It has been generally supposed that the best possible illumination for microscopic work is diffused sunlight obtained from a northern direction. No matter from what direction diffused sunlight is obtained, it will be found suitable for microscopic work. In no case should direct sunlight be used, because it will be found blinding in its effects upon the eyes. Natural illumination--diffused sunlight--varies so greatly during the different months of the year, and even during different periods of the day, that individual workers are resorting more and more to artificial illumination. The particular advantage of such illumination is due to the fact that its quality and intensity are uniform at all times. There are many ways of securing such artificial illumination, no one of which has any particular advantage over the other. Some workers use an ordinary gas or electric light with a color screen placed in the sub-stage below the iris diaphragm. In other cases a globe filled with a weak solution of copper sulphate is placed in such a way between the source of light and the microscope that the light is focused on the mirror. Modern mechanical ingenuity has devised, however, a number of more convenient micro lamps (Fig. 30). These lamps are a combination of light and screen. In some forms a number of different screens come with each lamp, so that it is possible to obtain white-, blue-, or dark-ground illumination. The type of the screen used will be varied according to the nature of the object studied. CARE OF THE MICROSCOPE If possible, the microscope should be stored in a room of the same temperature as that in which it is to be used. In any case, avoid storing in a room that is cooler than the place of use, because when it is brought into a warmer room, moisture will condense on the ocular objectives and mirrors. Before beginning work remove all moisture, dust, etc., from the inner and outer lenses of the ocular, the objectives, the Abbé condenser, and the mirror by means of a piece of soft, old linen. When the work is finished the optical parts should be thoroughly cleaned. If reagents have been used, be sure that none has got on the objectives or the Abbé condenser. If any reagent has got on these parts, wash it off with water, and then dry them thoroughly with soft linen. The inner lenses of the eye-pieces and the under lens of the Abbé condenser should occasionally be cleaned. The mechanical parts of the stand should be cleaned if dust accumulates, and the movable surfaces should be oiled occasionally. Never attempt to make new combinations of the ocular or objective lenses, or transfer the objectives or ocular from one microscope to another, because the lenses of any given microscope form a perfect lens system, and this would not be the case if they were transferred. Keep clean cloths in a dust-proof box. Under no circumstances touch any of the optical parts with your fingers.