CHAPTER XXI 

 EXAMINATION AND PRESERVATION MEDIA 



438. Introductory. We comprehend under this heading all the 

 media in which an object may be examined to advantage. 



All preservative media may be used for mounting, though the 

 only media that will afford an absolutely sure preservation of soft 

 tissues are the resinous ones. 



439. Refractive Indices of Examination Media. An examination 

 medium should be of such a refractive index as to afford a due 

 degree of visibility of colourless {unstained) elements. The 

 visibility of these is inversely as their transparency when pene- 

 trated by the medium. It is directly proportional to the difference 

 between the refractive indices of the object and of the medium in 

 which it is mounted. The greatest transparency is obtained 

 when the refraction of the medium is the same as that of the 

 tissue elements. Media having a lower index than that of the 

 tissues give diminished transparency, but greater visibility. 

 Media having a higher index than that of the tissues give great 

 transparency, but diminished visibility of (unstained) details. 

 Now the index of refraction of most tissue elements, after fixation 

 and dehydration, is occasionklly higher than that of Canada 

 balsam : so that media of the greatest clearing power {i.e. giving 

 the greatest transparency) must be looked for amongst reagents 

 having an index superior to that of balsam, whilst for enhanced 

 visibility of detail we must employ less refractive media, such as 

 castor oil, glycerin, or water. 



The following short list, extracted from Behrens' Tabellen 

 zum Gebrauch bei 77iikrosko2)ischen Arbeiten, Braunschweig, 181)2, 

 p. 42, and other sources, may be useful as a guide to the 

 optical effects of various media. The figures give the approxi- 

 mate indices of refraction. They should be accepted with 

 some caution, on account of the variability of samples. 

 The figures given for balsam refer evidently to the resin in 

 the solid state and not to the solutions used for mounting, 

 which are certainly much lower, according to the lower index 

 of the solvent. 



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