Page:Popular Science Monthly Volume 16.djvu/543

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E 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