some Manne Invertebrata. 185 



surface of the sea is tranquil, as ia a well-protected harbour, the 

 Noctiluca, because of their small specific gravity, form a conti- 

 nuous bed, and the least movement is sufficient to cover that dark 

 surface with a brilliant mantle. When the movement of a vessel 

 at once breaks in upon this mass of Xoctihica, and also calls out 

 their simultaneous phosphorescence^ the myriads of bright points 

 lying in the trough of the wave present one universal hue. From 

 a distance, the eye sees throughout a uniform brilliancy, and near 

 by distinguishes only the most brilhant scintillations, or those 

 thrown out by the animals at the immediate surface of the water. 

 These brilliant waves are like so many nebulae resolved by the 

 eye only in part. 



Third Part. — Observations and Experiments on the Light of 



the Noctilucse. 



[Instead of giving a full translation of this Part of the memoii', 

 as has been done of the preceding, we offer here an abstract pre* 

 senting in brief the conclusions of the author. — Eds.] 



1. In a sea rendered puosphorescent by Noctilucse, the light 

 proceeds only from the body of these Animals. — This proposition 

 is proved by direct microscopic examination ; and by the water's 

 being deprived of all light when the Xoctilucee are filtered out, 

 and becoming luminous again when they are restored to it. In 

 a tube of the seawater, the Noctiluca, if left quiet, soon form a 

 layer at the top of the liquid, and the light is confined entnely 

 to this layer of the animals. 



2. The production of light is independent of contact with the 

 air. — The flashes of light that are produced with the breaking 

 of every wave might seem to show that the access of the animals 

 to the external atmosphere was essential to the result. But on 

 the contrary, it is found by observation that in a vase of seawater 

 containing the JVoctilucce, the bed of these animals that collects 

 at the top of the base is equally luminous in every part. 



3. Colour of the light. — When the Noctilucce are in fvdl vigour 

 of life in quiet water, the colour is a clear blue. But on agita- 

 ting the water, or in the waves of the sea, the light becomes 

 nearly or quite white, or like silver sprinkled with some greenish 

 or bluish spangles. 



4. Intensity of the light. — M. de Tessan states that in some 

 tropical seas, the phosphorescence is so bright from the breaking 

 waves, that he could read ordinary type at a distance of fifteen 

 paces. The light from the Xoctiluae cannot compare \\-ith this. 

 At the head of the cove of the " Pare aux Huitres " at Boulogne, 

 it was not possible to tell the hour with a watch when the waves 

 were breaking at the observer's feet. With a tube 15 millimetres 



Ann. ^' Mag. N. Hist. Ser.2. Vol. xii. 13 



