332 ANNUAL REPOET SMITHSONIAN INSTITUTION, 1925 



found to reduce biological action very greatly, as is shown when the 

 lethal power of the screened rays is tested against that of the un- 

 screened on infusoria or the skin. The photographic method is so 

 exceedingly sensitive that deductions can not safely be drawn from 

 it alone. 



It is claimed that immunity is set up in the epidermis by one 

 exposure to a subsequent one, and this long before pigment is 

 formed (Perthes). Thus if an area of the skin be exposed for five 

 minutes and again for five minutes some hours later, and a second 

 area be given 10 minutes exposure all at one time, the erythema will 

 be much more marked in the second area. Choosing a small dose, 

 a second one given a few hours later increases erythema and soreness. 

 Maximal er3^thema, of course, can not be further increased by a 

 second dose, but this seems to be true for soreness also. The im- 

 munity is no doubt due to coagulation of the outer layer of living 

 cells, whence comes peeling. When pigmentation is still well marked 

 weeks after an exposure, susceptibility of the epidermis may be 

 shown to have returned by the erythema following a further dose 

 of ultra-violet rays. 



To measure the therapeutic action of the ultra-violet rays we can 

 use the lethal dose for infusoria contained in a quartz cell at 

 15-20° C, or the erythema-producing dose for the skin of the 

 average white arm, or the rate of bleaching of a standard solution 

 of acetone and methylene blue. The last has been standardized 

 againsfe the tw^o former, and each degree on the scale is twice to four 

 times that required to produce a moderate erythema. (A. Webster, 

 L. Hill, and A. Eidinow.) The acetone blue solution is exposed 

 in a quartz tube of standard diameter, and after exposure the degree 

 of bleaching determined by comparison w^ith a set of blue tubes of 

 depths of tint 10 to 3. The acetone solution absorbs the ultra- 

 violet rays shorter than 360 nfi, and the chemical reaction set up in 

 it bleaches the blue. Observations have been taken daily with full 

 exposure to sun and sk}" at various places and show the intensity 

 of ultra-violet radiation in clean air and the effect of smoke and 

 pollution. During a fine summer day the quartz tube ma}^ have to 

 be changed two or three times in the day, and the liighest total 

 reading last summer at Peppard, Oxon, equaled 23. In the Alps a 

 reading of 41 was obtained in one day. In dull cloudy weather of 

 winter the reading may be 1 or 2 and in smoke polluted towns 0. 



Using in addition a clock to keep moving the quartz tube together 

 with a small screen to shade it from the sun but not from the sky 

 except that immediately round the sun, we have found that the 

 total ultra-violet radiation from the sky is far more than from the 



