112 



ELEMENTARY CHEMICAL MICROSCOPY 



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 



FIG. 53. Biltz-Thomae Cell. ver Y 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 liquid can en- 

 ter the observing 

 microscope. The 

 cell is usually at- 

 tached to the 

 microscope, ob- 

 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 



FIG. 54. Illuminating Rays in the Cell of the Slit 

 Ultramicroscope. 



