INTRODUCTION. 19 



We have, then, in using glycerin as a mounting 

 medium which renders tissues transparent, to guard 

 against its too excessive action ; and this may be 

 done by mixing with it, in varying proportions, 

 some less refractive substance, such as water. For 

 permanent preservation, however, most tissues are 

 to be put into pure or nearly pure glycerin. 



Canada balsam is for many tissues a most excel- 

 lent mounting medium. It possesses to a still 

 greater degree than glycerin the power of rendering 

 them transparent, obscuring proportionately, in the 

 manner above described, certain of their minute 

 structural features. This difficulty can, however, be 

 to a considerable extent obviated with balsam as 

 with glycerin, by the judicious use of coloring 

 agents. Thus, for example, suppose we have an 

 object containing cells, the exact outlines of which 

 we wish to bring clearly into view ; if we stain with 

 haematoxylin alone, and then mount the object in 

 balsam, the nuclei will be distinctly seen, because 

 they have a violet color ; but the cell body will in 

 many cases be wellnigh invisible, because it has 

 almost the same refractive power as the balsam 

 which surrounds it. If, however, before mounting 

 in balsam we stain with eosin, which colors the cell 

 body, this will be distinctly visible under any cir- 

 cumstances. We shall stain most specimens which 

 are to be mounted in balsam first with haematoxylin 

 and then with eosin, and throughout this manual, 



