PIIOTOMICROGR.IPUY .IXD TECHNICAL MICROSCOPY 125 



or cell cliamher and tlie top of the contlcnscr by a stratum of immersion 

 fluid free from air bubbles. Cedar oil or pure water is uscti for this 

 purpose. The chamber for the cardioid condenser is illustrated in 

 Fig. 22. The chamber is made of quartz glass and consists of a 

 circular disc having on one side a circular groove and an optically 

 plain central portion within the groove about 2 n below the plane 

 outside the grcwve. A drop of the fluid to be examintnl is placed 

 on this depressed central portion and a cover glass of quartz placed 

 over it. The excess fluid is expelled to the annular groove and a 

 stratum about 2 /i in thickness is retained in the central portion of 

 the chamber for microscopic examination. The cell is assembled 

 in the metal mount which has a clamping ring and a recessed member 



Fig. 25 — Diagram of the rays in a cardioid condenser. 



to receive it. The ver>- brilliant illumination resulting from the 

 cardioid condenser would cause glass to fluoresce and for this reason 

 a quartz cell is used. Moreover, glass is more liable to be affected 

 by corroding agents than is quartz. 



The utmost care must be taken to prepare the cell chamber. This 

 includes washing with alcohol and water; dipping in boiling sulphuric 

 and chromic acid solution; washing in tap water; rinsing in distilled 

 water and then in redistilled alcohol; drjing in a hot air current and 

 finally cooling under a bell jar; all of which is necessary to insure 

 absolute cleanliness. 



An automatic arc lamp is used as a source of illumination and the 

 image of the crater is projected by a projection lens onto the mir- 

 ror of the microscope from which the rays are reflected upward into 

 the cardioid condenser. 



