. Contributions from the Botanical Laboratory and the Morris Arboretum of the University of Pennsylvania, vol. 14. Botany; Botany. y^M^^MW/A V////^^/^/W///. ^ riG. 5. The optical system of a Spierer lens: 0 = objective m = Spierer mirror 1 = lens = reflected rays = scattered rays e = cover slip f = material g = slide c = cardioid condenser d = diaphragm a = aperture for direct rays = direct rays for Spierer lens b = slot for cardioid condenser rays = cardioid condenser rays light unevenly, , there are more rays given off in one direction than in any other, namely, from
. Contributions from the Botanical Laboratory and the Morris Arboretum of the University of Pennsylvania, vol. 14. Botany; Botany. y^M^^MW/A V////^^/^/W///. ^ riG. 5. The optical system of a Spierer lens: 0 = objective m = Spierer mirror 1 = lens = reflected rays = scattered rays e = cover slip f = material g = slide c = cardioid condenser d = diaphragm a = aperture for direct rays = direct rays for Spierer lens b = slot for cardioid condenser rays = cardioid condenser rays light unevenly, , there are more rays given off in one direction than in any other, namely, from that side which is away from the source of illumi- -rtS- v >*' ^ 1 >, « * T i> p J • -<pn %. SEIFRIZ: PHYSICAL METHODS OF RESEARCH ON PROTOPLASM 107 nation. Degree of visibility of submicroscopic particles thus depends upon the angle of the illuminating ray. If a colloidal particle is viewed toward the source of illumination instead of at right angles to it, as in the slit ultra-microscope, or at 45 degrees to it, as in the dark-field (cardioid) con- denser, the observer will then see the maximum amount of scattered light. A brighter picture, smaller particles, and a finer structure will conse- quently be discernible. In order to accomplish this, Spierer placed a tiny metal (silver, gold, platinum, or aluminum) mirror (m, fig. 5) within an oil-immersion objective. This metallic reflector covers but a small part of the lens surface. Light comes directly from below, and passes through the colloidal matter. It then enters the lens, where it strikes the mirror from which it is reflected downward, thus illuminating the particles a second time from above. The first illumination is, however, the important one for it directs the maximum brilliancy of the ellipse (or rather the three-dimen- sional ellipsoid) of scattered light into the lens. In the usual dark-field condenser a less bright portion of the ellipse enters the objective. The presence of a mirror in the lens pr
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