272 Professor Ward [April 27, 



The plates I have shown you are really photographs in living 

 bacteria of the solar and electric spectra, only instead of being recorded 

 by the various stages of decomposition of the invisible particles of 

 silver salts, which are then treated by chemical solutions, &c, in the 

 processes of development, fixing, &c, they are recorded by the various 

 capacities for germination of the invisible spores, the image — i. e. con- 

 trast effect — being then developed by my favouring their germination 

 and further growth into opaque colonies by the application of slight heat. 



If this is so, it ought to be possible to use such a film of bacteria 

 spores as a sensitive plate for printing from a negative, according to 

 the principles laid down, and just as a piece of paper or glass, 

 covered with a film impregnated with silver salts, &c, is used in 

 contact printing in photography ; and this, too, I have succeeded 

 in doing. The chief difficulty to be overcome here is to bring the 

 film impregnated with bacteria sufficiently close to the gelatine 

 surface of the negative to be printed from, for of course it is 

 impossible to bring them in actual contact, since the bacterial film 

 must be kept covered by sterilised glass, &c, and the bacterial culture 

 kept safe from contamination by spores in the air. By employing 

 sheets of extremely thin glass, such as that used for cover-slips, how- 

 ever, on which to support the bacterial films, I have overcome this 

 difficulty, and now throw on the screen a photograph in living bacteria 

 of a piece of English landscape, printed from an ordinary negative by 

 throwing the solar rays, reflected irom a polished mirror of speculum 

 metal, through the negative, the gelatine surface of which was only 

 separated from the bacterial film by such an extremely thin plate of 

 glass as I have described. 



The bacterium employed was a beautiful purple one obtained 

 from the Thames, and which is very sensitive to light — so much so, 

 indeed, that I am convinced that with thin quartz plates I could get a 

 print in half an hour on a clear day. In the case shown the exposure 

 was only two hours. 



It only remains to add that similar results with stencil-letters, 

 the solar and electric spectra, and photographs such as that described 

 have been obtained with anthrax, typhoid, several ordinary river bac- 

 teria, and fungi and yeasts, and that — apart from details which need 

 not enter into discussion here — the general results are the same. 



In order to try and obtain a further insight into the physiological 

 and pathological phenomena concerned in this bactericidal action of 

 light, however, and in view of various disadvantages inseparable from 

 the employment of cultures in the mass, I have attempted with success 

 to trace the effects of the light on the individual bacterium cell itself, 

 isolated under the microscope. To do this, some form of culture 

 chamber is necessary in which the organism can be grown in a minute 

 drop of food-material — such as broth, gelatine, or even water — and 

 can be kept under a high power of the microscope for some hours, or 

 even days if necessary. This is managed as follows : — 



[Demonstration of culture cells and methods of using them.] 



