Micrometry

MICROMETRY (from the Greek mikros—small and metron—measure), the measurement of objects under a microscope. The unit of length adopted is 0.001 mm, called a micron and designated by µ. Measurement under a microscope is most easily performed using an ocular micrometer, the value of whose divisions must be determined for each objective and a specific tube length. For this purpose, a stage micrometer with specific divisions, for example 1 mm divided into hundredths, is placed on the microscope stage, and an ocular micrometer with divisions, e.g., of 0.1 mm, is inserted into the eyepiece. Having focused the eye lens of the eyepiece on the divisions of the ocular micrometer, we will find the image of the stage micrometer. Then both scales will be visible simultaneously, and they can be set so that the first division of the stage micrometer coincides with some division of the ocular micrometer; we will look for another division that also coincides in both scales. Let us assume that 5 divisions of the ocular micrometer cover exactly 3 divisions of the stage micrometer. These 3 divisions are equal to 0.030 mm, i.e., 30 µ; consequently, each division of the ocular micrometer corresponds to 6 µ. By placing the object to be measured in place of the stage micrometer, one can determine its dimensions in microns. If Zeiss apochromats are used, then without significant error, one can consider each division of the ocular micrometer in microns to be equal to the focal length of the apochromat expressed in mm; for example, with a 16 mm apochromat, each division of the ocular micrometer can be considered equal to 16 µ, with a 2 mm focus—2 µ, etc. This makes it possible to obtain the desired value in microns by multiplying the number of divisions of the ocular micrometer by the value of the focal length of the apochromat. Recently, the Zeiss firm has been attaching special tables to microscopes, which further facilitate the use of the ocular micrometer. For precise measurements, it is better to use a screw stage micrometer, which provides values that do not require conversions and corrections. This is especially convenient for large objects that do not fit into a single field of view of the microscope. It is also possible to photograph the stage micrometer and then, with the same camera extension and the same objective, photograph the object, and then measure it with this scale. For vertical measurements in the direction of the microscope axis, one can use a micrometric screw with divisions on the head, but in this case, it is necessary to take into account the refractive index of the medium between the coverslip and the objective. With a dry system, the found value must be multiplied by 1.5151 (the refractive index of glass); with an oil immersion system, when the rays are not refracted after passing from the glass into the cedar oil, the measured value can be considered the true one. For measuring angles, an ocular goniometer with a pointer and a circle divided into degrees is used, which makes it possible to determine the angle between two lines in a specimen. Usually, the divisions of the ocular micrometer are visible as faint lines. When measuring intensely stained specimens, one can use a contrast or stepped micrometer. The former represents a series of black squares of a specific size, arranged in a straight line and touching each other at their corners; the latter represents a series of black quadrilaterals of different lengths, arranged with their bases on one straight line, so that rows of steps are obtained.