128 THE FLOATING-MATTER OF THE AIR. 



colour, two were milky or white, and two of a dull 

 brownish hue. 



Cohn mentions the bluish tinge of the infusion by 

 reflected and its yellow tinge by transmitted light when 

 the Bacteria are incipient. This is due to a dichroitic 

 action, similar to that which produces the blue of the 

 sky and the morning and evening red. The blue, how- 

 ever, though discernible, is not pronounced, for the 

 Bacteria are too large to scatter the colour in any high 

 degree of purity ; but with a ' muddy ' infusion a very 

 fair red may be obtained from transmitted light. I 

 have used the Bacterial turbidity for photometric pur- 

 poses. On the 9th of October, for example, I ac- 

 companied Sir Eichard Collinson and a Committee of 

 the Elder Brethren of the Trinity House to Charlton, 

 with the view of comparing together two lights 

 mounted at the Trinity Wharf at Blackwall. To imi- 

 tate a foggy atmosphere, I employed an infusion cloudy 

 with Bacteria and placed in a glass cell. With it the 

 beams could be toned gradually down to complete ex- 

 tinction. 



Note II. Fluorescence of Infusions, 



All the animal infusions, both flesh and fish, showed 

 the same fluorescence. It was the same green hue 

 throughout, though of varying degrees of intensity. 

 In wild duck, grouse, snipe, hare, partridge, and 

 pheasant the fluorescence was fine — sometimes exceed- 

 ingly fine. In rabbit it was less fine than in hare, and 

 in a tame rabbit less fine than in a warren rabbit. 

 Fishes also difi'ered from each other. Mullet, for 

 example, was finer than cod, herring, or haddock. 

 Beef, mutton, heart, liver, all showed the same green 

 fluorescence. 



Led up to it by a series of remarkable experiments 



