ULTRAMICROSCOPES 6l 



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. 27. No light 

 other than that diffracted from the particles in suspension in the 

 liquid can enter the observing microscope. The cell is usually 

 attached to the microscope objective by a special cell holder; 



FIG. 27. Illuminating Rays in the Cell of the Slit Ultramicroscope. 



this, however, is open to the serious defect of difficulty in focus- 

 ing 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. 24. This arrange- 

 ment 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 microscope. 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 W^ 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 

 with a proper solvent and long washing is imperative. Before 

 introducing a liquid for examination it is always best to pour a 

 little alcohol through the cell and to follow this with the alco- 

 holic solution to be studied, or if aqueous suspensions are to be 



