ELEMENTS.
proposed to apply the term phosphorescence to that curious property the nervous elements possess, of remaining for a longer or shorter time in the state of vibration into which they have been thrown by the arrival of external excitations—as we see phosphore scent substances illuminated by solar rays continue to shine after the source of light which has illuminated them has disappeared.
We know, indeed, now, thanks to the works of modern physicists, that the vibrations of the ether, in the form of luminous undulations, are capable of being prolonged by phosphorescent bodies for a longer or shorter time, and thus surviving the cause which has produced them.
Niepce de Saint-Victor, in his researches on the dy namic properties of light, has arrived at results much more precise and unexpected ; since, in a series of reports,* he has shown that luminous vibrations may be to some extent garnered up in a sheet of paper, and remain as silent vibrations for a longer or shorter period, ready to appear at the call of a revealing sub stance. Tnus, having kept in darkness some prints pre viously exposed to the solar rays, he, several months after this insulation, succeeded in demonstrating, by means of special reagents, persistent traces of the photographic action of the sun upon their surface.
On the other hand, the daily practice of photo graphic reproduction by means of dry collodion, is an irrefragable demonstration of the aptitude which certain substances gifted with special elective sensibility have for preserving persistent traces of the luminous vibra tions that have for a certain time affected them. In fact, when we expose a plate of dry collodion to the luminous rays, and several weeks after such exposure develop the latent image it contains, we produce a resur rection of the persistent vibrations and obtain a record of the absent sun ; and this is so true, in this case of persistence of a vibratory movement which has but a limited duration within which it must be seized, that if we pass the prescribed limits and wait too long, the movement gradually becomes enfeebled, like a source of heat which cools and ceases to be able to reveal its existence.
This curious property, which inorganic substances possess, of preserving for a longer or shorter period a species of prolongation of the impressions which have first set them in motion, is found once more under new forms, with special phenomena, it is true, but essentially the same, when we come to study the dynamic pheno mena of the life of the nervous elements.
These also are gifted with a sort of organic phospho rescence, and are capable of vibrating and storing up external impressions, of remaining for a certain time in a sort of transient catalepsy, in the vibratory state into which they have been incidentally thrown, and of causing the first impressions to revive after the lapse of time.
We all, indeed, know that the cells of the retina con tinue in a state of vibration after an excitation has ceased. It has been calculated by Platau that this per sistence of impressions may be estimated at from thirty two to thirty-five seconds.* To this persistence of vibrations, and that special retentive force which the nervous elements possess, is due the fact that two suc cessive and rapid impressions become confounded, and thus give a continuous impression : that a live coal whirled round at the end of a string gives the impres sion of a circle of fire : that a disc, painted with the colours of the spectrum, when in rotation gives only the sensation of white light, because all its colours are confounded and form for us an unique resultant, which is the idea of white. All those who occupy themselves with histology know that after prolonged work the images seen in the focus of the microscope live in the fundus of the eye, and that sometimes, after several hours' work, shutting one's eyes is sufficient to cause them to reappear with great distinctness.