[Illustration: FIG. 1.--The microscope: 1, Eye-piece; 2, draw-tube; 3, main tube; 4, nose-piece with objectives attached; 5, objective in position; 6, stage; 7, substage; 8, adjustment of substage; 9, mirror; 10, coarse adjustment; 11, fine adjustment.] Illumination.--Good work cannot be done without proper illumination. It is difficult to lay too much stress upon this point. The best light is that from a white cloud. {18} A northern exposure is desirable, since direct sunlight is to be avoided. Good work can be done at night with a Welsbach light. Ordinary gas-light and the incandescent electric light are unsatisfactory, although the latter gives good results when subdued with a heavily frosted globe. The writer uses a frosted electric bulb in a dark-room lantern, and tones the light to the proper degree for low powers by means of frosted-glass plates which slide into the grooves which have held the ruby and orange glasses. One of these plates is made of blue glass, to overcome the yellow of the artificial light. It is not generally advised to do so, but it will be found convenient to use the Abbé condenser for all routine work. With daylight it is best to use the plane mirror: with artificial light, the concave mirror. To obtain best results, the light must be focused upon the object under examination by raising or lowering the condenser. Illumination may be either _central_ or _oblique_. Central illumination is to be used for all routine work. To obtain this, the mirror should be so adjusted that the light from the source selected is reflected directly up the tube of the microscope. This is easily done by removing the eye-piece and looking down the tube while adjusting the mirror. The eye-piece is then replaced, and the light reduced as much as desired by means of the diaphragm. Oblique illumination is to be used only to bring out certain structures more clearly after viewing them by central light: as, for example, to show the edges of a hyaline cast by throwing one of its sides into shadow. Oblique illumination is obtained in the more simple instruments by swinging the mirror to one side, so that the light enters the microscope obliquely. The more {19} complicated instruments obtain it by means of a rack and pinion, which moves the diaphragm laterally. Beginners frequently use oblique illumination without recognizing it. If the light be oblique, an object in the center of the field will appear to move from side to side when the fine adjustment is turned back and forth. The amount of light is even more important than its direction. It is regulated by the diaphragm. _It is always best to use the least light that will show the object well._ Unstained objects require very subdued light. Beginners constantly use it too strong. Strong light will often render semitransparent structures, as hyaline casts, entirely invisible (Figs. 2 and 3). Stained objects, especially bacteria, require much greater light.