266 Professor Ward [April 27, 



consequence of poisoning from outside. It is evidently due to the 

 light-rays directly inducing some changes in the substance of the 

 spore itself. 



The practical hygienic importance of this discovery is considerable, 

 and would not be diminished even if it had turned out to be true that 

 the light-action was really on something at the immediate surface 

 of the spore ; for it is clear that when spores escape into the air, and 

 are exposed to the direct rays of the sun, they run exactly the risks 

 I here subjected them to, and, as matter of fact, we find it is of the 

 utmost importance in bacteriological investigations not to allow the 

 spores of bacilli, fungi, &c. (even when fully ripe and otherwise fit 

 for keeping in dry tubes, &c, for years), to be exposed to the light, 

 for under such circumstances they gradually deteriorate. 



Now we can see why it is so essential to health that our streets, 

 houses, clothes, &c, should be thoroughly exposed to the sunshine. 

 Moreover, as will be clearer in the sequel, these experiments render 

 intelligible why it is that epidemics of parasitic fungi are so often 

 connected in people's minds with dull, cloudy, sunless weather, and 

 the Italian proverb with which I opened this lecture has for us a 

 meaning far deeper and more significant than it had before. 



It is evident that the foregoing method is capable of application 

 in several different directions, and I now proceed to attack the 

 problem as to which rays of light are most effective in the process of 

 destroying living germs, on entirely new lines. 



Starting from the thought that all the older attempts to compare 

 the growth in two or more tubes exposed to different coloured lights 

 are open to the criticism that it is impossible to be sure that each 

 tube-culture is alike at the commencement, and extremely difficult to 

 contrast them as growth proceeds, it was obviously a step forward to 

 compare the effects of different coloured lights on one and the same 

 plate culture. This I did in various ways, but the following is one 

 of the most instructive. 



A block of vulcanite is cut in the shape of a Roman capital E lying 

 on its back LlJ , and a thin glass plate is cemented to each side : this 

 gives a double celled screen, of which each cell is equal as regards 

 depth of liquid poured in, kind of glass, and so on, points of more 

 importance than may appear at first sight. Having prepared such a 

 screen with, say, clear water in one cell and the orange-coloured solu- 

 tion of bichromate of potassium in the other, an agar plate of spores 

 is made and two stencil-letters placed on it, so arranged that one letter 

 is covered by each cell of the screen. 



[Screen apparatus of various kinds shown and explained.] 



On exposing such a screened plate, one of course gets the com- 

 parative effects of the same light, acting at the same temperature, for 

 the same period on the same culture : the one condition altered being 

 that certain rays — in this case the blue-violet — are cut off by the 

 bichromate screen. 



[Plates thus exposed and photographs of such shown.] 



