A series of experiments ultimately brought to light a mineral substance, Fluorite, which, when combined in the proper proportion, one part to two of German crown and flint glass, was found to have the qualities looked for, and to possess different relations of a dispersive and refractive power. From Professor Abbe’s researches, begun in 1876, we have had the aperture of the objective greatly enlarged, and the homogeneous system brought into general use.
Previous to this date the best made objective merely approximated to colour correction. Undoubtedly the chief object to be obtained was the removal or diminution of the secondary colour aberration. This, together with other residual errors Abbe pointed out in 1880, led to the improvement of the optical quality of the glass used in the manufacture of all optical instruments, the chief difficulties being surmounted in the Jena glass factory, whereby a complete revolution was effected in the microscopic objective. The apochromatic glasses of Zeiss, Powell, Beck, Ross, Watson, Swift, and other makers, in which the secondary spectrum has been totally eliminated, or only a negligible tertiary spectrum remains—that is to say, the objectives of these makers—are now corrected for three spectrum rays, and not two, as in the older objectives; and only those who look forward for making further discoveries in the intimate structure of bacilli or for resolving the finest diatom markings can be said to fully appreciate the importance and value of the investigations of the late Professor Abbe, and which have, so to speak, entirely changed old empirical views as to the value of high aperture, and demonstrated that high amplification, unless associated by proportionally high aperture, necessarily produces untrue images of minute structures. It was he also who introduced a practically perfect system of estimating apertures, known as the “numerical aperture notation,” by which not only can an accurate comparison be made of the relative apertures of any series of objectives, whether dry or immersion, but their resolving power under the various conditions of the kind of light employed. Their penetrating power and their illuminating power can now be estimated with mathematical exactness.
Fig. 117.—Diagram of an Apochromatic Combination.
The practical advantages, then, secured by the adoption of the homogeneous system were, on the whole, greater than any before made or believed to be possible, and when taken into account in connection with the improvement of the eye-piece (also due to Abbe), almost perfect achromatism and homogeneity between objective, object, and eye-piece is secured, together with a sharp definition of the image over the whole visual field. These, with an increase of working distance between the object and the objective, and other important results, have been placed within the reach of the microscopist by men of science, and the outcome is the general adoption of the homogeneous system, termed by Carl Zeiss, a fellow-worker with Abbe, the[22] apochromatic system of constructing objectives.
Relative Merits of the English and German Objectives.
As to the relative merits of German-made objectives, no superiority can be claimed for them over those made by English opticians.
The Continental form of the 1⁄12-inch oil-immersion objective, shown in [Fig. 118], on the scale of 6 to 1, consists of four systems of lenses, namely, the front, a deep hemispherical crown lens of high refractive index; the second front of the system, an achromatic lens of such a form that it gathers the light from the hemispherical front; the middle lens, a single meniscus; and the back an achromatised lens, the second front of the back being connected in such a way as to compensate for the spherical and chromatic aberrations of the front lens.
The first homogeneous immersion objective which came under my observation was manufactured in the well-known Jena workshop of Carl Zeiss, December, 1877. This had a very considerable increase of numerical aperture, upwards of 50 per cent.; a clear gain, as an oil angle of even 110° proved to be of greater value than an angle of 180° in air, while the resolving power of the objective was increased in like proportion. There does not at present appear to be a bar to the construction of objectives of yet higher power, with increase of aperture. The available course open in this direction is the further discovery of another vitreous material and a suitable immersion fluid with an index of 1·8 or 1·9, and glass with a corresponding index, so as to ensure homogeneity of the combination. Zeiss asserts that in the more difficult departments of microscopical research the apochromatic lenses will supplant the older objectives, yet there are many problems in microscopy awaiting solution which do not demand the highest attainable degree of perfection in the objective, and in the majority of cases the older achromatic objective is all that is needful, provided it is good of its kind. The achromatic objectives and eye-pieces of the older type have still an advantage, as, owing to their simpler construction, really good lenses of the class required can be purchased at considerably lower prices than the objectives of the new series. These, from being more complicated in construction, involve a greater amount of skilled manual labour.
