DAYLIGHT I.V THE SCHOOLROOM. 517 



DAYLIGHT IN THE SCHOOLROOM. 



By M. JAVAL. 



WE are all agreed in preferring the light 6>f 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 bj^ 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 dilata- 

 tion and contraction of the iris, the sensitive surface does not vary in 

 a greater proportion than that of one to a hundred. The power re- 

 sides 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 fluc- 

 tuations which are constantly taking place. 



Still, we must not demand of our organs the maximum of adapta- 

 tion 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 Avill 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 sud- 

 den 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 equiva- 

 lent 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. Lender these condi- 

 tions, 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 



