At the present day it has become a difficult matter to classify the various aquariums of Europe, since they present so wide a range in size, quality, and purpose. Some are destined solely for the public use and can not be said to be of aid to scientific studies; others are devoted almost entirely to the advancement of biological research; and others still vary widely between these extremes, with an equally wide range in the character of their financial support. Many of the aquariums of the orthodox biological stations, however, have been situated in out-of-the-way places, convenient for the purposes of the student, but inaccessible to the general visitor. These may be admirably arranged and maintained—among them, for example, the aquarium room of the French station at Banyuls, on the Mediterranean, near Spain—yet they can not be strictly regarded as belonging to the class of public aquariums. For this reason, partly, more than a score of biological laboratories might at once be omitted from discussion. On the other hand, the Stazione Zoologica of Naples, while devoted to the highest type of research work, must be given the foremost rank among popular aquariums. And the Amsterdam Aquarium, holding rank on the popular side probably second to Naples, is also of value as a purely scientific station, although lower in caste than the Stazione. So, too, should the Plymouth Biological Laboratory be mentioned as of interest in its well-equipped aquarium. Together with those that have just been mentioned, the more strictly popular aquariums of Europe should include those of Paris, Berlin, and Brighton.
These aquariums are so widely separated from each other that they have come to differ not a little in the details of their equipment and management. And it is, indeed, only when the visitor has examined a number of these institutions that he begins to realize that there is a common principle underlying their general construction. Thus, for example, he would find in each the great darkened corridor, from which on every side, as through large windows, he may look into the brightly lighted tanks. Through these he may peer to a distance of twenty feet before his view is stopped by the rough, rock-cut background; nor does the line showing the surface of the water appear against the glass to destroy his illusion of ocean depth. The cunning builders have taken pains to have this line higher than the windowlike opening of the tank, so that the water surface, instead of marring the effect, in reality aids it, for the eye of the visitor, at a lower plane than the surface, sees upward but the totally reflected images of the forms below. Not that the glass fronts of the aquaria are exceedingly large—their height is rarely more than four or five feet in view of the danger of breakage through water pressure—although the idea of water depth is certainly not
