Fig. 204a.—Outer Membrane of Lower Plane of Beads thrown from remaining holes of grating; on raising the focus white interspaces turn into red beads.
Objective used, Zeiss’s apochromatic 1⁄12-inch oil-immersion, numerical aperture 1·40, magnifying power 1,750 diameters.
A well-known skilled observer of test objects[41] says: “Practically the resolving power of our achromatic objectives on lined objects reached their maximum in the late Dr. Woodward’s hands. Amphipleura pellucida was then, as now, the finest known regular structure of the diatoms. There appeared then nothing more to be gained in resolution when one of the apochromatic 1⁄12-inch objectives of Zeiss, with its entire absence of colour, passed into my hands, and I soon became convinced that it possessed the power of separating the different layers of structure in the valve, beyond the grasp of the dry-objective. The result of this increase of power enabled me to split up, as it were, the one plate of silex forming the valve of Pleurosigma formosum into three layers, and which had never before appeared to be possible; proving, in fact, that magnification without corresponding aperture is of little or no account.”
“The intimate structure of these test objects,” says Mr. Smith, “is built up on one plan, each being composed of two or more layers, (1) a valve with two layers, as in Pleurosigma balticum; (2) two layers with a grating and secondary markings placed diagonally, as in Pleurosigma formosum; (3) with two layers of a net-like structure, as in Pleurosigma angulatum, the fineness of the striæ or gratings of which measure the 1⁄50000th of an inch. Five other diatoms afford evidence of this compound structure. The presence of beads or hemispheres in one of the focal planes, and depressions or pits in another, are emphasised in the micro-photograph itself; reduced portions of the valve are represented in Figs. 204 and 204a.”
A portion of a diatom valve, Pleurosigma angulatum, micro-photographed on a higher scale of magnification, 4,500 diameters, is given further on.
Fig. 205.—Sections of an old-fashioned Glass Tumbler, from photographs by the late Mr. R. Beck.
Errors of interpretation arise either from the small cones of illumination afforded by the dry-objective, or the oblique illumination formerly resorted to for the resolution of these difficult test objects, and several of the lights and shadows resulting from the refractive power of the object itself. But the most common error is that produced by the reversal of the lights and shadows resulting from the refractive powers of the object itself. To make this clear, I reproduce two reduced photographs of a small section of an old-fashioned glass tumbler, covered externally with numerous hemispheres, illuminated by transmitted light ([Fig. 205]).
This illustration well emphasises the difficulty there is in determining structure under precisely similar conditions to those we are accustomed to of examining valves of diatoms under the microscope. If these photographs be held in front of a strong light, they at once convey different impressions to the mind, the hemispheres appearing depressions in the one, and raised beads in the other. Both are prints from the same negative, but in mounting are reversed; and therefore the apparent dissimilarity is due to a slight inequality of illumination, which the mind accepts as light and shade.