Vision in health and disease; the value of glasses for its restoration, and the mischief caused by their abuse. Being the substance of lectures delivered at the Central London Ophthalmic Hospital / by Alfred Smee ... ; illustrated by one plate and numerous woodcuts.

Alfred Smee

Date:: 1847

Credit: Vision in health and disease; the value of glasses for its restoration, and the mischief caused by their abuse. Being the substance of lectures delivered at the Central London Ophthalmic Hospital / by Alfred Smee ... ; illustrated by one plate and numerous woodcuts. Source: Wellcome Collection.

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$at a given distance perfectly, whilst at other places it would he imperfect. It sometimes happens that the vertical refraction of the rays differs from the horizontal, so that the rays are brought to a focus at unequal distances. (90.) Sometimes double refraction exists, either horizontal or vertical, as though the refracting humours had two optical centres. I am acquainted with a gentleman of distinction, who has an affection of this character in both eyes, which causes him to see two objects when he looks at hut one, and a line appears double, like the last words of this paragraph :—■ The extent of separation varies in different cases. The extent of separation varies in different cases. (91.) Sometimes a single object appears double. According to the law of vision which I have already detailed, the defect always exists when the figure of the object is so thrown upon the retina, that the real position of it is not vertical to a tangent of the part of the nerve excited. Hence, whenever the optic axes are not acting in exact accordance, double vision ensues. When the axes of the eyes are not simultaneously directed to the same object, the affection is termed squinting, or strabismus; and whenever there is strabismus there will be double vision when both eyes are em¬ ployed. Nature, however, practically gets over the difficulty by neglect¬ ing the impression of one organ. A slight derangement of the stomach wall sometimes throw the eyes out of their true direction and cause squint¬ ing, Double vision may always be produced by gently pressing the eye out of its proper position with the finger. Double vision with one eye may occur if two apertures exist in the iris, or if the cornea be divided in the median line and the slit be not accurately joined together in the pro¬ cess of reparation. From my own experience, I should say, that, however ap¬ parently exact the juncture of a slit in the median line of the cornea may be, the conjunction, nevertheless, is never absolutely perfect. (92.) Treble vision may occur in one eye in certain cases where three apertures exist, or with two eyes if double vision exist in one, and the position of the object as seen by the other is not in its true place. It is, however, a state far more rare than that of double vision. (93.) In one or two instances I have met with manifold vision, the party seeing numerous objects where one alone existed. This state of vision would be invaluable to the miser, who values gold more for its quantity than its utility. (94.) In contradistinction to double, treble, and manifold vision, we have occasionally but half vision. The great Wollaston had once or twice this affection, for, in looking at the word “Johnson” over the door, he only saw son. This peculiar state sometimes occurs from indigestion, organic disease in the brain, or other causes. The case of Dr. Wollaston was one of internal hemiopia; that is to say, he saw with the external half of the retina, and not with the internal. But I have now a very curious case under$