VOYAGE TO ALGERIA 



179 



powerfully condensed beam, its condition being revealed 

 through the light scattered by its suspended particles. 

 "Care is taken to defend the eye from the access of all 

 other light, and, thus defended, it becomes an organ of 

 inconceivable delicacy." Were water of uniform density 

 perfectly free from suspended matter, it would, in my 

 opinion, scatter no light at all. The track of a luminous 

 beam could not be seen in such water. But "an amount 

 of impurity so infinitesimal as to be scarcely expressible 

 in numbers, and the individual particles of which are so 

 small as wholly to elude the microscope, may, when ex- 

 amined by the method alluded to, produce not only sen- 

 sible, but striking, effects upon the eye.' 1 



The results of the examination of nineteen bottles filled 

 at various places between Gibraltar and Spithead are here 

 tabulated: 



Here we have three specimens of water, described as 

 green, a clearer green, and bright green, taken in Gibral- 

 tar Harbor, at a point two miles from the harbor, and off 



