VISION. — (Fr. vision, from Latin visio, from video, virus, sight.) — The faculty of seeing is one of the chief means by which living creatures are brought into relation with the world around, and is the especial means by which they are enabled to appreciate the wonderful pheno mena which flow directly and indirectly from the creation of light. When in obedience to the Divine command, " There was LIGHT," there were organs created for its perception ; and it is interesting to observe that the re storation of this gift of perception, when lost, was among the most frequent, and certainly not the least striking of the manifestations of miraculous power displayed by the Saviour of mankind. The vastness of the field over which the faculty of vision gives us command, the precision and permanence of this class of our perceptions, the variety and accuracy of the information it conveys, and the delight it affords, lead us irresistibly to regard it as the most perfect of our senses. In the in vestigation of this subject a train of minute adaptation and wonderful contrivance is dis closed to us, in which are combined the ex tremes of grandeur and of delicacy. There is no department of science that possesses a more absorbing interest than the laws of optics when applied to the eye, and certainly none which points with a steadier hand to the wisdom of an omnipotent Creator.
Very curious and unexpected information respecting the early condition of the surface of this planet and the ancient atmosphere has been afforded by an investigation into the structure of the organs of vision with which the earliest marine animals were supplied. In the eloquent language of Dr. Buckland, " with respect to the waters wherein the Trilobites maintained their existence throughout the entire period of the transition formation, we conclude that there could not have been that imaginary tur bid and chaotic fluid, from the precipitate of which some geologists have supposed the ma terials of the surface of the earth to be de rived : because the structure of the eyes of these animals is such, that any kind of fluid in which they could have been submerged at the bottom must have been pure and trans parent enough to allow the passage of light to organs of vision, th6 nature of which is so fully disclosed by the state of perfection in which they are preserved. With regard to the atmosphere also, we may infer that had it differed materially from its actual condition, it might have so affected the rays of light, that a corresponding difference from the eyes of existing Crustaceans would have been found in the organs on which the impressions of such were then received. Regarding light it
self, also, we learn, from the resemblance of these most ancient organizations to existing eyes, that the mutual relations of light to the eye and of the eye to light, were the same at the time when Crustaceans endowed with the faculty of vision were first placed at the bot tom of primteval seas as at the present mo m ent." the opinions of the ancients on the subject of light but little allusion need be made, as they were but crude and vague con jectures. One, for instance, supposed that the eyes emitted emanations of some sort by which objects were, as it were, felt. Others ima gined that visible objects were constantly throwing out from them spectral resemblances of themselves, which, when received by the eye, produced an impression of those objects; but in these fanciful notions there is little satisfaction, and we proceed at once to the hypothesis of our illustrious countryman, Sir Isaac Newton. According to his theory, light was an imponderable substance, whose inconceivably minute particles produced the sensation of light by their action on the eye : moving with immense velocity, they were nevertheless acted on by attractive and repul sive forces residing in all material bodies, and by these forces were liable either to be turned aside from their natural straight course, re fleeted by the repulsive force, or penetrating between the particles of bodies, to take a direction on quitting them finally determined by the position of the surface at which they emerged. About the same time, however, a very different hypothesis was advanced by Huyghens, to the effect that all space is filled with an extremely elastic and rare ether, and that light is the result of the undulatory movements communicated to this ether by self-luminous bodies, which movements being transmitted to the optic nerve, give rise to the, sensation of light. The beautiful experi ments of Dr. Thomas Young strongly con firmed the truth of this theory, which is based upon the supposition that light acts by vibra tions upon the retina, in the same manner as the undulations of the air striking upon the tympanum excite the sensation of sound.