EESINS AND BALSAMS. 207 



With the objects with which I have experimented I have not found this 

 to be the case. 



394. Stephenson's Biniodide of Mercury and Iodide of Potassium 



(Journ. Roy. Mic. Soc., 1882, p. 167). Interesting, as giving a solution 

 which when saturated has an index of 1*680, the highest index of any 

 known aqueous fluid. I have experimented both with strong and weak 

 solutions, and doubt whether much practical advantage can be derived from 

 them. Tissues are well preserved, but the preparations are ruined by a 

 precipitate which forms in the fluid. 



395. Monobromide of Naphthalin (see Journ. Roy. Mic. Soc., 1880, p. 

 1043 (ABBE and VAN HEUBGK), and Zool. Anz., 1882, p. 555 (MAX FLESCH). 



Resinous Media. 



396. Resins and Balsams, Resins and balsams consist of a 

 vitreous or amorphous substance held in solution by an essen- 

 tial oil. By distillation or drying in the air they lose the 

 essential oil and pass into the solid state. It is these solidified 

 resins that should in my opinion (and that I believe of the 

 best microscopists) be employed for microscopical purposes. 

 For the raw resins always contain a certain proportion 

 of water, which makes it difficult to obtain a clear solution 

 with the usual menstrua, is injurious to the optical properties 

 of the medium, and to its preservative qualities, and further, 

 especially hurtful to the preservation of stains. I therefore 

 recommend that all solutions be made by heating gently the 

 balsam or resin in a stove until it becomes brittle when cold, 

 and then dissolving in an appropriate menstruum. 



FOL (Lehrb., pp. 138-9) is of a different opinion. 



Solutions made with volatile menstrua, such as xylol and 

 chloroform, set rapidly, but become rapidly brittle. Solutions 

 made with non-volatile media, such as turpentine, set much 

 less rapidly and pass much less rapidly into the brittle state. 



As to the old dispute about the respective merits of damar 

 and balsam, the case appears after all to lie in a nutshell. 

 Damar gives the better definition of delicate detail ; balsam 

 has greater clearing action, and affords perhaps more solid 

 mounts. 



It may be remarked here that for some of the purposes for 

 which these media are employed, Oil of Cedar may be found 

 preferable. The mounts need not be closed (except for im- 



