tion, and then a downward course in a long curve. The specks assume different shapes; sometimes they are mere dark points, at other times they consist of a series of dark points held together in lines and angles like a loose web or net-work. Again they appear as rings, and not unfrequently as disks with a dark centre, resembling the magnified red corpuscles of the blood so closely that they might pass for them. Rings of this cellular shape occur singly, or in line, or in broken network. Musca volitantes appear in yet another variety of shape, namely, in straight, dark lines, arranged in zigzag forms, with the dark edges slightly illuminated. Muscæ are, as a rule, seen most frequently in the sunlight, and as if at some yards distance. They appear greatly exaggerated in a fog, and then seem to be within arın's-length of the observer. In extreme instances they are visible by gas and candle-light. They do not indicate actual organic disease of the structures of the eye, but are supposed to be from débris of cells. floating in the vitreous humor.

Albinism.-A condition of disease existing in persons who are often called Albinos, and in whom the dark pigmentary substance of the eyeball at the back of the iris and in the middle or choroid coat of the eye is not duly secreted. In albinism the ere is seen of a red color within the pupil, and vision, during daylight, is extremely embarrassing and painful. The disease is hereditary, and is peculiar to some families of men and to some lower animals.

Diseases of the Crystalline Lens and its Capsule.—Cataract.

The word cataract is an unfortunate but long-used term for defining opacity of the crystalline lens of the eyeball. In cataract the lens becomes white and opaque at some point of its surface, or in the capsule which envelops the albuminoid substance,globuline, of which the lens is composed. The opacity extends until, at length, the whole of the lens may be involved, and blindness rendered complete. The process of development is usually slow in its progress, except in the case of saccharine or diabetic cataract, which may be developed in a few days. The process has been called the "ripening" of cataract by the surgeons, who have an operation for the removal, or depression, or destruction of the opaque lens, and who are accustomed to wait until the cataractous change is complete or ripened before they

perform the operation. A few years ago Dr. Weir Mitchell of Philadelphia discovered that cataract could be artificially produced by charging the blood of an inferior animal with sugar. In continuation of his most important research, I observed that the same condition could be produced by nearly all crystalloidal substances, salts, which would enter the blood and increase its specific weight.

There are several varieties of cataract :

Hard cataract, in which the lens has become firm, condensed, and dark. The cataract of the aged.

Soft cataract, in which the lens is of bluish-gray tint, sometimes of fractured appearance and swollen or large. The cataract of all ages of life.

Fluid cataract, in which the lens is more aqueous than in soft cataract.

Traumatic cataract, in which the disease has occurred from an injury to the eyeball.

Diabetic or glucose cataract.-A cataract in which the disease is produced by the presence in the blood, in large quantity, of glucose or grape sugar.

Congenital cataract.-A form of soft cataract in which the opacity is developed at birth.

The tendency to cataract is hereditary, but the disease does not generally occur until late in life. Cataract is common to the lower animals, herbivorous and carnivorous, as well as to man.

Diseases of the Vitreous Body.

Synchysis.-The most distinct affection of the vitreous humor is synchysis, in which the membranous cells are broken up, and the humor, unnaturally fluid, is charged with minute scales. The eyeball is soft, and the iris, or curtain, hangs flaccid. Before vision is lost, bright starlike spots, due to the floating scales, are constantly seen. There is no chance, according to our present experience, of recovery. The disease may be due to a blow, or to slow inflammation, or to wounds inflicted on the eyeball during operations.

Morbid deposits.-The vitreous humor is the seat of morbid deposits. Flakes, or débris of its own membranous structure are the commonest deposits.

GENERAL AFFECTIONS OF THE EYE, AND DEFECTS OF SIGHT.

Glaucoma.

Glaucoma is an affection in which the globe of the eye becomes large, dull, and tense, with dilated fixed pupil, greenness of color beyond the pupil, and excavation of the optic disk. The disease, attended with much pain followed by loss of vision, exists in two varieties, the acute and the chronic. It is rather a common affliction, occurring more frequently in men than in women, and not usually commencing before the period of middle life. It has an hereditary character, afflicting those who are of worn or feeble constitution chiefly. It is frequently connected with a gouty or rheumatic constitutional taint.

Hydrophthalmia.

