CHAPTER XIX. 



EXAMINATION AND PRESERVATION MEDIA. 



399. Introductory. I 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. 



400. 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 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 occasionally 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 mikroskopischen Arbeiten, Braunschweig, 1892, p. 42, 

 and other sources, may be useful as a guide to the optical effects of 

 various media. The figures give the approximate indices of refrac- 

 tion. 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. 



