250 BACTERIA AND SUNLIGHT. 



Ire(_nieiith slirouded in mist that the necessity' for careful 

 attention to cleanhness and comfort is most urgentl^' felt. 

 AMiile we clean our rooms most carefulh' and cHsinfect 

 ever\\\liere. the southerner opens wide his windo\\'s and 

 lets the sun's ra^•s penetrate throug-hout the house. For 

 this it is necessar^• that the sk\' continue clear. The 

 germs of bacteria that are exposed t-o intense sunlight 

 can onh' \\-ithstand its effect for a short time. Even the 

 germs of Bacillus aiitliracis, that dangerous bacterium 

 which produces a fatal distemper in sheep and cattle, 

 loses its power of multiphing in a verA' few hours. An 

 English botanist, Marshall Vt'ard. concei^'ed the idea of 

 demf)nstrating this effect of light on "bacteria germs by 

 photography. He spread gelatine containing germs of 

 bacteria on a glass plate and placed this in the sun 

 behind a perforated sheet of tin. After a fe^^' liours lie 

 brought the glass plate into a dark \\arm room where 

 it remained for some time. In all the spots on tlie glass 

 plate that \\ere behind the perforations in tlie tin, 

 and which consei|uenth- the sunlight would affect, the 

 bacteria had de\cloped no luither and ^^ere killed; but 

 on the shaded parts the^' not onh had not suffered, but 

 were increasing rajiidh' and marked the gelatine accord- 

 ingh . Thus it ^^as that the pattern of the perforations 

 was distincth' recognised on the gelatine. l'ositi\'e prints 

 can be obtained b\ means of bacteria even from ordinary 

 jihotographic negati\'es. provided the exjieriment be per- 

 formed with ])articiilarh' sensiti\'e germs. Purple bacteria 

 from the Thames, when placed behind glass negatives, 

 have ]iroduced pictures ol English landscape.s which 



