62 ELEMENTARY CHEMICAL MICROSCOPY 



employed, displace the alcohol with distilled water free from all 

 fatty or greasy matter and then introduce the colloidal solution. 

 This process is usually essential in order that the liquid to be 

 examined shall come into perfect contact with the windows of 

 the cell with no interfering film and no air bubbles. 



A much cheaper and simpler cell is shown in Fig. 28. 1 It 

 consists of a tube of black glass with central swelling and win- 

 dows at right angles to 

 each other. These win- 

 dows are either of glass 

 or of quartz, the latter 

 being preferable, since 

 glass is slightly fluores- 

 cent. For use, two pieces 

 of rubber tube are at- 

 FIG. 28. Simple Cell for Use with Slit tached as shown by the 



Ultramicroscopes. dotted Hnes Thege j-^ 



cells give excellent results with gases and vapors and may also be 

 employed for the study of such solutions as will not be affected 

 by contact with rubber. For preliminary examinations they are 

 far more convenient than the Biltz cell and like it can easily be 

 held in place on the type of stage shown in Fig. 24 by thin metal 

 clamps or rubber bands. Moreover, these cells are more easily 

 cleaned and are relatively inexpensive. 



When solids are to be examined, as, for example, specimens 

 of glass, it is important that there be two sides of the preparation 

 which meet at as nearly right angles in as sharp an edge as is 

 possible. The reason for this will readily be understood by re- 

 fering to the diagram, Fig. 29. If the sides do not meet in a 

 sharp edge as shown at a, but form an obtuse angle or rounded 

 edge b, the beam of light must be lowered below b. If this 

 is done, the beam of light R will lie too low to be focused, even 

 if the lower lens of the objective is brought into actual contact 

 with the upper surface of the object. In this case the beam lies 

 beyond the working distance of the objective. Should we at- 

 tempt to bring R within the range W, as indicated in the lowest 

 1 Made by E. Leitz, Wetzlar. 



