INTRODUCTIOX. 11 



green opacity pervades them, which renders them not only 

 unpleasant to the observer, but unwholesome to the living 

 beings confined in them. This arises from the too rapid 

 development of vegetation ; germs of confervoid growth ac- 

 cumulate throughout the water, besides what settles down 

 in green and slimy incrustation on the rock work and 

 glass. When this happens, it is a sign that the light has 

 been admitted too directly and too freely ; for this growth 

 is developed by hght, and when that is shut off it soon 

 disappears. Light, therefore, in marine tanks should be 

 admitted sparingly and indirectly, and it has recently been 

 found that it is best to pass it through a coloured medium. 

 Thus, by far the best construction and position for a shore- 

 vivarium will be that already indicated, — a sloping back 

 towards the light, and a top of Uuisli-green glass. To 

 represent the comparative darkness of deep water, Mr. 

 AVarrington^s deep rectangular tank has the top and sides 

 darkened with a deepish blue. This gives a rather ghastly 

 appearance to the objects, but not more so than might be 

 expected from a visit, if we could pay it, to the same crea- 

 tures in their homes. Eresh-water iVquaria will admit mo re 

 light with impunity. 



