Page:Popular Science Monthly Volume 1.djvu/780

760 first nearest approach the radius of either star's nebulous envelop was greater than the distance between the centres of the two bodies, the atmospheric resistance would tend to transform the parabola or hyperbola, in which the body was moving, into an ellipse. Each subsequent return would shorten the period, until, in the process of cooling, the stellar atmosphere had so far contracted as no longer to involve any part of its companion's.

It would be an interesting question whether some of the double stars, whose apparent distance apart has seemed too great to justify the hypothesis of a physical connection, may not afford other instances of motion, either as parabolas or hyperbolas.

School-Life and Eyesight.—In a communication to the Mechanics' Magazine, Prof. Liebreich describes the injuries to the eye incident to school-life, pointing out their causes, and the means to be adopted to avoid them. The changes in the functions of the visual organ, developed under the influence of school-life, are three in number: First, decrease of the range of vision; second, decrease of the acuteness of vision; third, decrease of the endurance of vision.

Decrease of the range, short-sightedness (myopia), is developed almost exclusively during school-life rarely afterward, and very rarely before that time. There is often an inherited predisposition to become short-sighted, and this is developed during school-life, more or less, according to certain external conditions. It is a common notion that short-sighted eyes are the most durable. This is founded on the fact that such eyes can see near objects distinctly without the aid of glasses, at an age when normal eyes require the assistance of convex lenses. But this is no proof of their durability. On the contrary, a high degree of short-sightedness is a diseased condition, caused by anatomical changes in the membranes of the eye, which involve a greater tendency to serious complication than the normal eye. Short-sightedness exerts an injurious influence on the general health by inducing the habit of stooping; and, from a national point of view, its increase is to be considered a serious evil.

Decrease of acuteness of vision (amblyopia) is a serious condition, generally the result of positive disease of the eye, which may exceptionally be induced at school. Amblyopia of one eye only, is, however, often produced by unsuitable arrangements for work, which disturb the common action of the two eyes, and weaken the eye which is excluded from use.

Decrease of endurance (asthenopia) is a frequent affection, that has destroyed many a career, prevented the development of many a fine intellect, and deprived many of the fruits of their laborious exertions. It arises principally from two causes: the first is a congenital condition, called hypermetropia, which can be corrected by convex glasses, and which cannot, therefore, be laid at the door of school-life; the second is a disturbance in the harmonious action of the muscles of the eye—a defect which is difficult to cure, and which is generally caused by unsuitable arrangements for work.

These three anomalies all arise from the same circumstances, viz., insufficient or ill arranged light, or from a wrong position during work. Where the light is insufficient, or badly arranged, we are obliged to lessen the distance between the eye and the book while reading or writing; and we must do the same if the desks and seats are not of the right shape and size, and suitably located. When the eye looks at a very near object, the accommodating apparatus, and the muscles which turn the eye, are brought into a condition of extra tension, and this is to be considered as the principal cause of short-sightedness and its increase. If the muscles of the eye are not strong enough to resist such tension for any length of time, one of the eyes is left to itself; and, while one eye is being directed on the object, the other deviates outwardly, receives false images, and its vision becomes indistinct (amblyopia). Or, perhaps, the muscles resist these difficulties for a time, become weary, and thus is induced the diminution of endurance (asthenopia).

In order to prevent these evils, the light must be sufficiently strong, and fall on the table from the left-hand side, and, as far as possible, from above. The children ought to sit straight, and not have the book