In our own seas the luminosity is undoubtedly caused principally by the presence of myriads of minute floating or free-swimming organisms that inhabit the surface waters. Of these each one has its own season, in which it appears in vast numbers. Some appear to live entirely at or near the surface, but others apparently remain near the surface only during the night, or only while certain conditions favourable to their mode of life prevail. And further, it is possible that these minute creatures, produced as they generally are in vast numbers at about the same time, and being more or less local, are greatly influenced by changes of temperature, changes in the nature of the wind, and the periodic changes in the tides; and it is probable that we are to look to these circumstances for the explanations of the sudden changes so frequently observed. In warmer seas the phenomenon of phosphorescence is much more striking than in our own, the brilliancy of the light being much stronger, and also produced by a greater variety of living beings, some of which are of great size, and embrace species belonging to the vertebrates and the higher invertebrate animals. Those interested in the investigation of this subject should make it a rule to collect the forms of life that inhabit the water at a time when the sea is unusually luminous. A sample of the water may be taken away for the purpose of examination, and this should be viewed in a good light, both with and without a magnifying lens. It is probable, too, that a very productive haul may be obtained by drawing a fine muslin net very slowly through the water. After some time the net should be emptied and gently washed in a small quantity of sea water to remove the smaller forms of life contained, and the water then examined at leisure. Of course it must not be assumed that all the species so obtained are concerned in any way with the phosphorescence of the sea, but any one form turning up in abundance when collected under the conditions named will probably have some connection with the phenomenon.