all ordinary needs. For the purpose of microscopic measurements an ocular micrometer is necessary. This may be obtained as a separate eye-piece, or as a round piece of glass with measured divisions marked upon it that may be put into an ocular. The value of the scale must be determined for every lens and tube-length by estimating the number of its parts covering one part of a stage-micrometer marked in hundredths of a millimetre. [Illustration: FIG. 50.—A satisfactory outfit for the working laboratory. Continental stand, medium-sized, with large stage, three objectives, etc.] An instrument of any one of the above-mentioned makes, carefully selected and tested, should last its owner a life-time if proper care is taken of it. It should receive the same careful attention accorded a good violin or piano. It should be protected from dust, action of chemicals, heat, sunlight, and rough usage. When carried it should be supported in such a way that its weight is not thrown upon the thread of the adjusting screws. The adjustment, draw-tube and iris diaphragm should be carefully oiled at intervals, using the least possible amount of the best microtome oil. It is not necessary here to enter into the construction and theory of the microscope, as this knowledge has usually been obtained before the pathologic laboratory is reached. Experience has shown me, however, that it is always necessary to remind students, even those experienced for some time in the use of the microscope, of certain fundamental principles in the adaptation of microscopic technique to pathologic work. The following rules are of value:— 1. Use a _low-power_ objective for all work except for the study of bacteria, microparasites and finer cell-structures. The aim should be to obtain as much of a bird’s-eye view of the “geography” of the section as possible. Contrast plays a very important part in pathologic diagnosis; and it is lost in high-power work, so far as the relations of cells and tissue-elements and pathologic products are concerned. The student almost invariably enters the pathologic laboratory with a fixed “high-power habit,” and he is usually greatly surprised to learn how much he misses with the high-power and how much he can see with the low-power. A motto used many years in my laboratory, “Low-power objective and high-power cortex,” is of greater educational value than may appear at first sight. A slide should be examined first with the naked-eye, as it is held against a window or light; then it should be examined under the low power. Rarely will it be necessary to use a high-power except for the purposes mentioned above. 2. Weak eye-pieces should be used; strong ones darken the field and tire the eyes. Only in the case of apochromatic lenses and compensation oculars can the strong ones be used without darkening the field too much.