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 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?
The answer is derived by multiplying the relative magnifications of the two objectives by the value of a graticule unit using the 2x objective: (2/10) x 200µm = 40µm
The graticule is used to measure distances, while the grid can be used to measure the number of items within a particular area, for example, the density of blood vessels or the numbers of cells within a defined area of tissue. The grid has to be calibrated, in the same way as the graticule, so that the area of each square is defined for a particular objective. When counting things within a grid square, some of the items will often lie on one of the grid lines. To account for this, it is conventional to count items that are on the top and right lines of a grid-square as 'in' and those on the bottom and left lines as 'out'.