112 
ELEMENTARY CHEMICAL MICROSCOPY 
Fig. 53. Biltz-Thomae Cell. 
cells the essential feature is the central dark glass chamber of 
about 3 millimeters internal diameter, provided with two small 
windows at right 
angles to each other 
—these two windows 
consist of either thin 
glass or, better, of 
very thin quartz 
disks cemented in 
place. The passage of the beam of light through one of these 
cells is shown in the diagram. Fig. 54. No light other than 
that diffracted 
from the particles 
in suspension in 
the hquid can en¬ 
ter the observing 
microscope. The 
cell is usually at¬ 
tached to the 
microscope ob- 
Fig. 54 . 
Illuminating Rays in the Cell of the Slit 
Ultramicroscope. 
jective by a special cell holder; this, however, is open to the 
serious defect of difficulty in focusing and that cells purchased 
at different times are not exactly of the same thickness of wall, 
and hence the center of the upper window will not fall in the 
optic axis of the microscope. For these reasons the author 
prefers to support the cells upon an elevating mechanical stage P, 
as shown in Fig. 51. This arrangement permits the shifting and 
easy adjustment of the cell, so that its upper window is exactly 
centered with respect to the optic axis of the observing micro¬ 
scope. The cell is held in place by the spring clips c. The 
stage supporting the cell U may be raised and lowered by 
means of a knurled nut q. The nut p clamps the stage in 
place while the screws Wi and W2 serve to move P forward 
or back and to the right or left. 
One of the most serious defects of the Biltz cell is the difficulty 
of properly cleaning it after use, especially when there has been 
deposition of a colloidal film upon the windows. Treatment 
