430 HISTOLOGICAL TECHNIQUE. 



sink in the clearing-agent. In order to transfer them from the 

 clearing-agent to a slide, the first step in mounting, a good method 

 is to slip a strip of cigarette-paper under the section, withdraw it 

 along with the section (using a platinum needle as aid, if necessary), 

 drain off the superfluous fluid, and then lay the cigarette-paper on 

 the slide, section side down. Light pressure will now squeeze out 

 considerable of the clearing-agent, when the paper can be stripped 

 from the section and slide, leaving the section nearly dry and with- 

 out folds or wrinkles. With a little care, this method of transferring 

 the section to the slide rarely fails. When such is the case the 

 manipulations must be repeated. 



A drop of the mounting-medium is now placed upon the section 

 and a cover-glass laid on and gently pressed down until it comes in 

 contact with the section and the excess of balsam or dammar is 

 expelled from beneath the cover. If the sections tend to raise the 

 cover, the latter may be weighted with a bullet placed in its centre. 

 Freshly mounted specimens are not so favorable for examination 

 with high powers as those that have been mounted for a few hours 

 or days. This is because the refractive indices of the clearing-agent 

 and mounting-medium are not identical. When these have become 

 thoroughly mixed, or the former has evaporated, the specimen is 

 impregnated with and surrounded by a homogeneous medium that 

 does not scatter the light passing through it. 



Canada balsam has a somewhat higher refractive index than 

 dammar. It therefore renders the sections a little more transparent 

 and more completely obliterates the structure-picture. When it is 

 desired to retain as much of the structure-picture as possible, 

 which is usually the case, dammar should be chosen for the mount- 

 ing-medium. It dries a little more slowly than balsam, but soon is 

 sufficiently dry at the edges of the cover-glass to preserve the sec- 

 tion from injury. If the slides are kept in a horizontal position, 

 in a warm place (40 to 50 C. ; 104 to 122 F.), for a couple of 

 days, they will be dry enough for storage, but for several weeks 

 must be handled with care. 



Stained sections may be examined in glycerin, having been 

 mounted by the same manipulations as those used for mounting 

 in balsam, without previous dehydration or clearing. Such mounts 

 are, however, difficult of preservation. The various cements that 

 have been recommended for fastening the edges of the cover-glass 

 to the slide are usually inefficient, as the changes of temperature 



