Action of Light Rays on Organic Compounds. 183 



The general production of formaldehyde by the action of light on bio- 

 chemical materials may also stand related to the important lethal effects of 

 sunlight and ultra-violet light upon micro-organisms, which is seen in the 

 sterilising action of sunlight upon many pathogenic organisms and in the 

 similar use of ultra-violet light installations for sterilisation purposes. 



The relationship of the lethal effects to the wave-length of tlie light has 

 been studied by many observers. Downes and Blunt* showed, at an early 

 period, not only that both direct and diffuse sunlight inhibited the appearance 

 and slowed the growth of organisms self-sown in Pasteur's cultivation fluid, 

 but that this action did not appear when the cultures were preserved behind 

 red or yellow glass screens, while blue or violet glasses allowed light to pass 

 which possessed as much deterrent action as light through clear glass. 

 Marshall Wardf was the first to invent an ingenious method of making the 

 organisms record their own destruction, which has again been independently 

 re-discovered by two other sets of investigators at intervals of several years. 

 The method consisted in throwing a spectrum, obtained by sending sunlight, 

 or the light of electric arcs in air, through a quartz spectroscope, upon an 

 agar-agar plate sowed over with the organism. A number of slots were 

 cut out on the covering lid of a shallow plate like a Petri dish, and some of 

 these were covered with quartz and others with thin glass strips. The 

 spectra were directed through these on to the agar-agar culture. The 

 remainder of the surface was protected by an opaque cover of tinfoil. After 

 exposure for 12 hours the plate was incubated for four days and then the 

 results were photographed. When the glass was interposed the only area 

 of destruction was that of the blue and violet, but when the quartz only 

 intervened between source of light and organisms the destruction passed far 

 on into the ultra-violet region. A quite similar method was used by Barnard 

 and Morgan,^ who found the lethal action in the ultra-violet so intense that the 

 bright spectral lines in the ultra-violet were mapped out as clearly almost as 

 on a photographic plate. These authors also determined the wave-lengths 

 of the lethal zone of the spectrum, and found its limits to lie between 

 3287 and 2265A. Quite recently the work has been independently repeated 

 with similar results by Browning and Euss.§ 



Some of the authors quoted, especially Marshall Ward, consider the nature 

 of the chemical reaction involved. Marshall Ward draws attention to the 



* 'Eoy. Soc. Proc.,' vol. 26, p. 488 (1877) ; vol. 28, p. 199 (1879). 



t 'Eoy. Soc. Proc.,' vol. 54, p. 472 (1893); other papers by Marshall Ward on this 

 subject, ' Roy. Soc. Proc.,' vol. 52, p. 39.3 (1892) ; vol. 53, pp. 26 and 164 (1893) ; vol. 56, 

 ■p. 345 (1894). 



+ ' Roy. Soc. Proc.,' vol. 72, p. 126 (1903) ; 'Brit. Med. Journ.,' November 14, 1903. 

 § 'Roy. Soc. Proc.,' B, vol. 90, p. 33 (1917). 



