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This page describes a very simple method of calibrating the fine focussing control on a microscope. The control itself has to be engraved with graduations and an index mark has to be provided so its exact position is defined and any changes made can be quantified. In the Swift polarising microscope which was used in this exercise, these graduations were already present and it became a matter of determining the relationship between the graduations of the focussing control and the linear vertical movement of the objective lens which corresponded to them.
To provide the necessary standard distances, a series of eight coverslips, 0.3mm thick were cemented to a microscope slide to form a staircase s shown in the lower figure to the right. A diamond scriber was used to scratch a thin line on each coverslip prior to assembly so that each step of the staircase would have a ruled groove along its edge upon which it was possible to focus when the assembled staircase was mounted on the stage of the microscope. A micrometer was used to measure the thickness of the slide and the total thickness of the slide and the 8 cemented cover slips. The slide was found to be 0.992mm thick and the total thickness, 3.540 mm thick. This meant that the average step-height of the staircase was 0.3185mm, accounted for by an average thickness of the coverslips (separately measured) of 0.3005 and a thickness of cement of 0.0185 mm per coverslip.
It was a simple matter to note the position of the fine focus control when the microscope was focussed on each of the steps of the staircase slide. The results are shown in the table below:
|Step number||Total thickness||focus control divisions|
The slope of the plot in the figure above is 204 divisions per millimetre. Since the fine focussing control has 20 divisions per turn, one complete turn of the control adjusts the focussing by 100 microns, and so a division corresponds to a 5 micron change in focussing.