Hydrophthalmia is dropsy of the eyeball from an accumulation of the fluid secreted in the space between the inner surface of the cornea and the crystalline lens, in which space, called the anterior chamber, the iris or curtain floats. In this disease the eye is large, tense, and staring, and the pupil is dilated. The cornea may or may not be rendered opaque, and vision, if not lost, is much impaired. The disease is due to irritation of the lining membrane of the anterior chamber.

Weak Sight. Asthenopia.

Weak or enfeebled sight, though a term commonly used, is not easily defined as connected with any special change in the eyeball. It may be present from an irritable condition of the retina; from a fluid state of the blood; from a want of power to accommodate the eye to distances; from nervous anxiety about the sight; or, from the commencement of one or other of the diseases of the structures of the eyeball. Most commonly, I think, the term weak sight is applied to one of the two defects of vision that come next before us, myopia or presbyopia, before they are fully developed or while they remain undetected.

Short Sight. Myopia.

A condition of vision in which, from the greater convexity or the position of the lenses of the eyeball, the natural focus of

vision is not maintained. The rays of light being focussed before they reach the retina, objects have to be brought nearer to the eyes than is natural or convenient, in order that they may be correctly seen. It has lately been pointed out by Mr. Liebreich that this condition may be induced in children by the habit of leaning forward, with the face almost on the paper, in the process of learning to read or write. The disease is of hereditary char

acter.

Long Sight. Presbyopia.

A disorder of vision, in which the conditions are the opposite from those which are present in myopia. The focus of vision is so deranged that objects have to be held at a distance inconveniently away from the eyes in order to be distinguished naturally. The disease is common to old age, hence the term presbyopia. Another term, hypermatropia, has been introduced to define true long sight. It is a common cause of asthenopia or imperfect vision.

Color Blindness.

A condition in which, from defect of vision, a person cannot distinguish colors correctly, nor tell one color from another. Dr. B. Joy Jeffries of Boston defines the condition as an inability to distinguish one of the basic colors "of the spectrum, red, green, and violet, from gray. A person may be color blind in any one of these colors, red blindness being most common, green next, and violet least." The same author computes that one person out of every twenty in the community is affected with color blindness. At one time it was assumed that the diseased state leading to color blindness was confined to the retina, but it is now known to be connected also with changes of disease in the optic nerve, or even in the brain itself. It can be produced by nervous shocks, and, strangest, perhaps, of all, it can be induced by some chemical substances taken into the body, specially by santonine and by alcohol. The disease is hereditary, and up to this time seemingly incurable in those who inherit it. The hypothesis as to its nature is that in the retina there is a corresponding nerve centre for each basic color, red, green, and violet, and that the color blindness is due to disease affecting these centres.

Astigmatism.

The term Astigmatism, irregular refraction, is applied to a defect of sight, by which the natural acuteness of vision is to some extent lost, owing to different refraction from different meridians of the eye; so that "rays of light derived from one point do not again unite into one point." The deviation of sight is monochromatic, that is to say, an aberration of rays of the same color. The cause of the aberration lies in the cornea or in the crystalline lens. There are two varieties, the regular and the irregular. In the regular variety the cornea is chiefly the seat of the irregularity. In the irregular variety the crystalline lens is the seat of the irregularity. Astigmatism is now very efficiently remedied, in many cases, by the scientific adaptation of artificial lenses or glasses.

Day and Night Blindness.

Day blindness is a term used to describe a disease in which the person affected can only see when the light is subdued, as at night. Night blindness, on the other hand, means a condition in which there is blindness except in full light. The causes of the two conditions are often vaguely defined, but night blindness is known to visit those, chiefly, who have been exposed to intense or tropical light. The term Nyctalopia is sometimes used to express the former and Hemeralopia to express the latter affection.

DISEASES OF THE LACHRYMAL APPARATUS AND EYELIDS.

The lachrymal gland placed at the upper and outer angle of the eyeball is subject to inflammation and to resultant structural changes. But the most common disease connected with the lachrymal apparatus is obstruction of the lachrymal duct, the duct which runs from the inner corner of the eyeball downwards into the nasal cavity, and which conveys the tears from the eyelids into the cavity of the nose to be dispersed there. The lachrymal duct, when obstructed, fails to allow the watery secretion to pass through it, and the tears then flow in continual current over the lids on to the cheek. Acute inflammation of the lachrymal sac, -lacryocystitis;-chronic inflammation of the sac; and abscess and fistula are other diseases of the lachrymal system.