98 



PROTOPLASM 



the objective's aperture. The beam of hght used for illumination 

 must be larger than the central stop. 



Fig. 74.— 

 Central stop 

 diaphragm. 



Fig. 75. — The unsym- 

 metrical scattering of light 

 by colloidal particles. 



An ingenious development of dark-field illumination is the 

 Spierer lens. It was designed by Charles Spierer and is based 



on the principle that a colloidal parti- 

 cle scatters light unevenly in such a 

 way that there are more rays given 

 off in one direction than in another, 

 viz., in the direction of the illuminat- 

 ing ray (Fig. 75). Obviously, then, 

 the visibility of ultramicroscopic 

 particles depends upon the angle of 

 the illuminating ray. If a colloidal 

 particle is viewed toward the source of 

 illumination, the observer will then see 

 the maximum amount of scattered 

 light. A brighter picture, smaller 

 particles, and a finer structure will 

 consequently be discernible. The 

 principle of the Spierer lens is simple ; 

 with its accouterments it is illustrated 

 in Fig. 76. Observation toward the 

 source of illumination, with at the 

 same time dark-field, are accomplished 

 by placing a tiny metal mirror of 

 silver, gold, platinum, or aluminum 

 (m. Fig. 76), within an oil-immersion 

 objective. This metallic reflector 

 covers but a small part of the lens 

 surface. Light comes up directly from 

 below through a small aperture, passes through and illuminates 

 the colloidal matter, and enters the lens. Here it strikes the 



Fig. 76. — The Spierer lens and 

 special (Zeiss-Spierer) cardioid 

 condenser: o = microscope ob- 

 jective; I = lower lens of the oil- 

 immersion system; m = 

 platinum (Spierer) mirror ; 

 r.r. = reflected rays from the 

 (Spierer) mirror, e = cover slip, 

 / = colloidal material; ^ = slide; 

 c = cardioid condenser; d = 

 special fixed diaphragm; a = 

 1.5-mm. aperture for direct light, 

 d.r., to Spierer lens; b = slit for 

 cardioid rays, c. r. 



