1893.] The Action of Light on Bacteria. 475 



lenses, &c., were so feeble that it was necessary to employ quartz 

 throughout. 



Secondly, the bactericidal effect is found to extend far into the 

 ultra-violet. The intervention of a thin piece of glass results in the 

 cutting off of a large proportion of effective rays (fig. 2). 



Thirdly, the most destructive rays end of blue and beginning of 

 violet are to some extent effective even after reflection from the 

 inner faces of the quartz plate covering the film and the glass on 

 which it is supported, and so a peculiar bellying out of the image 

 of the illuminated slot is observable during the early stages of incuba- 

 tion the figure being thus made to show its own curve of intensity 

 as it were. 



The plates employed are ordinary agar cultures in shallow glass 

 dishes, covered with a glass plate in which one or more slots about 

 3 ins. long by -J- in. wide are pierced. Over the slots a quartz plate 

 is secured, and all covered with black paper and foil, except the slots. 

 The exposures are made on ice. 



The author is also using plates divided in two halves, so that two 

 similar films containing bacteria of different species can be exposed 

 simultaneously to the same spectrum. 



These results suggest evidently that the naked arc light may prove 

 to be a very efficient disinfecting agent in hospital wards, railway 

 carriages, or anywhere where the rays can be projected directly on to 

 the organism. The author has elsewhere shown the evidence on 

 which it is concluded that the action is direct and on the cell con- 

 tents ; but even if the action took place at the surface of the cells, 

 the above conclusion would still be true in practice. 



It is extremely desirable that experiments should be made on the 

 action of light on living cells of animals e.g., Infusoria, ova, &c 

 since results would probably be obtained of importance as regards sun- 

 burn, sun-baths, and other matters.* 



exposures to the solar spectra were made by the author himself, and he is indebted 

 to Professors McLeod and Stocker for the use of apparatus and for valuable advice. 

 * Eaum, in ' Zeitschr. f . Hygiene,' 1889, has collected some literature bearing on 

 this subject. 



