COPAL VARNISH. 251 



Commercial Venice turpentine is mixed in a tall cylinder glass with an equal 

 volume of 96 per cent, alcohol, allowed to stand in a warm place for three 

 or four weeks, and decanted. It is stated that preparations may be mounted 

 in this medium without previous clearing with essential oils or the like. 

 The index of refraction being lower than that of the above-named balsams, 

 delicate details are more distinctly brought out. Stains keep well in the 

 medium, and Vosseler states that he possesses preparations made fifteen 

 years ago that are perfectly well preserved. 



This medium is also recommended by SUCHANNEK (ibid., vii, 4, 1891, 

 p. 463). He advises that it be prepared with equal parts of Venice turpen- 

 tine and neutral absolute alcohol (obtained by treating commercial absolute 

 alcohol with calcined cupric sulphate and quicklime). The mixture should 

 be agitated frequently and kept in a tile stove for a day or two until clear 

 and sufficiently inspissated. 



424. Copal Varnish. I have seen tissues very instructively 

 mounted in this medium, which is probably worthy of further 

 study. " Berry's Hard Finish/' which is an easily obtainable 

 copal varnish, has been highly praised for mounting purposes 

 (see Journ. Roy. Mic. Soc., 1887, p. 1064). 



425. Castor Oil. This has been lately recommended as a mounting 

 medium for certain delicate tissues (sections of eyes of Cephalopods) by 

 GBENACHEB (Abhandl naturf. Ges. JfaWe.a.-fif.,Bd.xvi; Zeit.f. wiss.Mik., 

 1885, p. 244). This was with the idea that its low refractive index (n = 

 1'49, whilst Canada balsam n = T54) would give a useful augmentation of 

 visibility for the more refractive elements of the tissues. 



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

 to be the case. 



426. Photographic Negative Varnish (for mounting large sections 

 without cover-glasses). See WEIGEET, Zeit.f. wiss. Mik., iv, 2, 1887, p. 209. 



427. Styrax and Liquidambar. See Journ. Roy. Mic. Soc., 1883, p. 

 741 ; ib., 1884, pp. 318, 475, 655, and 827 ; and the places there quoted. 

 Also Bull. Soc. Beige de Mic., 1884, p. 178 ; and FOL, Lehrb., p. 141. 



428. Tolu Balsam (see Zeit.f. wiss. Mik., iv, 4, 1887, p. 471). 



429. Frankincense (see Journ. Boy. Mic. Soc., 1892, p. 901). 



