The so-called phosphorescence of most inorganic bodies is one of a totally different nature from that exhibited in organic forms.  The diamond shines for a time in the dark after it has been exposed to the sun; so do pieces of quartz when rubbed together, and powdered fluor-spar when heated shines with considerable brilliancy.  Various artificial compounds, such as sulphide of calcium (Canton’s phosphorus, as it is called from the discoverer), sulphate of barium (Bologna stone, or Bologna phosphorus), sulphide of strontium, etc., after being illuminated by the rays of the sun, give out in the dark a beautiful phosphorescence, green, blue, violet, orange, red, according to circumstances.  The luminous paint which has recently attracted so much attention is of the same nature.  In these cases what we have is either a conversion of heat rays into light rays (as in the powdered fluor-spar), or the absorption and giving out again of sun-rays.  In the latter case the phenomenon is essentially the same as fluorescence, in which the dark rays of the solar spectrum beyond the violet are made visible.

But we must now return to the other questions that have been started in relation to phosphorescence in animals.  There has been much speculation as to the object of this light, and to the purposes it serves in Nature.  Probably no general answer can be given to this question.  It is no doubt impossible to show why so many animals have been endowed with this remarkable property; but we may consider some of the effects which the possession of it has in different cases.

In the first place, it will undoubtedly serve in many cases to afford light to enable the animal to see by, and in the Lampyridae it would seem that the degree of luminosity is related to the development of the vision.  In that family, according to the Rev. H.S.  Gorham, the eyes are developed, as a rule, in inverse proportion to the luminosity.  Where there is an ample supply of this kind of light the eyes are small, but where the light is insignificant the eyes are large by way of compensation.  And moreover, where both eyes and light are small, then the antennae are large and feathery, so that the deficiency in the sense of sight is made up for by an unusual development in the organs of touch.

But it is none the less certain that the presence of this light cannot always be designed to serve this purpose, for many of the animals so endowed are blind.  The phosphorescent centipedes are without eyes, like all the other members of the genus (Geophilus) to which they belong, and probably the majority of phosphorescent marine forms are likewise destitute of organs of sight.