212 CHEMISTRY OF PLANT LIFE 



by passing the rays from the source of light through a series of 

 condensing lenses which are adjusted at the proper distance and 

 angles to bring the image of the illuminating body within the tube 

 containing the substance which is to be examined and in the line of 

 vision of the microscope. Obviously, this results in intense illu- 

 mination of any particles in the solution which come within this 

 brilliant image of the sun, or arc, and therefore renders visible 

 particles which are of less diameter than the wave-length of 

 ordinary light (450/iM to 760;u/x for the visible spectrum) and, 

 hence, are not visible by the ordinary means of illumination in the 

 direct line of vision. It will be apparent that what is seen in the 

 field of the ultramicroscope is not the particles themselves, but 

 rather the image of the sun (or other illuminating body) falling 

 upon the particles which come within the image, just as one does 

 not see the paper but only the image of the sun when the rays from 

 the sun are brought to a focus upon a sheet of paper through any 

 ordinary convex lens, or " burning glass." Hence, the ultra- 

 microscope gives no idea of the shape, color, or size of the par- 

 ticles upon which the image falls; but it does permit the counting 

 of the number of particles within a given area, and a study of 

 their movements, from which it is possible, by mathematical com- 

 putations, to calculate the relative size of the particles themselves. 

 Repeated studies have shown that particles of the sizes between 

 5juM and 250/zM in diameter, which are visible under the ultra- 

 microscope, are sufficiently small to bring about the surface 

 phenomena which are known as properties of colloidal solutions. 

 Further, the ultramicroscope permits the observation of the 

 growth, or disintegration, under various chemical reagents, of the 

 individual colloidal particles, which appear as scintillating points 

 in the field of the microscope; and the study of changes in rela- 

 tionships during gel-formation, peptization, etc. 



The " Tyndall Phenomenon." Colloidal solutions exhibit this 

 phenomenon; that is, if a bright beam of light be passed through a 

 sol which is contained in a clear glass vessel having parallel 

 vertical sides, and the solution be viewed from the side, it appears 

 turbid and often has a more or less bluish sheen. This effect is 

 due to the small particles in the sol, of polarizing the light which is 

 reflected from them, the blue rays being bent more than are those 

 in the other part of the spectrum. The Tyndall phenomenon is 

 similar in its effect in making the tiny particles of the sol visible 



