246 THE AMERICAN MONTHLY [Sep 



already given an example, when speaking of the discov- 

 ery of the Bacillus tuberculosis ; the elements thus freed 

 from stain can be restained with another color, those 

 which have resisted the washing out process being unaf- 

 fected by this second staining, thus we have what is call- 

 ed a double stain. 



These double stains are not confined to just that branch 

 of research from which our example is taken, but are 

 equally applicable to animal and vegetable histology ; nor 

 is it always necessary to wash out one stain before apply- 

 ing the next, as some stains have the power, when appli- 

 ed subsequently, of replacing the original stain in those 

 elements for which they have a special affinity; others, 

 again, may be mixed in one solution, and thus applied 

 simultaneously, each picking out its own especial element ; 

 there are also some valuable triple stains belonging to 

 this "one solution" class, by means of which our knowl- 

 edge of the leucocytes of the' blood has been greatly in- 

 creased. 



The last point needing mention, the theory of staining 

 with mordants, we have already explained at some length; 

 we cannot here enter into the question of the prelimina- 

 ry treatment of the object to be examined, which, accord- 

 ing to the various reagents used, favorably or unfavor- 

 ably affects the subsequent staining. 



Here then we have the salient points, at least, of that 

 science, without which the microscope, despite the recent 

 improvements in both brass and glass, would still be lit- 

 tle more than a scientific plaything, useful, no doubt, for 

 exhibiting the beautiful and the curious, but incapable of 

 supporting by its evidence any of those numerous sciences, 

 which to-day look to it as their main stay. 



For our own part, we are convinced, that in every one 

 of these sciences the advance of the future will depend 

 more on the mastery of the technique of staining, than 

 upon any optical improvement of any kind.— .4/i. of Micr. 



