Calibration

Day 34: While it's a fairly simple matter to measure a dissection specimen or other object seen under low magnification, measuring under high power is another story. Note the slide in the foreground. Inside the circle in the center, there is a line which is almost invisible to the naked eye. Seen through the microscope, it reveals itself to be a ruler one millimeter in length. This ruler is divided into 100 smaller units of 10 micrometers each. Since we don't want to lay a specimen directly on this slide (somewhat confusingly referred to as a "stage micrometer") and in any case, would find it impossible to prod a teeny-tiny sample into the proper location, we are going to need another tool. Enter the eyepiece reticle.

The eyepiece reticle is equipped with a grid in its lens, also divided into 100 sections. If you install the reticle and use it to observe the stage micrometer, you'll probably discover that the divisions are very different from those in the reticle and don't align. By figuring out the ratio between the stage micrometer divisions and the reticle divisions, a little simple math will provide a conversion to use in conjunction with the eyepiece reticle. The stage micrometer is only used to calibrate the microscope.

When using a different objective, the magnification factor changes. Therefore the ratio of stage micrometer divisions and reticle divisions changes accordingly. We must recalibrate for each objective, and if we have more than one microscope, we'll have to calibrate them as well. In the upper right, I have shown the two sets of divisions. Although you can't see it in the photo, using the 40x objective of my brightfield microscope, 30 reticle divisions equal 75 stage micrometer divisions, i.e., 750μ, therefore a single reticle division is equal to 25μ. In lichen identification, a diagnosis sometimes hinges on the size and/or shape of ascospores. To give this perspective, the spores of Boletus edulis (one of my favourite edible mushrooms) are 12-17μ in length by 4-6μ wide.