Measuring with Microscopes

Measuring with microscopes We can measure the size of objects using an eyepiece graticule. This scale is typically 10mm long and is divided into 100 sub-divisions. The scale on the eyepiece graticule cannot be used directly to measure the size of objects under a microscope’s objective lens because each objective lens will magnify to a different degree. The graticule must be calibrated for each different objective lens. It is sensible to record the results of the calibration for a particular objective lens attached to the microscope.

Calibrating the eyepiece graticule To calibrate an eyepiece graticule you need to use a microscope slide called a stage micrometer. This slide has a scale etched on to it – our school micrometers are 10mm long, have 1mm subdivisions and smallest subdivisions of 0.1mm. You have to line up the eyepiece graticule scale and the stage micrometer scale to calculate the length of the divisions on the eyepiece graticule. • You can calculate the scale for different objective lenses by dividing the differences in magnification. For example, if an objective lens magnifying x40 gives a calibration of 250µm per graticule unit, then an objective lens magnifying x400 (10x greater) will mean a graticule unit is equivalent to 250µm ÷ 10 = 25µm.

1 2 3 4 5 6 7 8 9 10

1 2 3 4 5 6 7 8 9 10 Low power (x40)

1 2 3 4 5 6 7 8 9 10 High power (x400)

BUT IT COULD EASILY BE DIFFERENT ON OTHER MICROSCOPES!! The scale on a microscope is in GRATICULE UNITS Objects at different magnifications look larger or smaller, but the graticule units stay the same. Therefore the metric value of a graticule unit varies. On our school microscopes, 1 graticule unit equals… 250 m (0.25 mm) at low power (x40) 100 m (0.1 mm) at medium power (x100) 25 m (0.025 mm) at high power (x400) BUT IT COULD EASILY BE DIFFERENT ON OTHER MICROSCOPES!!

Low power (x40)

Low power (x40) 4 gu = 1 mm 1 gu = 1/4mm 1gu = 0.25 mm 1gu = 250µm

Med power (x100)

Medium power (x100) 10 gu = 1 mm 1 gu = 1/10mm 1gu = 0.01 mm 1gu = 100µm

High power (x400)

High power (x 400) 4 gu = 0.1 mm 1 gu = 0.1/4 mm 1gu = 0.025 mm 1gu = 25µm

Low power (x40) 4 gu = 1 mm 1 gu = 1/4mm 1gu = 0.25 mm 1gu = 250µm X 40  x 100 (Higher magnification by x2.5) therefore divide by 2.5. Medium power (x100) 10 gu = 1 mm 1 gu = 1/10mm 1gu = 0.01 mm 1gu = 100µm High power (x 400) 4 gu = 0.1 mm 1 gu = 0.1/4 mm 1gu = 0.025 mm 1gu = 25µm X 100  x 400 (Higher magnification by x4) therefore divide by 4.