A reference to the above table will at once show that the nomenclature of objectives expresses at once the initial magnifying powers, but as makers have great difficulty in so calculating their formulæ so as to obtain the _exact_ power, these figures must be taken as approximate. Thus a 1/4-inch, which should magnify 40 diameters if true to its description, might actually magnify a little more or less. The magnifying powers of Zeiss’s and other apochromatic objectives can be ascertained by dividing the focal length of the objective in millimeters into 250 mm. (the distance of distinct vision), thus Focus millimetres 24 16 12 4 3 2 1·5 Initial magnifying power 10·5 15·5 21 63 83 125 167 diameters. The total magnification, when any eye-piece is working in conjunction with an objective, is ascertained by multiplying the initial power of the objective by that of the eye-piece. The above calculations are all for a 10-inch tube-length. Should, however, a shorter or longer length of body be employed, the magnification can at once be ascertained by a proportion sum. If the magnification be 180 with 10-inch tube-length, what would it be with a 6-inch body--10 : 6 :: 180 = 108 diameters. Abbe designed three different forms of eye-pieces: 1, the searcher eye-piece; 2, the working eye-piece; and 3, the projecting eye-piece. The _Searcher_ is a negative form of low power. The working is both negative and positive, the positive form of which is constructed on a newer principle; while the projection is chiefly intended for microphotography, its field being small and its definition superlatively sharp. These are severally explained among eye-pieces. 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.