ZOOLOGY AND BOTANY^ MICKOSCOPY, ETC. 445 



cement three white glass beads with hardened balsam. Put a small 

 quantity of soft balsam in the centre of the cell, and gently warm it 

 over a spirit-lamp. Take the object, a wasp's or blow-fly's head, for 

 example, and place it upon the previously warmed balsam, arranging 

 it in the required position. Now take a clean cover-glass, the diameter 

 of which should be a little less than that of the cell, and holding it 

 between the points of a pair of forceps, place a large drop of balsam 

 in its centre, and allow it to fall upon the object. The edge of the 

 cover should rest upon the three beads. If the quantity of balsam 

 under the cover-glass is not suflficient to fill up the whole of the space 

 between it and the slide, a little more must be allowed to run in, and 

 if the object has become displaced, it may be rearranged by means of a 

 fine blunt needle, introduced beneath the cover-glass. A clip should 

 be used during the last operations, but only to prevent displacement 

 of the cover. The slide must now be put aside in a warm place, until 

 the balsam is hard enough to allow the superfluous portion to be 

 removed safely. Sufficient balsam should be left to form a sloping 

 edge around the cover-glass, and it should be hardened for a few 

 days after cleaning. Be sure that the balsam is quite hard before 

 applying brown cement. The ease with which an object can be re- 

 arranged, or a chance air-bubble removed, without distui'bing the 

 cover-glass, constitutes the chief advantage of using beads. A supply 

 of different sizes should be kept, and the size used must be regulated 

 by the thickness of the object. Pure balsam in collapsible tubes is 

 to be strongly recommended, on account of the nicety with which the 

 quantity of balsam required for mounting a slide can be regulated. 

 The neck of the tube should be wiped with a clean cloth moistened 

 with benzole before the screw-cap is replaced, in order to prevent the 

 possibility of a little balsam hardening in the screw, and so prevent 

 the easy removal of the cap when next required. 



Reagent for Simultaneous Staining and Hardening.* — In view 

 of the objections to the various combinations of staining and hardening 

 reagents hitherto employed, E. Pfitzer, in order to meet the require- 

 ments of vegetable microscopy, has devised a fluid which both hardens 

 and stains. It consists of the colouring matter, nigrosin, dissolved 

 with picric acid, in water or alcohol. 



a. To a concentrated watery solution of picric acid is added a small 

 quantity of a watery solution of nigrosin ; if the object to be studied 

 contains much water, some crystals of the acid are added, in order 

 to maintain the strength of the liquid. 



The deep olive-green fluid kills with great rapidity. After some 

 hours' immersion of the object which is to be examined, it may be 

 transferred to common spirit, especially if it is desirable to dissolve 

 out chlorophyll, &c., or if the object has to be kept for some time. 

 By this means the denser masses of protoplasm are stained pale violet, 

 the chromatophores darker, while the pyrenoid, nucleoli, and other 

 coloured parts of the cell-nucleus come out deeply stained; thin 



* Ber. Deutsch. Botan. Ges., i. (1883) pp. 44-7. 



