Nebulous doubling with oblique illumination indicates over-correction of the marginal zone; indistinctness of the edges without marked nebulosity indicates under-correction of the zone; an alteration of the focus for oblique and central illumination points to an absence of concurrent action of the separate zones, which may be due to either an average under or over correction, or to irregularity in the convergence of the rays. [Illustration: Fig. 121.--Zeiss’s Cover-glass Gauge.] COVER-GLASS GAUGE. Zeiss has gone a step further to lay the microscopist’s ghost of the cover-glass. He invented a measurer (Fig. 121) whereby the precise determination of thickness of glass-covers can be obtained. This measurement is effected by a clip projecting from a circular box; the reading is given by an indicator moving over a divided circle on the lid of the box. The divisions seen cut round the circumference show 1/100ths of a millimeter. This ingenious gauge measures upwards of 5 mm. This necessary and important digression has led me away from the consideration of the achromatic objective, and to which I shall now return. English Immersion and Dry Objectives. The homogeneous immersion system met with its earliest as well as its staunchest advocates among English opticians. Among its more energetic supporters were Messrs. Powell and Lealand, who were the first to construct a 1/8-inch immersion objective on a formula of their own, and which was found to resolve test-objects not before capable of resolution by their dry objectives. This encouraged them to make a 1/16-inch, acquired by Dr. Woodward for the Army Medical Department, Washington, and subsequently a 1/25-inch; neither of which surpassed their 1/8-inch in aperture, and a new formula was tried in the construction of their first oil-immersion objective. This had a duplex front, and two double backs; but even this did not quite accomplish what was expected of it, and another change was subsequently made; the anterior front combination became greater than a hemisphere--a balloon-lens. This at once gave an increase of aperture to a 1/12-inch objective of 1·43 numerical aperture. After some few more trials a more important change of the formula took place. The front lens was made of flint-glass, and the combination took the form represented in diagram (Fig. 122). This, on an enlarged scale, represents Powell’s 1/12-inch numerical aperture 1·50. It is a homogeneous apochromatic immersion of high quality and very flat field. It will be noticed that in this combination the four curves of the lenses are very deep compared with those of other opticians. [Illustration: Fig. 122.--Powell and Lealand’s 1/12-in. Oil-immersion Objective, drawn on a scale of 6-1.]