SECTION II.
Application of Galvanism to the organs of hearing and of sight.
By applying Galvanism to different parts of the face, a flash of light is excited in the eyes, which is stronger or weaker according to the nature of the parts to which it is directed. These organs, though delicate, are always affected in such a manner that the mechanism of the eye sustains no injury from the metallic arcs when they are made to communicate with only two plates of different metals. I received an account of this phænomenon at Milan, several years ago, from the celebrated Volta, who produced it by applying a conductor of zinc in such a manner, that one end of it touched the bulb of the eye, and the other the tip of the tongue armed with a plate of tin. In administering Galvanism for diseases of the eyes, it is much better to employ the pile. To excite the appearance of a flash of light, it is not necessary that the eyes should be open; it takes place even if they are shut, and covered with a bandage in a darkened apartment. Some of the partisans of Euler, perhaps, may here maintain, that this phænomenon of Galvanism is a deception, and that no light can be really excited in such cases, as the production of light depends on the emanation
of rays from a luminous body which penetrate into the interior part of the eye. But without entering into the examination of this question, which is foreign to my principal object, I shall only observe, that long before the theory of Galvanism was known, the celebrated Darwin had found that when the eyes are shut, there are certain internal stimulants which are capable of producing the appearance of light and of colours.
In no case is the difference between Galvanism and electricity more apparent than in the application of the former to the sight. Very little benefit had been obtained by means of a metallic point inclosed in a glass tube, which directed the electric charge from a jar to the cornea; and besides, the shock of a Leyden jar applied directly to so delicate an organ as the eye could not but be attended with danger. Galvanism applied to the exterior part of the eye, in the same place where a stream of electricity would have had no influence whatever, has always produced a certain effect on the organ of sight, as is proved by the following experiments.
EXPERIMENT I.
If you touch with one hand the bottom of the pile, and at the same time apply to the summit different parts of the face moistened with salt water, a flash of light will be excited in the eyes. The same result will be obtained, if instead of touching the bottom of the pile with the hand you touch
it with the sole of your foot. No flash of light is observed when the Leyden flask is employed in the same manner.
EXPERIMENT II.
Having observed the preceding phænomenon in myself, and excited it in others, I was desirous of proving it in regard to several persons at the same time during the course of my public lectures, and for that purpose I made use of the following apparatus. I arranged two metallic plates in a horizontal position, at the distance of nine inches from each other, so that six persons with their hands dipped in salt water could touch the lower plate, and the upper one with the tips of their tongue. A charged Leyden flask placed between the two plates being then discharged, the whole of the persons experienced a violent shock, but perceived no flash of light. It is well known however that, when a similar arc is formed with the interposition of a very strong pile, a flash of light is constantly observed; though the force in the second case is much less than that excited by the explosion of the Leyden flask. The same result will be obtained by bringing the upper plate into contact with the nose.