The method of finding and determining when the screw-collar adjustment of the high-power objective has arrived at a point of perfect definition and magnification is as follows:—
Select any dark speck of dust, or an opaque portion of the object, and carefully focus this small particle by working the screw of the fine adjustment, move the screw up and down until you are satisfied the image is the sharpest and blackest that can be obtained, then once more test the focus a little above and a little below while closely scrutinising the effect on the image. It will now be seen that whereas in focussing on one side of the best focus the object disappears in a fog, by focussing on the other side it remains in view for a longer period, but alters its appearance; it is now no longer a black dot, but a bright dot of light surrounded by a black margin. The effects being thus dissimilar on different sides of the best focus, show that the objective is not perfectly adjusted for the cover-glass in use.
The next step is to find out whether the bright image is above or below the best focus, as on this depends the direction in which the adjustment-collar should be turned. To determine this it is only necessary to ascertain which way the slow-motion milled head of the microscope turns when moving the objective upwards.
In the case under consideration, the bright image will be above the best focus, which shows that the cover-glass in use is thicker than that for which the objective is adjusted, consequently the adjustment-collar must be moved in the opposite direction.
Fig. 207.—Podura Scale Test.
If the collar be turned too far in the opposite direction, it will be found that the bright image is below the best focus, and the cover-glass is then thinner than that for which the objective is adjusted. The collar must then be turned back again until the effect on each side of the best focus is exactly similar. This effect in the case of a circular speck of dust will be that the object disappears equally rapidly on either side, and does not instantly vanish into fog, on either side presenting the bright spot appearance, though not in so marked a degree on either side. When the object is in perfect adjustment the expansion of the outline is exactly the same, both within and without the focus.
A different indication, however, is afforded by such test-objects as the finer diatoms, and the podura scale, in which we have to do with a set of distinct dots and other markings. If the dots have a tendency to run into lines when the object is without the focus, the glasses should be brought closer together; on the contrary, if the lines appear when the object is within the focal point, the glasses should be farther separated.
The adjustment of the objective by the screw-collar in the case of the podura scale should be carried out in the way described, when the following effects will be observed to take place, usually in the order of their arrangement.
[Fig. 1] shows the appearance of a podura scale when the adjustment of the object-glass is correct, and [Fig. 2] shows the effect produced on each side of the exact focus. [Fig. 3] shows the way in which the markings individually divide when all the adjustments are correct, and when the focus is altered the least possible amount only each way.