Ger. Prithee hold thy peace.
We have now glanced at the principal illusions to which the senses of sight and hearing are liable, and the bearing which they have on the subject of spectral apparitions and other phenomena which it has been customary to regard as manifestations of the supernatural.
But a false appreciation of sensations excited by natural objects is not the only mode in which we are liable to be deceived, for we are apt to regard sensations excited by the action of the mind, or by a disordered condition of the nervous system, or both combined—subjective sensations—as sensations excited by natural objects—objective sensations.
To the erroneous perceptions arising from this source the term hallucination has been given, and the phantasmata to which they give rise are more important than those arising from illusions, since the judgment is often unable to correct them, and they may impose equally on the wisest and the most ignorant.
It is a law in physiology that a nerve of special sensation, (including in that term its central as well as its peripheral terminations,) in whatever manner it may be excited, can only produce that sensation to which it is appointed. Thus the nerve of sight, whether it be excited by natural or artificial light, or mechanical stimulus from without, or by morbid changes within, can only give rise to the sensation of light; the nerve of hearing, sound; the nerve of smell, odours; and so on.
If the ball of the eye is pressed upon (say by the finger at the inner angle) when the eyelids are closed, or the light otherwise excluded, certain luminous figures will be perceived. This arises from the pressure exciting the inner coat of the eye (the retina), which is formed principally by the expansion of the nerve of light (the optic nerve), and is the tissue in which the changes necessary for the production of the sensation of light are induced by the rays of light from without.
The luminous figures caused by mechanical excitation of this, the peripheral termination of the nerve of sight, vary in intensity in different individuals and at different times. They are sometimes very brilliant, and have been observed to be iridescent. In form they are circular, radiating, or regularly divided into squares, which have been compared by Purkinje to the figures produced by the vibrations communicated to a fine powder scattered on a plate of glass, along one edge of which a violin-bow is drawn; or to the rhomboidal figures formed on the surface of water in a glass, thrown into vibration by the same means.
A familiar illustration of the excitation of a sensation of light by mechanical stimulus is the brilliant sparks of light, starlike figures, &c., caused by a blow on the eye, or by a fall on the head.
A sensation of light may also be caused by the passage of a current of electricity through the eyeball; by mental emotion, as grief, passion, &c.; and by a morbid state of the brain or optic nerve. It is often also induced by a disordered state of the health, and under this condition the luminous appearance occasionally assumes a bluish, green, yellow, or even red tint.