1 8 BOTANICAL MICROTECHNIQUE. 



'the various manipulations on slides with hollows ground in 

 them. 



3. The Use of Other Strongly Refractive Mounting Media. 

 (a) Dammar Lac. 



26. Dammar lac is best dissolved in equal parts of benzol 

 and oil of turpentine. Its use is the same as that of Canada 

 balsam, from which it differs in its somewhat lower refrac- 

 tive index. Thus differences of structure that depend on 

 differences in refractive power often become more conspic- 

 uous. The glasses described in 22 may, of course, be used 

 for this medium. It appears to have been employed little 

 in botanical microscopy. 



(b) Venetian Turpentine. 



27. This mounting medium proposed by Vosseler (II) is 

 prepared by thinning the resin obtained from the apothecary 

 under this name with an equal volume of alcohol and then 

 Avarming it on a water-bath, shaking it energetically and 

 finally filtering. The filtrate is then somewhat thickened 

 on the water-bath. 



The fluid so obtained has the advantage of mixing with- 

 out cloudiness with 90$ alcohol, and thus it makes prelim- 

 inary clearing with clove-oil unnecessary in cases of incom- 

 plete dehydration. Besides, even with delicate objects, like 

 Spirogyra, transfer from alcohol to Venetian turpentine much 

 less often causes collapse than does transfer to clove-oil, 

 Canada balsam, and the like. One may avoid the crumpling 

 of very delicate objects by placing them first in a mixture 

 of 10 parts turpentine and 100 parts alcohol, and then per- 

 mitting a gradual concentration of the turpentine over an- 

 hydrous calcium chloride, according to the method, of 

 Pfeiffer (I, 30). If small dishes are used for this purpose, 

 they maybe provided with ridges of paraffme, to prevent the 

 turpentine from rising on their sides, by simply dipping 

 them to the proper depth in melted parafifine. 



The refractive index of Venetian turpentine lies between 



