Guide to the practical teaching
of biology and genetics

Optical planes, measuring the size and thickness of microscopic objects

TASK 1: Optical planes and measuring of the thickness of the objects

PP: insect wing - "křídlo hmyzu"

Rotate the fine focus adjustment knob in one direction and note the first sharp image (a hair), medium (a major branch) and the last (a hair) = upper, middle and lower optical plane. Note which unit of the fine focus adjustment knob corresponds to the first optical plane and which corresponds to the last one (measure it against a fixed point on the coarse knob) and calculate the difference. What is the thickness of the object in µm?

Fig.: Optical planes (A - upper, B - middle, C - lower).

TASK 2: Measuring the length of microscopic objects

PP: bird red blood cells - "ptačí krev/krvinky"

Put the objective micrometer on the stage and observe it using 4×, 10×, 40× and 100× magnifying objectives. The left margins of both scales must be aligned. Read the corresponding numbers of units on both scales where they meet and calculate the micrometric coefficient for each objective. When changing to the next magnification, readjust the centering, brightness, focus and realign the left margins of both scales. Note these coefficients for future measuring of microscopic objects. Measure the length of the red blood cells.

Fig.: Comparison of the length of different cells.