This very comprehensive term includes a variety of very different diseases; and we must here consider not only imperfect or depraved vision, but the causes of the total loss of sight. Imperfect vision proceeds from many sources. We have noticed, in different parts of the work, that which arises from the rays of light converging before they reach the retina, or beyond it; the species occasioned by diseases of the lids. and obtuscations or ulcers on the cornea; those which arise from opacity of the lens, and from a palsy of the optic nerve. Little.therefore remains but to notice partial transitory obstructions, or imaginary appearances. In nervous diseases the sight is sometimes for a time lost; and though this deprivation occurs without danger, and is temporary only, yet we ought to reflect that it is often the forerunner of a fatal apoplexy or a palsy. The muscae volitantes, as they are called, motes floating before the eye, and for a time obscuring the sight, are equally signs of an approaching cataract. Yet, after a strong light, or from transitory debility, they occur with little danger or permanence. There is another imperfection of vision from fulness of blood; and in this case the sight is obscured by lines, with apparent intersections. An author, whose name has escaped us, mentions it as an impression on the retina, from the passage of the blood through vessels not usually conveying the red globules. We remember the disease occurring in a man who could represent his sensations by a pencil. He drew the figures that appeared to him. and they formed an exact representation of the circulation of the blood, as seen through a miscroscope. Bleeding and low diet completely removed this complaint.

There are many imaginary appearances in the eyes, not only from fulness, but from nervous affections. Double vision is not uncommon: to see objects inverted is an occurrence not indicative of any considerable disease. False representations are generally morbid: to see angels round the bed, wild beasts with open mouths ready to devour, flames curling round and scorching, are the effects of fever, or an imagination greatly disturbed. The organ is not affected; but the impression on the sensorium is not consonant to that on the nervous extremities, or the associated idea is stronger than that from the impression.

Blindness is seldom complete. Strong lights are often perceived; the forms of objects not uncommonly: but the colour, the shade, and the minuter forms, are in many instances imperceptible. In this case every sense is alive to supply the imperfection. Spalanzani has shown, that a blinded bat can avoid objects in its way; and we know, from the blind people who can describe their feelings, that they can distinguish a crowded from an empty room; one furnished from another unfurnished; windows opening to the country or a street; tall from short persons; and even in a theatre, an able and judicious actor from a pretender to the art. The feelings, the breathing, the hearing, in short, a combination of all the senses, almost a new sense, contribute to their information. It has been supposed that the blind can distinguish colours by feeling, but this is not true. We remember Dr. Moyes observing, that an old blind man was brought to him who professed to distinguish colours. He had been a dyer; but in his determinations he was often wrong, and when correct, was assisted by the smell. Dr. Reid, in his 'geometry of Visibles,' endeavours to show what ideas a blind man would entertain of different objects, though with little success. People blind from infancy have been restored to sight; and we might suppose that from their observations much might be collected. We have, however, only two well authenticated instances of persons restored to sight, who never remember to have seen. One, the case so often quoted from Mr. Cheselden; the other, more lately, in the Philosophical Transactions by Mr. Ware. They unfortunately differ in many respects; but we must be allowed to hint our suspicions, that Mr. Ware's patient must have remembered seeing, for he knew a cloth on the table to be green. This indeed he might have heard, but he ascertained the distance: this a blind man could not have done. Had he in any instance been able to distinguish objects either from their brilliancy or their shape, this faculty might have been acquired, but in no other way. We remember seeing an account of the feelings of another person who never had seen, restored to sight by the extraction of a cataract, in the papers of an old surgeon. He could not distinguish distances; and when carried near a river, was eager to walk on that beautiful plain. The various resources for the amusement of blind persons, and their mode of assisting their acquisition of the different abstract sciences, are scarcely a part of our subject. Perhaps we have already strayed from it; but literally, in our situation, nihil humani nobis est alienum.