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] Working Accessories. TROUGHS--LIVE-CAGES--COMPRESSORS. _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. [Illustration: Fig. 208.--Varley’s Live-box.] _Troughs and Live-box._--These are made of various materials, glass, vulcanite, brass, &c., expressly for examining infusoria and live animals. They should be so constructed as to admit of the use of a medium power, a 1/2-inch at least, under the microscope. They should also admit of being easily cleaned and repaired when broken; matters rarely thought of by those who construct them. An early devised _live-box_ (Varley’s, Fig. 208) consists of two circular pieces of brass tubing, one sliding over the other carrying a disc of glass and fitting over another glass with bevelled edges to prevent the fluid flowing away. [Illustration: Fig. 209.--Ross’s Compressorium.]