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.
Figs. 4 and 5 show the two appearances on one and the other side of the best focus when the adjustment is incorrect, Fig. 6 showing the appearance of the same at its best focus.
_The scales_ are magnified 1,300 diameters, and each square measures ·001 of an inch.
This method, however, of finding the best focus of an objective can scarcely be accomplished without a sub-stage condenser. It may therefore be of service to the student, and to those who are not disposed to purchase expensive forms of condensers, to know that either an inch or an inch and a half objective, or convex-lens mounted on a simple wooden ring with a flange, can be arranged to slip in the place of the diaphragm under the stage. This kind of condenser will prove to be of considerable value with a 1/2-inch, a 4/10-inch, and a 1/4-inch; while a still more excellent achromatic condenser can be made out of a Steinheil’s _aplanatic-loup_ arranged to drop into the central fitting of the sub-stage. As without a condenser of some kind it is hardly possible to enter upon any course of histological or scientific research.[43]
_A glass plate_ with a ledge, and some pieces of _thin glass_, although applicable for many purposes, are specially designed for objects in fluid. Thus a drop of fluid containing the object sought for is placed upon the slide and covered by a piece of thin glass; or, the object being put upon the glass slide and the thin glass over it, the fluid is applied near one side, and runs under by capillary attraction.