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.'* 



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 



