It has been already explained that two objectives, one of much greater power than the other, but both having only the same numerical aperture, will show only the same amount of detail; the higher power on a larger scale. That is, supposing with a ¼-inch objective of 1·0 numerical aperture certain structure is resolved, then a 1⁄8-inch substituted with exactly the same numerical aperture, but with double the magnification, no more resolving power will be found in the latter objective than in the former. For this reason a doubt has been expressed as to whether high-power objectives—especially the more expensive oil-immersions, made to transmit large pencils of light through their larger apertures—are so well adapted for ordinary research as the best series of dry achromatic objectives, or even, in some instances, the medium aperture lenses; undoubtedly, for histological (physiological and pathological) work, the latter will be found to meet the students’ requirements quite as well as the former.
The student or amateur will do well to commence with moderate or medium powers, a 2-inch, a 1-inch, a ½-inch, a 4⁄10-inch, or ¼-inch. These, together with the A and B eye-pieces, will give a range of magnification from 30 to 250 diameters.
Penetration in the objective is a quality for consideration, as the adjustment of high powers is a work of delicacy, and in some cases their penetration is impaired by the arrangement made to obtain finer definition. The value, however, of penetration in an objective is always considered to be of more or less importance. It is a quality whereby, under certain conditions, a more perfect insight into structure is obtained. As a rule, the objective having the largest working distance possesses the better penetration. Theoretically, the penetration of an objective decreases as the square of the angular aperture increases. For this reason the medical student will be justified in choosing the objectives I have named, since these will be better adapted to his work and pursuits. The penetration of the objective is a relative quality assessed at a different value by workers whose aims are widely different. But for the observation of living organisms, the cyclosis within the cell of the closterium or valisneria, for instance, preference will undoubtedly be in favour of the objective with good penetration.
Resolving Power.—This is a quality highly prized by the bacteriologist. In the case of the high-angled apochromatic oil-immersion, with its compensating eye-piece, its resolution is found to be of very considerable advantage, because of its capacity to receive and recombine all the diffraction spectra that lie beyond the range of the older achromatic objective, with its smaller angular aperture. The actual loss of resolving power consequent upon the contraction of aperture from 180° to 128½° is ten per cent., if not more. Resolution depends, then, upon the quality and quantity of the light admitted, the power of collecting the greatest number of rays, and the perfection of centring. In other words, upon the co-ordination of the illuminating system of the microscope—mirror, achromatic condenser, objective and eye-piece. If diatoms are employed as test-objects, it should not be forgotten that there are great differences, even in the same species, in the distances their lines are apart. For this reason ruled lines of known value, as Nobert’s lines, are to be preferred. The following example will suffice to show the value of a dry 1⁄8-inch objective of 120° in defining the rulings of a 19-band plate, which is equivalent to the 1⁄67000th of an inch. This objective, with careful illumination, showed them all; but when cut down by a diaphragm to 110°, the eighteenth line was not separable; further cut down to 100° the seventeenth was the limit, to 80° the fourteenth, and to 60° the tenth was barely reached.
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]
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.