Science, Maths & Technology

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Introduction to microscopy

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# 2.3 Grids and graticules

Sometimes it is important to measure the size of a cell, the distance between cells or the number of cells in a given area. Most light microscopes have the option to add a graticule (scale bar) or a grid into the light path to allow such measurements. These elements are usually added by exchanging one of the eyepieces for one with a graticule. The appearance of the graticule is therefore fixed according to the eyepiece that is used - changing the objective does not change the appearance of the graticule. (This feature is simulated on the digital microscope.) A consequence is that the graticule has to be calibrated for each objective. This is most readily done by viewing a slide that has rulings etched into its surface, and observing how many units on the graticule correspond to (for example) 1mm on the slide (Figure 6).

Figure 6 Calibration of a graticule. Each division on the graticule corresponds to 0.2mm (= 200µm), using this objective.

## SAQ 3

Figure 6 shows the result using a 2x objective: one graticule unit = 200µm. If a 10x objective was used, what distance would one unit on the graticule correspond to?