GENERAL METHODS. 9 



ble to make considerable masses of tissue as transparent 

 as possible. For this clearing of preparations, vegetable 

 anatomists have heretofore used chiefly strong dissolving 

 or disorganizing reagents, like caustic potash, chloral hydrate, 

 etc., which exert a clearing effect chiefly by the solution or 

 swelling of substances that hinder observation. 



Clearing media play an important part, especially in all 

 stained preparations ; but here the above-named reagents 

 are not applicable, since they would destroy most stains. 

 In this case one must avail himself almost exclusively of the 

 methods for some time employed by zoologists and anato- 

 mists, which consist in placing the preparations in strongly- 

 refractive media like clove-oil, Canada balsam, etc. These 

 clear less by destroying than by equalizing the refractive 

 differences. 



We may thus conveniently distinguish between chemical 

 and physical clearing, even though a perfectly sharp line 

 cannot be drawn between the two processes. Indeed the 

 reagents that are primarily chemically active often have 

 besides a clearing effect due to their higher refractive 

 indices. Yet here there are always two essentially different 

 methods involved, and a separate consideration of them 

 seems to me justified. I will, however, remark that the 

 physical clearing methods are in no way limited to stained 

 preparations, and can be used with the best results, especially 

 in investigations with polarized light. 



A. CHEMICAL CLEARING METHODS. 



12. Formerly caustic potash was almost wholly used 'for 

 the chemical clearing of preparations. More recently vari- 

 ous other clearing media, especially phenol, chloral hydrate, 

 and eau de Javelle, have been recommended, and in most 

 cases decidedly deserve preference. According to the 

 object of the investigation, one may use sometimes one and 

 again another medium with the best results. As to the 

 manner of using these reagents, the following may be said. 



I. Potassium Hydrate (KOH) is used mostly in aqueous 



