68 ELEMENTARY CHEMICAL MICROSCOPY 



in place the liquid forms a thin film q, the excess of liquid being 

 forced into the groove o. The quartz cell is held in position 

 upon the stage of the microscope by means of a brass chamber B 

 consisting of a bed-piece into which the cell fits, a funnel shaped 

 section / pressing gently upon the quartz cover, and a top sec- 

 tion t screwing down upon the section /. When much work is 

 to be done with this device it is best to have all the screw threads 

 but three turns cut from the bed-piece and a slight recess cut 

 as shown at i. This permits a rapid removal of t, and/ is easily 

 lifted out. As furnished by the makers/ is flush with the threads 

 of the bed-piece b and being smooth with no milling is hard to 

 remove. A small pin in/ fitting into a hole in 6, not shown in the 

 diagram, prevents / from turning when t is being screwed down. 



It is absolutely essential that both cell and cover be absolutely 

 clean and free from all dust particles. Unless so clean that 

 when the cover is laid upon the cell and very gently pressed 

 Newton's rings can be seen the device is unfit for use. To pre- 

 pare Q for use wash very thoroughly both pieces, immerse in 

 hot chromic-sulphuric cleaning mixture, rinse with distilled 

 water, follow by purified alcohol, and dry in a current of warm 

 air, next support upon a loop of platinum wire and heat to a 

 bright red in a Bunsen burner. As soon as the pieces are cool, 

 lay in place in B, and use them at once. When employing the 

 quartz cell and cardioid condenser, never use anything but 

 water as immersion fluid between condenser and cell. 



Use only sufficient liquid to form a thin layer q and not quite 

 fill the groove o. 



The objective must be centered by means of the adapter A 

 (Fig. 30), so that the bright spot of light formed in Q will fall 

 in the center of the field. 



Always raise or lower the cardioid condenser so as to ascertain 

 the proper position for the blackest background and brightest 

 diffraction images. 



See that the beam of light from the radiant falling upon the 

 mirror of the microscope is of sufficient diameter to fill the aper- 

 ture of the condenser. 



Use an arc of not less than 15 amperes. 



