8o 



PRINCIPLES OF GENERAL PHYSIOLOGY 



microscope made for the examination of liquids, a very intense but small beam 

 of light is projected horizontally from the sun or an arc lamp, by means of 

 a system of condensing lenses, into the liquid contained in a small cell with 

 a flat side towards the light and a flat top towards the observer. The track 

 of the beam is examined from above by means of a water-immersion lens forming 

 the objective of an ordinary microscope. If the solution contains particles, 

 these are seen as bright discs with vigorous Brownian movement. The limit 

 of visibility depends on the intensity of the illumination. The finest particles 

 cannot be distinguished separately, but are indicated by a haze. Zsigmondy 

 uses the name, " submicron," for elements seen as separate discs, although 

 invisible in the ordinary microscope, and " amicron " for those which even the 

 ultra-microscope can only indicate as a diffuse illumination in the track of the 

 beam. Fig. 36 shows the course of the light rays ; Fig. 37 the arrangement of 

 the apparatus, in Zsigmondy and Bachmann's (1914) pattern, and Fig. 38 shows 

 the illuminated field seen by the observer. 





Fio. 36. 



DIAGRAM OK THE COURSE OF TUB RAYS OF LIGHT IN 

 THE ULTRA-MICROSCOPE. 



The correct interpretation of all the phenomena seen by this method has not been arrived 

 at as yet, and much caution must be exercised in drawing conclusions. There are one or two 

 points which a little experience in its use with a variety of solutions has taught me, that it 

 may be well to call attention to. It is a matter of some difficulty to obtain water that does 

 not show a few particles, so that in the preparation of colloidal solutions foreign particles are 

 almost unavoidable. Now these may be mistaken for the substance under examination. To 

 take an example, a dilute solution of Congo-red, even the purest, is almost certain to show a 

 few bright discs of light ; but, on close observation with the most intense light that the 

 apparatus can give, it will be noticed that the track is filled by a faint haze. In this case 

 it can easily be shown that the few particles seen are not the dye itself ; the addition of a 

 little acid to the solution splits off the free dye acid and this forms a colloidal solution witli 

 comparatively large particles, so that the whole track of the beam appears densely packed 

 with bright diffraction images. It is unnecessary to repeat that this appearance of densely- 

 packed particles is really due to the disparity in size between the objects and their diffraction 

 images. What is clear is that the dye salt is not resolvable into particles, it consists of 

 "amicrons," whereas the free acid consists of " submicrons." 



Again, if the 

 phase, the system 

 is present. This a 



If a given solution cannot be resolved, it must not, therefore, be assumed that 

 it is a true solution. It may consist of particles too small to be seen by the 



