THE COLLOIDAL STATE 99 



mirror from which it is reflected downward again, thus illumi- 

 nating the particles a second time from above. The first 

 illummation is, however, the important one, for it directs the 

 maximum brilliancy of the ellipse of scattered light into the lens. 

 The presence of a mirror in the lens prevents direct light from 

 passing beyond the objective to the eye of the observer, but it 

 does not hinder scattered light from the particles entering the lens 

 around the mirror. In order that only as much direct light shall 

 enter as can be reflected by the mirror, the aperture of the iris 

 diaphragm below the microscope stage must be at least as small 

 as the mirror. 



The Spierer lens is a dark-field system in itself; it may, there- 

 fore, be used with an ordinary Abbe condenser if the diaphragm is 

 closed to a pinhole. But it is more convenient to use it with a 

 special cardioid condenser (c, Fig. 76) which has a fixed aperture 

 {d) of correct size (as small as the mirror). The use of a cardioid 

 dark-field condenser with the Spierer lens adds still another 

 ellipse of scattered light. The Spierer optical system has brought 

 to light an interesting structure in cellulose (page 256). 



All dark-field methods of illumination depend upon diffraction 

 phenomena for their usefulness. This is both an advantage and 

 a disadvantage. The diffraction of light by ultramicroscopic 

 structure makes that structure evident, but the picture revealed 

 may not be an exact duplicate of the actual structure. It may 

 be merely a diffraction image. This is, however, not necessarily 

 true, but one must be cautious in interpreting dark-field pictures. 

 The great .advantage lies in the fact that if interference in any 

 form is obtained, then colloidal structure is certain to be present, 

 and usually the dark-field picture tells the type, even if it does 

 not always give an exact counterpart of the structure. 



Structural features in protoplasm that are invisible and others 

 that are but faintly visible with direct light are brought out 

 vividly with the aid of dark-field illumination. Such illumination 

 also greatly facilitates the observation of bacteria. 



Orders of Magnitude and Particle Size. — In order better to 

 grasp the dimensions that characterize the colloidal state, it will 

 be well to stop a moment and recall the ultramicroscopic scale. 

 A micron is one-mfllionth of a meter, or one-thousandth of a 

 millimeter, and has the symbol n. This unit does for microscopic 

 objects; thus, a human blood corpuscle or an average globule of 



