262 Professor Ward [April 27, 



this question. The first, employed by Downes and Blunt, Arloing, 

 Janowsky, Geissler and Kotljar, was to use screens of coloured 

 glass, or coloured solutions, &c, behind which tubes of broth or 

 other liquids containing the bacteria were placed, and to judge of 

 the effect by comparing how rapidly the broth became turbid ; or 

 behind which gelatine tubes, or agar or potato cultures were 

 exposed, and a rough estimate made as to how well or ill the bacteria 

 grew on the surfaces. 



The second was to place such tubes in the various regions of the 

 spectrum, and compare the results, as before, by judging of the tur- 

 bidity of the broth, or the rate of growth on potato, &c. &c. This 

 method was employed by Arloing, Janowsky, Chmelewsky and 

 Geissler. 



Quite apart from other drawbacks to any such methods as these, 

 and while fully recognising their historical necessity as pioneer work, 

 it is obvious that no very accurate results could be expected from 

 them, and as matter of fact we find up to 1892 that opinions were 

 so divided on the question which are the effective rays, that no 

 definite conclusion could be drawn. For instance, Arloing could not 

 distinguish the action of any one particular set of rays from that of 

 others, while Gaillard concluded that all the rays have a feeble effect. 

 Santori concluded that neither the red nor the violet rays are the 

 active ones, whereas Janowsky thought both the red and blue-violet 

 were the effective rays. 



Eoux and Duclaux thought the action was due to a poisoning of 

 the bacteria by products of oxidation of the food-medium, while 

 Arloing and others insisted it was a direct action of the light. Some 

 denied the facts altogether, or believed that all the light did was to 

 retard the germination of the spores ; while others insisted that the 

 bacilli are more easily destroyed than the spores, and that it was all 

 an obscure matter of temperature. In fact, previous to 1892, the 

 utmost confusion existed, and all kinds of statements were current 

 on the subject among the very small band of observers who concerned 

 themselves at all with it ; the best indication of the want of definite 

 knowledge being, perhaps, the silence of the text-books on the 

 matter. 



The fact is that the methods in use up to that period were inade- 

 quate for the solution of the problems that had arisen, and many 

 cautious bacteriologists were consequently sceptical as to the bacteri- 

 cidal effects of the sunlight, or indeed any light at all. 



During the course of my investigations into the vitality of the 

 anthrax bacillus when its spores are left in water, I was struck with 

 the rapidity with which the bacillus disappears from tubes exposed 

 to the sunlight, a fact often observed before, and recorded by Straus, 

 Buchner, Momont, Frankland and others, and usually supposed to 

 be due to the spores germinating out into tender bacteria, which are 

 then killed off. This is by no means the case, however, for the spores 

 are themselves so acted on by the light-rays that they become incapable 



