144 MICROSCOPIC TECHNIQUES 



permit lower incineration temperatures which result in less volatili- 

 zation of chlorides, nevertheless it is sufficient as a rule to carry out 

 the treatment in air. 



4. Microscopic Exainiiiation and Interpretation 



Observations of incinerated sections should be made under the 

 microscope with dark-field illumination provided by a cardioid 

 condenser. A proper light source is an important factor and Scott 

 (1937) pointed out that a carbon arc seems to produce excessive 

 longitudinal aberration, while a Spencer illuminator fitted with a 

 500 watt projection lamp or a Zeiss Point-0-Light lamp, with proper 

 centering of the condenser and adjustment of the mirror, are suitable. 

 Unscreened light is best for observation of minute particles, but 

 the use of a daylight filter or ground glass is more restful to the eye. 

 Mercury vapor lamps enable high resolution of small particles but 

 make recognition of colors difficult. 



The use of a comparing ocular with two microscopes is recom- 

 mended for simultaneous observation of both an incinerated section 

 and its stained control. Of course, the stained section is illuminated 

 in the ordinary bright field. 



If the incineration has been carried out properly, there will be no 

 black or brownish carbon deposits. The topographic disposition of 

 the ash no doubt fairly represents the distribution of mineral con- 

 stituents in the fixed tissue. That this distribution is exactly main- 

 tained in the living tissue cannot be said with certainty; however, 

 Scott ( 1932 ) has found the parallelism that histological sites in 

 living tissues that absorb ultraviolet radiation (275 m/x) are those 

 which yield large amounts of ash on incineration. 



The only elements that can be identified in the ashed sections 

 with any measure of certainty are considered in the following 

 sections. 



Sodium and Potassium. It has been assumed that sodium and 

 potassium yield a fine-grained, faintly bluish-white ash. Policard 

 and Pillet (1926) attempted the identification of sodium and 

 potassium as the sulfates by exposing sections, before incinerati(^n, 

 to the fumes of sulfuric anhydride in order to convert the chlorides 

 to sulfates. The sulfates are resistant to volatilization during the 

 ashing, while the chlorides are apt to be lost. 



