Popular Science Monthly/Volume 16/February 1880/Daylight in the Schoolroom
|DAYLIGHT IN THE SCHOOLROOM.|
By M. JAVAL.
WE are all agreed in preferring the light of day to any other. In spite of the extreme variations which take place in its intensity and sometimes in its coloring, we seldom think of modifying it or softening it for healthy eyes except when they are exposed to entirely unaccustomed conditions. The eye is capable of accommodating itself to most astonishing changes in the brightness of light. The light of the sun is about a million times more intense than that of the full moon; yet the eye can distinguish objects by the light of a star. The changes in the diameter of the pupil contribute only in a small degree to the faculty of adaptation, for, between the extreme dilatation and contraction of the iris, the sensitive surface does not vary in a greater proportion than that of one to a hundred. The power resides chiefly in the retina, the sensibility of which is blunted in day-light and intensified in darkness. In consequence of this remarkable aptitude, the eye is the reverse of a good photometric apparatus. Enormous changes in the intensity of light pass unperceived by it, and we are able to attend to our occupations undisturbed by the fluctuations which are constantly taking place.
Still, we must not demand of our organs the maximum of adaptation of which they are susceptible. If we read a book with the sun shining directly upon it, even if we do not injure the eyesight, we will disarrange the rate of adaptation, so that we will not be able for some time to see in a demi-obscurity. On the other hand, if we stay long in the dark, we may increase the sensibility of the retina so that a sudden return to daylight will be painful. Bearing these facts in mind, we should keep the direct rays of the sun out from workshops and schoolrooms, where the place of each person is fixed, and should not make our bedrooms too dark, lest the eye be worried by sudden changes. On a similar principle, we should flood with diffused light the rooms in which numerous workmen are to be gathered, some of whom must be far from the windows. With a good light, or one which is equivalent to several million candles a yard off, we use in reading only a fraction of the cornea, and the contraction of the pupil has the effect of greatly diminishing the diameter of the circles of diffusion which are liable to produce in the retina faults of vision. Under these conditions, a badly formed eye may perform good service, and is subject to only a moderate degree of fatigue. The brilliancy of the light may vary greatly without our losing the benefit of the clearness which an extreme contraction of the pupil assures. But as the day declines and the image on the retina becomes insufficiently luminous for clear vision, the pupil becomes dilated, and the inequalities of different eyes are manifested. The diminution of light goes on almost unnoticed by well-formed eyes, for it is compensated by the increase of the used surface of the cornea. Less perfect eyes, on the other hand, not being able to perform their functions with ease, have to make fatiguing efforts to accommodate themselves to the situation, which tend to increase existing defects or to induce short-sightedness.
Adults are less liable than children to suffer injury from insufficient light, for several reasons: 1. Their pupils being less dilatable, they are obliged to desist from their work sooner when the light diminishes; 2. They make more frequent use of glasses; 3. They are less often confined, like school-children, and compelled to continue their labors after the light has become insufficient; and, 4. The coverings of their eyes are less extensible, and, if they have so far escaped myopia, they have more chances of continuing free from it.
A good management of the daylight is especially important with regard to the construction of schoolhouses. It is not enough to lay down a rule establishing the proportion which the surface of glass should bear to the number of pupils; attention must also be given to the direction whence the light comes to each pupil. The darkest point in the room must be light enough, and for this it is necessary that each desk shall receive a sufficiency of light direct from the sky. Every one who has practiced photography knows that the sky acts more strongly than any terrestrial body upon the sensitive surface. The least favored place in the room should be within the reach of this light. Nevertheless, the direct rays of the sun should be avoided, for they will dazzle. Where such an arrangement is otherwise practicable, the advantage of a diffused light may be gained by opening windows on the north side. Then, if the seats are placed perpendicularly to the wall occupied by the windows, so that the pupils may receive the light from the left side and from above, the result will be satisfactory provided the width of the room does not much exceed the height of the tops of the windows above the floor; for, under this condition, the least favored seat will still look upon about one twentieth of the surface of the sky. With ceilings of the ordinary height, unilateral lighting answers very well for rooms that do not exceed twelve or thirteen feet in width. For larger rooms, windows may also be opened on the other side, or behind the pupils, but never in front of them.
If we open windows in opposite sides of the room, we must arrange it so that they shall not be on the south side, for that would let in the glare of noon. For this reason it would be preferable to direct the axis of the room north and south, in which case it may be expedient to temper the forenoon and afternoon sunshine with transparent curtains. This arrangement will also give us the advantage of a better lighting in the morning and afternoon during the short days of winter. A certain latitude in orientation is admissible, which may extend to forty degrees on either side of the north and south line, as the disposition of the ground may require. An inclination toward the north-northeast is preferable to one toward the north-northwest for general hygienic reasons, for with it the room may receive the sun for the longer time in the forenoon. The teacher should face the south, so that the pupils facing the north may receive the stronger light from behind. In the northern part of the country a window might be allowed at the top of the southern wall, to be covered during sunshine and used during dark weather.
We have still to consider the possibility of the schoolroom being shadowed by neighboring buildings. This must be prevented by acquiring enough ground to keep the buildings away. Even after we have properly proportioned the height of the windows to the size of the room, if there is a neighboring building the height of which is precisely half the distance between its base and the middle of the schoolroom, the worst situated scholars will receive the light from only the upper half of the windows, and not enough of it. We have, then, to establish the rule that a free space must be reserved on either side of the schoolroom, the width of which, measuring from the middle of the room, shall not be less than twice the height of the largest building that is likely to be put up near it. The inconvenience arising from the shade of trees is modified by the absence of leaves in the winter and their welcome presence in summer, and does not call for general rules.—Revue Scientifique.