INTRODUCTORY REMARKS. 223 



CHAPTER XVIII. 



CLEARING AGENTS. 



342. Introductory Remarks. Clearing agents are liquids 

 one of whose functions it is to make microscopic preparations 

 transparent by penetrating amongst the highly refracting 

 elements of which the tissues are composed, the clearing liquids 

 themselves having an index of refraction not greatly inferior 

 to that of the tissues to be cleared. Hence all clearing agents 

 are liquids of high index of refraction. The same substances 

 have also a second function, which consists in getting rid of 

 the alcohol in which preparations are generally preserved, and 

 facilitating the penetration of the balsam or other resinous 

 medium in which preparations are, in most cases, finally 

 mounted. Hence all of the group of bodies here called 

 " clearing agents " must be capable of expelling alcohol from 

 tissues, and must be at the same time solvents of Canada 

 balsam and the other resinous mounting media. 



It is important to note again, notwithstanding some repeti- 

 tion, the manner of employing these agents. The old plan 

 was to take the object out of the alcohol and float it on the 

 surface of the clearing medium in a watch-glass. This plan 

 was faulty, because the alcohol escapes from the surface of 

 the object into the air quicker (in most instances) than the 

 clearing agent can get into it ; hence the object must shrink. 

 To avoid or lessen this cause of shrinkage, clearing is now 

 generally done by the method suggested by Griesbrecht, which 

 consists in putting the clearing medium under the alcohol 

 containing the object. This is done in the following manner. 

 Take a test-tube, and pub into it enough alcohol to contain 

 the objects (a watch-glass will often do well, but a test-tube 

 is safer). With a pipette carefully put under the alcohol a 

 sufficient quantity of clearing medium (or carefully pour the 

 alcohol on to the clearing medium). Then put the objects into 



