The conditions most favorable for respiration produce the greatest exhibitions of light; thus, if the water is constantly aërated, or disturbed so that the air has access, the gleam is intensified. If the animal is touched with the point of a needle, the light is quickly visible; and just before death it is continuously luminous, the phosphorescence disappearing just after dissolution. Experiments have shown that in a vacuum the light diminishes,--carbonic gas producing the same result. Humboldt refers to his luminous appearance after bathing in water abounding in _Noctilucæ_; and among the curious experiments might be mentioned one where print was read by a gobletful of these little creatures which rendered them living lamps, literally. M. de Tessan, a French observer, has recorded a phenomenon, which, I should judge, was due to _Noctilucæ_, with perhaps the additional light of other forms. The accompanying picture on plate II. was made from his description, showing the light, and people upon the shore endeavoring to read by it. He writes: “On the 10th of April, in the evening, the sea in the roadstead of Simonstown, Cape of Good Hope, presented an extraordinary phosphorescence of a most vivid character. At whatever points the phosphorescence was greatest, the water was colored on the surface as red as blood; and it contained such an immense quantity of little globules that it had the consistency of sirup. A bucket of water taken up at one of these points, and filtered through a piece of linen, left in the filter a mass of globules greater in volume than the water that had passed through; in other words, the globules constituted more than half of the whole quantity of sea water taken up in the bucket. Viewed under the magnifying-glass, these globules presented the appearance of little transparent and inflated bladders, having on their surface a black point surrounded with equally black radiating _striæ_.... The least agitation or slightest contact made them throw out a vivid greenish light.” [Illustration: PLATE II. M. DE TESSAN READING BY LIGHT OF PHOSPHORESCENT SEA.] As the waves washed in, M. de Tessan describes the light as appearing like the vivid flashes of lightning. “It lighted up the chamber that I and my companions occupied in the house of Mr. Ball, though it was situated more than fifty yards distant from the breakers. I even attempted to write by the light, but the flashes were of too short duration.”