_Flatness of Field._--This quality in the objective has, by the introduction of the immersion system, lost much of the importance formerly attached to it. Some writers assume it to be an “optical impossibility.” The compensating eye-piece has had the effect of contracting the visual field, consequently the peripheral imperfections of the objective are of a less disturbing character. It has, however, not been made perfectly clear whether the highest perfection of the two primary qualities of a good objective, _defining power and resolving power_, can be always obtained in one and the same combination of lenses. Doubtless, _defining_ power can be more satisfactorily determined by the examination of a suitable object, and the perfection of the image obtained; to assist in securing which, a solid axial cone of light equal to about three-fourths of the aperture of the objective must be employed. To sum up, then, “the focal power of all objectives depends in their perfect _definition_, a property on which their converging power depends, and in turn their magnifying action is dependent; again, focal power is the curvature imprinted by the lens on a plane wave, and is reciprocal of the true focal length. It is appropriately expressed in terms of the proper unit of focal curvature, the _dioptric_; a unit of curvature.”[39] [Illustration: Fig. 202.--Seiler’s Test Slide.] It may be taken as an axiom with microscopists that “neither the penetrating power nor the high-power defining objective is alone sufficient for every kind of work. The larger the details of ultimate structure, the narrower the aperture--and the converse; the minuter the dimensions of elementary structure, the wider must be the aperture of the objective.” Every worker with the microscope must have satisfied himself of the truth of this statement, when engaged in the study of the movements of living organisms, or defining the intimate structure of the minuter diatoms, or of the podura scale. _Test for Illumination._--Dr. C. Seiler recommends the human blood corpuscle as the best test of good illumination. He prepares the object in the following manner: Take for the purpose a clean glass slide of the ordinary kind, and place near its extreme edge a drop of fresh blood drawn by pricking the finger with a needle. Then take another slide of the same size, with ground edges, and bring one end in contact with the drop of blood, as shown in Fig. 202, at an angle of 45°; then draw it evenly and quickly across the underslide, and the result will be to spread out the corpuscles evenly throughout. Blood discs being lenticular bodies, with depressed centres, act like so many little glass-lenses, and show diffraction rings if the light is not properly adjusted.[40]