongly brown. After 5 weeks the 



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erythema has subsided; the 2967 A exposure is slightly pigmented, and 

 the 3850 A exposure has developed into a deep brown pigmentation. 



As yet the photochemical reactions involved in these biophysical 

 activities are only slightly understood. The trend of the evidence (Blum 

 [1941]) seems to indicate that an active substance is liberated which is 

 responsible for the erythemal response as an indirect capillary reaction. 

 Apart from the vasodilatation, which appears after a latent period, the 

 structural injuries are limited almost entirely to the first 0.1 mm when 

 wavelength 3000 A was used. Penetration by 7500 A is only about 2.5 

 mm, although a wavelength of about 11,500 A can penetrate to a depth 

 of 5 mm. 



Artificial Sources of Ultraviolet Radiation 



A typical mercury-vapor arc is illustrated in Fig. IV-4. This is a 

 Cooper Hewitt quartz-enclosed mercury-vapor arc. The luminous tube 

 of the burner is made of clear fused quartz of high ultraviolet trans- 



Anode 



Fig. IV-4. Sectional view of a " Cooper Hewitt " quartz mercury arc. 110-volt 

 burner. Length 6f in. Luminous tube diameter f in. 



mission. The leads are flexible stranded wire insulated with porcelain 

 beads because of the high temperature attained by the terminals. The 

 anode, or positive electrode, of the burner consists of a tungsten wire 

 coiled into a target. In operation it is heated white hot by the arc which 

 extends from it to the surface of the mercury cathode-, or negative 

 electrode. The cathode chamber serves to keep the mercury at the 



