3. In the use of higher powers see that the tube is drawn out to the proper length, as indicated in the directions sent with the instrument. 4. For pathologic work an Abbé condenser is essential. It should be pushed up to its proper position beneath the stage, and the plane mirror should be used with it, reflecting the light from a cloud if possible. Daylight is always the best light. When this cannot be obtained an incandescent or Welsbach lamp with ground-glass globe can be used. The concave mirror should then be used. The yellowish tint of artificial light may be avoided by the use of a piece of blue glass placed beneath the condenser, or a vessel of copper sulphate solution may be interposed. Especial lamps designed to meet the requirements are offered by the trade. 5. The iris-diaphragm should be adjusted by the same hand that moves the slide, usually the left one. With unstained preparations the diaphragm should be nearly closed; when using the oil-immersion it should be fully opened, as is also the case when stained preparations are studied with the lower power. With higher magnification the aperture is diminished somewhat, although color-effects are best shown with open diaphragm. In the study of pigments the diaphragm should be fully closed for a few moments to see if the pigment shows any color by reflected light. It is then examined by open diaphragm. In the study of sections the mirror and diaphragm should be manipulated in various ways to bring out all of the detail of the preparation, and should be adjusted to suit each preparation. 6. Objectives must never be screwed down until they strike the slide or stage. The higher-powers are frequently ruined in this way. When running the objective down always examine from the side to see that there is no danger of its striking the stage. In the use of the oil-immersion place the drop of oil upon the slide or cover-glass, and lower the objective by turning the coarse adjustment until the oil spreads out between the lens and the glass; then focus with the fine adjustment until a well-defined field is obtained. The oil-immersion lens should not be allowed to stand many consecutive hours in the oil. The oil should be cleaned from the lens by wiping the latter with lens-paper or a soft cloth; if the lens is sticky the paper or cloth may be moistened with benzol. The lens itself should never be _wet_ with benzol, xylol, alcohol or any cleaning-fluid, because of the danger of softening the balsam in which the lenses are imbedded.