1877.] 



the Effect of Light on Bacteria fyc. 



495 



sent anv further than to state the results of one or two of the numerous 

 experiments which we hare made in this direction. The first question 

 which presents itself is, with what rays of the spectrum is this property 

 of light coincident ? Is it localized in any one part, or is the unbroken 

 pencil of rays necessary? The most definite result which we have 

 hitherto obtained is embodied in the following experiment. 



Obs. 13. — On Oct. 8th eighteen small test-tubes were partially filled 

 with a solution, which was purposely made of twice the strength given in 

 the Appendix, and were then plugged with cotton wool. 



Three of these tubes were placed in each of four boxes, the sides of 

 which were made of blood-red, yellow, deep blue, and ordinary glass 

 respectively. Of the remaining six tubes, three were encased, and three 

 were simply exposed to the light. All were placed as usual on an outside 

 window-ledge facing south-east. 



Oct. 11. — The encased tubes became turbid with innumerable Bacteria. 



Oct. 12. — The tubes in the yellow box were all clouded with Bacteria. 



Oct. 13. — The tubes in the red box showed signs of commencing tur- 

 bidity, all the other tubes remaining up to this time quite pellucid. 



Oct. 14, — All the red tubes were very turbid ; two of the tubes in the 

 box of ordinary colourless glass were also slightly turbid. 



Oct. 17. — One blue tube has become slightly clouded with Bacteria. 



The tubes as they broke down were all carefully examined with a T y 

 immersion. Xo organisms other than the ordinary rod-shaped Bacteria 

 were seen in any, nor did these differ as regards their apparent vitality 

 and activity. 



The remaining tubes in the blue glass case, those simply exposed to 

 the light (i. e. not placed in any glass case), and the surviving tube of the 

 three exposed in the box of ordinary window-glass have remained per- 

 fectly clear to the date of writing (Oct. 17th). This experiment points 

 forcibly to the actinic rays of the spectrum as the active agents ; we hesi- 

 tate, however, to affirm this positively for the present. 



In the course of our investigation we have repeatedly had occasion to 

 notice the proneness of mycelial fungi to appear (to the exclusion of 

 Bacteria) in solutions which were themselves of a yellow colour, or were 

 subjected to yellow fight. Thus tubes containing urine exposed to light 

 have several times developed a tuft of mycelium either during insolation 

 or afterwards when encased. 



The following observation illustrates the same fact as regards the 

 artificial solution with which most of our experiments have been 

 made. 



Obs. 14. May 30. — Three small test-tubes containing solution A were 

 suspended in three larger tubes containing solution of picric acid of such 

 strength as to represent three gradations of colour from a barely per- 

 ceptible yellow to a deep tinge of the same. Three corresponding arrange- 

 ments were made with pure distilled water in place of the picric acid 



