How can you determine the actual size of a specimen under a microscope?

You can determine the actual size of a specimen under a microscope by using a microscope graticule and stage micrometer.

To accurately determine the size of a specimen under a microscope, you need to calibrate your microscope with a stage micrometer. A stage micrometer is a glass slide with a precisely measured scale etched onto it. This scale is usually 1mm long and divided into 100 divisions, so each division is 0.01mm.

The microscope graticule is a small glass disc that fits into the eyepiece of the microscope. It has a scale etched onto it, but the size of the divisions on this scale will vary depending on the magnification used. Therefore, it needs to be calibrated against the stage micrometer.

To calibrate the microscope, you place the stage micrometer on the stage of the microscope and align it with the graticule in the eyepiece. You then adjust the magnification until one division on the graticule corresponds to a known number of divisions on the stage micrometer. This gives you a conversion factor for that magnification.

For example, if at a certain magnification, one division on the graticule corresponds to two divisions on the stage micrometer, then each graticule division represents 0.02mm (since each stage micrometer division is 0.01mm).

Once you have this conversion factor, you can then measure the size of your specimen. You simply count how many graticule divisions the specimen spans, and then multiply this by the conversion factor to get the actual size of the specimen.

Remember, the conversion factor will change if you change the magnification, so you need to recalibrate the microscope each time you adjust the magnification. Also, be aware that the calibration is only accurate for the centre of the field of view, as lens distortion can cause inaccuracies towards the edges.