High-Power Objectives.
Points of Importance for securing the best results with High-power Objectives.—Always give to the body-tube of the microscope the length for which the objective is corrected, 0·160 mm. for the short continental tube, and 0·250 mm. for the English tube (10-inch). Employ both dry and immersion objectives mounted for correction, commencing with a numerical aperture of 0·75 (that is about 100° in air). If the graduation is not given in thickness of cover-glass apply to the maker to correct this omission.
With the homogeneous oil-immersion objective it is highly necessary to utilise all marginal pencils of light, to optically unite the upper lens of the condenser with the preparation as well as the front lens of the objective by means of a liquid having the same index of refraction or at least equal to that of the immersion. Cedar Oil has been generally adopted for the purpose mentioned, the better way of using which is as follows: place a drop on the centre of the front objective, or on the top of the cover-glass, and then lower the objective by means of the coarse adjustment until it comes in contact with the oil, and carefully bring into focus by the fine adjustment. If the slide is held between the finger and thumb of one hand and moved from side to side, while the other hand is working the fine adjustment, there can be no danger of injuring either the objective or the specimen. Before putting the microscope away, take a fine camel-hair brush dipped in ether, alcohol, or methylated spirit, and carefully remove the oil from the objective and the glass cover of the object; a soft chamois leather or cambric pocket handkerchief will dry it off, or a piece of fine white blotting paper answers equally well. Should the lens come accidentally into contact with the Canada balsam, it must be very carefully removed either by ether or alcohol. The former is by far the safest, as alcohol, if not very carefully used, quickly dissolves out the balsam and loosens the cover-glass of the object.