ACTION OF LIGHT ON MICRO-ORGANISMS 375 



perpendicular and oblique rays respectively, Procacci 

 investigated their action separately as well as together. 

 When the vessels were exposed to the perpendicular as 

 well as the oblique rays of the sun, the bactericidal 

 power of the light was unimpaired at the bottom of the 

 vessel, at a depth of half a metre ; but when the perpen- 

 dicular rays only were admitted, this power ceased, the 

 bacteria near the surface only being destroyed, whilst 

 in the lower layers they remained nearly as numerous 

 as in the cylinder which was not insolated at all. 

 The following table shows the actual results obtained : 



Effect on Bacteria of Sunshine Passing Vertically through 60 cm. 

 of Drain-water (Procacci) 



That the oblique rays rendered important service in 

 the destruction of the bacteria at the bottom was further 

 shown by a special examination of portions of the water 

 in the immediate vicinity of the sides of the cylinder, 

 for on freely admitting these rays and excluding the 

 perpendicular, it was found that the smallest number of 

 bacteria was present in those parts of the water which 

 were nearest to the glass. 



A systematic series of experiments on the bacteri- 

 cidal action of light on organisms in water was made 

 by one of us l in connection with the behaviour of spori- 

 ferous anthrax introduced into various waters. These 

 results are of especial interest, as not only was the vitality 



1 ' Experiments on the Vitality and Virulence of Sporiferous Anthrax 

 in Potable Waters,' Percy Frankland, Proc. Roy. Soc., vol. liii., 1893, 

 p. 204. 



