1627 
sometimes straight, sometimes bent, so that they resemble bundles 
of sea-weed as found on the shore. 
The preparation has a strong inclination to deposit such flakes; 
after a few hours all movement has ceased. This is not astonishing 
at all. If we assume that the breadth dimension of the particles lies 
just at the border of the ultramicroscopic visibility, this, according 
to ZsYGMONDI *), must (for metallic oxides) be judged at 30 uu, the 
length dimension will, therefore, lie in the order of 500 uu. If we 
consider that the distance between the two quartz glasses only 
amounts to 1 or 2 g it will not cause any surprise that the par- 
ticles do not keep on moving for a long time. 
In connexion with what now follows it may be observed that the 
particles lie in the field of vision in completely arbitrary directions. 
4. Observations in the slit-ultramicroscope with a Bir cuvette. 
Source of light as above. Objective Zeiss D*. Ocular Huyerns 3 
Now also we notice beside the round images which are in vivid 
motion elongated slowly moving particles. But a remarkable 
difference is shown: we only notice particles in a position 
perpendicular to the axis of theilluminating beam 
with deviation of at most 30°. Particles with their axes parallel to 
that of the illuminating beam are not to be seen, no more than 
those that exhibit deviations therefrom up to about 60°. 
As it could not be supposed that in these circumstances the par- 
ticles were already ranged, it became obvious that there should be 
an optical reason why those particles are invisible which are situated 
more or less parallel to the direction of the source of light (we 
shall in future call the position of these particles “sagittal” in con- 
trast with that of the visible ones which lie “frontally”; for the ray 
of light arrives straight at the observer). This surmise was in 
agreement with the above-mentioned fact that with a cardioid con- 
densor (which causes an all-round illumination of the preparation) 
the particles were seen in quite an arbitrary direction. 
Subsequently, I have observed the preparation while it was 
passing slowly through the cuvette. A capillary exit tube which 
could be placed higher or lower by means of an elevator made it 
possible to regulate this velocity of currency at will. If indeed the 
particles are ranged by such a currency parallel to the direction of 
the current, the invisible sagittal particles should be rendered visible 
by the frontal flow. As a counting of the particles in a running 
liquid was very difficult, the clearing up of the field of vision was 
1) Kolloidchemie, Leipzig 1912, blz. 11. 
