Many types of polarization microscopes have been constructed; those of a more elaborate form are used for research investigations; others of smaller design for routine investigations. CHAPTER III MICROSCOPIC MEASUREMENTS In making critical examinations of powdered drugs, it is frequently necessary to measure the elements under observation, particularly in the case of starches and crystals. [Illustration: FIG. 21.--Ocular Micrometer] OCULAR MICROMETER Microscopic measurements are made by the =ocular micrometer= (Fig. 21). This consists of a circular piece of transparent glass on the centre of which is etched a one- or two-millimeter scale divided into one hundred or two hundred divisions respectively. The value of each line is determined by standardizing with a stage micrometer. STAGE MICROMETER [Illustration: FIG. 22.--Stage Micrometer] The =stage micrometer= (Fig. 22) consists of a glass slide upon which is etched a millimeter scale divided into one hundred equal parts or lines: each line has a value of one hundredth of a millimeter. STANDARDIZATION OF OCULAR MICROMETER WITH LOW-POWER OBJECTIVE Having placed the ocular micrometer in the eye-piece and the stage micrometer on the centre of the stage, focus until the lines of the stage micrometer are clearly seen. Then adjust the scales until the lines of the stage micrometer are parallel with and directly under the lines of the ocular micrometer. Ascertain the number of lines of the stage micrometer covered by the one hundred lines of the ocular micrometer. Then calculate the value of each line of the ocular. This is done in the following manner: If the one hundred lines of the ocular cover seventy-five lines of the stage micrometer, then the one hundred lines of the ocular micrometer are equivalent to seventy-five one-hundredths, or three-fourths, of a millimeter. One line of the ocular micrometer will therefore be equivalent to one-hundredth of seventy-five one-hundredths, or .0075 part of a millimeter, and as a micron is the unit for measuring microscopic objects, this being equivalent to one one-thousandth of a millimeter, the value of each line of the ocular will therefore be 7.5 microns. [Illustration: FIG. 23.--Micrometer Eye-Piece] With the high-power objective in place, ascertain the value of each line of the ocular. If one hundred lines of the ocular cover only twelve lines of the stage micrometer, then the one hundred lines of the ocular are equivalent to twelve one-hundredths of a millimeter, the value of one line being equivalent to one one-hundredth of twelve one-hundredths, or twelve ten-thousandths of a millimeter, or .0012, or 1.2_µ_. It will therefore be seen that objects as small as a thousandth of a millimeter can be accurately measured by the ocular micrometer.