The three factors, heating, humidity, and air motion, must be considered together. Every person requires each hour about 3000 cubic feet of air, and the problem of heating and ventilating is that of providing this amount in gentle motion, at a temperature of about 65° F., and of humidity from 50-55%. Higher temperatures and stagnant air cause disinclination to work, headache, nausea, restlessness, or sleepiness, and if continued are likely to result in loss of appetite, and anemia. The tuberculosis movement has clearly shown the benefits both for the sick and the well of living in the open air, and has caused great and beneficial changes within a generation. The more time spent in the open air the better; since however most persons who work must spend the greater part of the day indoors, ventilation is a matter of great importance. Although fresh air enthusiasts are still too few, yet some go to the extreme and think that because cool air in motion is good, the colder the air and more violent the motion the better. On the contrary, chilling the whole body or a part of the body lowers resistance. Draughts of air have no bad effects upon persons in good health, particularly those accustomed to changes in temperature. But draughts are likely to be injurious to aged or sick persons and babies, by diminishing their resistance to such infections as common colds and pneumonia. It should be remembered that draughts or cold alone cannot cause colds; the specific germs must be present. LIGHTING.--Amount and direction of light are physiologically important. Defects of the eyes, too prolonged use, and insufficient light are the commonest causes of eye strain. Most eye defects can be relieved by glasses. Children's eyes should be examined upon entering school, and as often afterward as the oculist advises. Prolonged use causes fatigue of the eyes, especially when the illumination is poor; within limits, the amount of light needed depends on the nature of the work. Light should come from the left side of right handed people; never from the front. Light reflected from snow, sand, glazed white paper of books, or other bright surfaces is fatiguing from its intensity, and from the unusual angle at which it enters the eyes. Too much light is harmful, and probably causes some of the effects, such as nausea and headache, commonly attributed to poor ventilation. Almost all blindness is preventable, and blindness due to industrial accidents and processes is no exception to this rule. Surely no individual precautions or legal measures are too great in order to guard against this saddest of all physical defects. CLEANLINESS OF HOUSES.--A clean, well-cared for house is desirable from every point of view, but certain kinds of cleanliness affect health more than others.