earth’s surface may be as high as 3000 foot-candles and on a “gloomy” day the illumination at the earth’s surface may be 1000 foot-candles.  When it is considered that mankind works under artificial light with an intensity of only a few foot-candles, the marvels of the visual apparatus are apparent.  But it should be noted that the eyes of the human race evolved under natural light.  They have been used to great intensities when called upon for their greatest efforts.  The human being is wonderfully adaptive, but it could scarcely be hoped that the eyes could readjust themselves in a few generations to the changed conditions of low-intensity artificial lighting.  There is no complaint against the range of intensities to which the eye responds, for in range of sensibility it is superior to any man-made device.

For extremely low brightnesses another set of physiological processes come into play.  Based purely upon the physiological laws of vision it seems reasonable to conclude that mankind should not work under artificial illumination as low as has been considered necessary owing to the cost in the past.  With this principle of vision as a foundation, experiments have been made with greater intensities of illumination in the industries and elsewhere and increased production has been the result.  In a test in a factory where an adequate record of production was in effect it was found that an increase in the intensity of illumination from 4 to 12 foot-candles increased the production in various operations.  The lowest increase in production was 8 per cent., the highest was 27 per cent., and the average was 15 per cent.  The original lighting in this case was better than that of the typical industrial conditions, so that it seems reasonable to expect a greater increase in production when a change is made from the average inadequate lighting of a factory to a well-designed lighting-system giving a high intensity of illumination.

In another test the production under a poor system of lighting by means of bare lamps on drop-cords was compared with that of an excellent system in which well-designed reflectors were used.  The intensity of illumination in the latter case was twenty-five times that of the former and the production was increased in various operations from 30 per cent. for the least increase to 100 per cent. for the greatest increase.  Inasmuch as the energy consumption in the latter case was increased seven times and the illumination twenty-five times, it is seen that the increase in intensity of illumination was due largely to the use of proper reflectors and to the general layout of the new lighting-system.

In another case a 10 per cent. increase in production was obtained by increasing the intensity of illumination from 3 foot-candles to about 12 foot-candles.  This increase of four times in the intensity of illumination involved an increase in consumption of electrical energy of three times the original amount at an increase in cost equal to 1.2 per cent. of the pay-roll.  In another test an increase of 10 per cent. in production was obtained at an increase in cost equal to less than 1 per cent. of the payroll.  The efficiency of well-designed lighting installations is illustrated in this case, for the illumination intensity was increased six times by doubling the consumption of electrical energy.