. Elementary chemical microscopy. Microscopy; Microchemistry. MICROMETRY â MICROMETRIC MICROSCOPES 185 in Fig. 117 in B. The value of the ocular micrometer scale must be determined for each objective in turn, adjusting the draw- tube in every case so as to avoid estimating fractions of a scale division and in each case the record must be kept of the tube length under which the observations were made. In the ordinary micrometer ocular it is often somewhat of an eye and mental strain to count the number of scale divisions, especially if the object is relatively large. To facilitate counting, Lei
. Elementary chemical microscopy. Microscopy; Microchemistry. MICROMETRY â MICROMETRIC MICROSCOPES 185 in Fig. 117 in B. The value of the ocular micrometer scale must be determined for each objective in turn, adjusting the draw- tube in every case so as to avoid estimating fractions of a scale division and in each case the record must be kept of the tube length under which the observations were made. In the ordinary micrometer ocular it is often somewhat of an eye and mental strain to count the number of scale divisions, especially if the object is relatively large. To facilitate counting, Leitz has placed upon the market a scale, part black, part light, in which the divisions are sharply differentiated in blocks of ten, both horizontally and vertically. This type of ruling has received the name of Step micrometer, and is far less fatiguing to employ than the older simple ruling. Fig. 118 shows part of the scale of a step micrometer. Instead of being ruled in tenths and hundredths of a mil- limeter as usual, such a value is used by Leitz that when Leitz objectives are employed on a Leitz microscope, it is only necessary to set the draw-tube at the point indicated for that particular objective. The ocular micrometer Method Em- value is obtained from a table, supplied with ployed in Ruling the the instrument. Calibration by means of a Leitz Step Mlcrom- eter Ocular. stage micrometer is therefore unnecessary. For measuring bright or self-luminous bodies, such as the incandescent filaments of lamps, etc., the Gebhardt Contrast Micrometer, Fig. 119, made by Zeiss, will be found useful. In place of line rulings, which would be practically invisible, the scale con- sists of a row of tiny black squares touching at their corners. A scale of this type will stand out sharply, no matter how bright the object may be. Filar Micrometers. ââ In microm- etry with oculars having fixed Fig. 119. Zeiss Contrast Microm- scaleg there ig always the probability eter Ocular for
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