108. We will add to the preceding considerations the examination of some optical effects which have given rise to the singular system of globular spirits. The air is not always of an absolute limpidity, and there are conditions under which the currents of aeriform molecules and their agitation produced by the heat are perfectly visible. Some persons have taken that for masses of spirits moving around in space ; the mere mention of this opinion is all that is necessary to refute it ; but there is another species of illusion, no less absurd, against which it is equally well to be forewarned.
The aqueous humor of the eye offers points, scarcely perceptible, that have lost their transparency. These points are like opaque bodies in suspension in the liquid, and whose movements they follow. They produce in the air, and at a distance, from the effects of enlargement and refraction, the appearance of small disks varying from one to ten millimetres in diameter, and which seem to swim in the atmosphere. We have seen persons take these disks for spirits, who follow and accompany them everywhere, and in their enthuse asm take for figures the shades of irisation, which is almost as rational as to see a figure in the moon. A simple observation, furnished by these people them selves, would bring them to the land of reality.
These disks, or medallions, they say, not only accompany them, but follow all their movements ; they go to the right, to the left, up, down, or stop, according to the movement of the head : that is not astonishing, since the seat of the appearance is in the globe of the eye ; it should follow all its movements. If they were spirits, it must be admitted that they would be confined to entirely too mechanical a part for free and intelligent beings — a very tedious part, even for inferior spirits, and certainly entirely incompatible with our ideas of superior spirits. Some, it is true, think the black points bad spirits. These disks, the same as the black spots, have an undulatory movement which never varies from a certain angle, and their not rigidly following the line of vision adds to the illusion. The reason is very simple. The opaque points of the aqueous humor, primary cause of the phenomenon, are, as we have said, held, as it were, in suspension, and have always a tendency to descend ; when they ascend, it is in consequence of the movement of the eye from low to high ; but, after reaching a certain distance, if the eye is fixed, the disks descend of them selves, then stop. Their mobility is extreme, for an imperceptible movement of the eye . is sufficient to make them change their direction and traverse rapidly the whole extent of the arc in the space where the object is produced. So long as it is not proved that an image possesses a spontaneous and intelligent movement of its own, there can be seen in it but a simple optical or physiological phenomenon. It is the same with the sparks, which are sometimes produced in sheafs and bundles, more or less compact, by the contraction of the muscles of the eye, and which are, probably, owing to the phosphorescent electricity of the iris, as they are usually limited to the circumference of the disk of that organ. Similar illusions can only be the result of incomplete observation. Whoever may have seriously studied the nature of spirits by all the means practical science gives, will understand their puerility. While we combat the theories by which the manifestations are attacked, when these theories are based on ignorance of facts, we should also seek to destroy the false ideas which exhibit more enthusiasm than reflection, and which, in that very way, do more harm than good with the skeptical, already so disposed to look for the ridiculous.