The exquisitely sensitive condition of the nervous coat known as the retina, is such that it appreciates a vibratory movement of the one-trillionth part of an inch in a second. These vibrations of the molecules of the medium of light convey to the brain all the forms and conditions of the surrounding universe. The photographic impression on the sensorium is permanent or transitory, in accordance with the state of existing conditions. During the early periods of life, the brain is much more sensitive to impressions than in after-life. So permanent are these impressions, that they never can be effaced. They may be modified or toned down by after experience or education, still it is these first impressions of early youth which form the basis on which all our subsequent ideas are built.

The first are the most simple impressions; they are conveyed through the optic nerve, in the same vibratory manner, to the brain, which is. constituted to receive the various images, and it is only by a reproduction of the identical vibratory state, that the conscious memory or the history of our past experience is reproduced.

It must be admitted, that as the external sense of vision is so sensitive as to receive the vibrations of colour or waves per second in a single inch of space without confusion, the brain itself, of which the eye is merely the servant, must at least be equally sensitive.

The following enormous numbers show the impulses per second on the retina: Extreme red........399,101,000,000,000

Soda yellow........509,069,000,000,000

Extreme violet.......831,479,000,000,000

The only analogical approach to this sensitive state is the iodized silver plate to the action of light; it requires the minutest fraction of a second to impress an image. The vitalized electrical effects far transcend any of the purely chemical phenomena of which we are cognizant. The functions of the nervous system are so rapid, that they cannot be measured by any ideas we possess of time. There are states of nervous sensibility so great that permanent impressions take place from causes apparently the most remote.

No surface with which we are acquainted can compare in point of extreme sensitiveness with that of the child in utero, when it is surrounded with fluid, the liquor amnii. Then every shock or impression which affects the mother's brain is vibratorily conveyed to the child; that is, the fluid receives the shock, and these vibrations represent the forms or images of things taken in through the sense of vision. Under ordinary or normal states of the mother, these influences have not a marked effect, as there is no nervous connexion. It is only when the brain of the mother receives a violent impression that it is, as it were, reflected or thrown off, the consequence being that the whole system receives the shock, and that portion which is most sensitive, and in a manner independent, receives the image.

We know that the filaments of the auditory nerve are suspended in a liquor which vibrates or undulates more than 8000 times per second in conveying to the brain high notes of sound; so that here is an analogous condition to that in which the liquor amnii is thrown when the nervous system of the mother has received an impression so severe as to be thrown off from the brain.

It is not to be supposed that this vibratory condition of the fluid is limited to 8000 per second, it may be 8,000,000. But the fact of the fluids being instrumental in the conveyance of impressions, is here demonstrated as being a functional property of the animal economy. The transmission of an image to the brain through the sense of sight, is effected by the undulations of the ethereal medium, as exemplified in the action of light on the retina. The eye is a perfect camera obscura; the same arrangement of lenses, the identical mode of producing the picture. The choroid coat represents the ground glass on which the picture is focused, from which it is vibrated through the optic nerve to the brain, which is a mass of nervous molecules arranged in a fibrous form. It is the vibration of these molecules in their various combinations which produces the thinking power. Thought is the motion of these particles of nervous matter charged with vitalized electricity. It must not be supposed that the eye alone is impressed with the sense of light; there are animal existences whose surface is so delicately and sentiently endowed that they are cognizant of light, though they possess no visual organs. These may even be capable of appreciating and detecting the forms and conditions of matter.

For example, the skin of the proteus anguinus, is so delicate as to be affected painfully on exposure to light; it is also aware of the approach of other living matter at comparatively remote distances by the sense of touch. The fact of the existence of the picture on the retina, shows the necessity for its existence, such an embodiment of the object seen being essential to mental or brain vision. And when it is remembered how extremely rapidly these transitions take place, that light travels at the rate of 186,000 miles per second, and that the waves or vibrations of violet are on a single inch 70,555, we arrive at the enormous figures of 831,479,000,000,000 impulses on the retina in a second. It demonstrates a sensitiveness which we have no parallel to reason from. That numberless variety of movements or vibrations of the nervous molecules take place corresponding with the impression on the senses, it is impossible to doubt.

In fine, it is not a speculative conclusion that the brain must receive impressions from the senses in harmony with the physical vibrations of light, sound, taste, smell, and touch. In order to do so, its anatomical construction must be such as to respond to these vibratory communications and impressions.

Memory is the reproduction of the vibratory configuration representing the original images conveyed to the brain. It is the combination and recombination of these vibratory movements which produce thought.