Figs. 119 and 119_a_ will doubtless make this clearer. If the objective of 140° works with water (Fig. 119), there would be a cone of rays extending up to 70° on both sides of the axis, _and this large cone would be submitted to spherical aberration at the front surface a_. But with homogeneous immersion (Fig. 119_a_) the whole cone of 112° is admitted to the front lens without any aberration, there being no refraction at the plane surface; and as the spherical surface of the front lens is without notable spherical aberration, the incident pencil is brought from the focus F to the conjugate focus F′, and contracted to an angle of divergence of 70°-90° _without having undergone any spherical aberration at all_. The problem of correcting a very wide-angled objective has thus been reduced by the homogeneous oil-immersion system, both in theory and practice.[23] Abbe’s Test-plate. Abbe designed the test-plate (Fig. 120) for testing the spherical and chromatic aberrations of objectives, and estimating the thickness of cover-glasses corresponding to the most perfect correction: six glasses, having the exact thickness marked on each, 0·09 to 0·24 mm., cemented in succession on a slip, their lower surface silvered and engraved with parallel lines, the contours of which form the test. These being coarsely ruled are easily resolved by the lowest powers; yet, from the extreme thinness of the silver, they form also a delicate test for objectives of the highest power and widest aperture. The test-plate in its original size is seen in Fig. 120, with one of the circles enlarged. [Illustration: Fig. 120.--Abbe’s Test-plate for estimating thickness of glass-covers.] To examine an objective of large aperture, the discs must be focussed in succession, observing in each case the quality of the image in the centre of the field, and the variation produced by using, alternately, central and very oblique illumination. When the objective is perfectly corrected for spherical aberration, the outlines of the lines in the centre of the field will be perfectly sharp by oblique illumination, and without any nebulous doubling or indistinctness of the edges. If, after exactly adjusting the objective for oblique light, central illumination is used, no alteration of the focus should be necessary to show the outlines with equal sharpness. If an objective fulfils these conditions with any one of the discs, it is free from spherical aberration when used with cover-glasses of that thickness. On the other hand, if every disc shows nebulous doubling, or an indistinct appearance of the edges of the line with oblique illumination, or, if the objective requires a different focal adjustment to get equal sharpness with central as with oblique light, the spherical correction of the objective is more or less imperfect.