CHAPTER XIX. 



EXAMINATION AND PRESERVATION MEDIA. 



399. Introductory. I comprehend under this heading till 

 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. 



400. Refractive Indices of Examination Media. An examina- 

 tion 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 penetrated 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 trans- 

 parency, 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 somewhat 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 BEHUENS' Tabellen 

 zum Gebrauch bei mikroJfopischen Arbeiten, Braunschweig, 

 1892, p. 42, and other sources, may be useful as a guide to 



