1900] MICROSCOPICAL JOURNAL. 245 



Now, scarcely any accurate observations are made, in 

 either Histology or Bacteriology, without the aid of stains; 

 and the literature of the subject has increased so largely 

 that it would be impossible, in the space at our command, 

 to give more than a very general outline of the theory, 

 underlying the hundreds of formulae recommended ; per- 

 haps, under the circumstances, it will be best to merely 

 draw together the threads of what has already been said. 



The chief reason for staining microscopic objects is to 

 differentiate them, that is, to exhibit one group of cells in 

 a different color or shade from another group (a histolo- 

 gical stain); or to demonstrate the nucleus, or granules, of 

 a cell by these same color differences (a cytological stain). 



Stains may be divided into two great groups (1) Gen- 

 eral, and (2) Specific ; the latter of which may be again 

 divided into three sub-divisions, after the classification of 

 Ehrlich, already mentioned, and into the details of which 

 we need not now enter. Those comprised in the first 

 group, the general stains, color the whole of the prepa- 

 ration, although all the elements are not equally affected; 

 the specific stains on the other hand, as their name im- 

 plies, color only certain groups of cells, or elements of 

 cells ; but there is scarcely any stain so specific that it does 

 not need careful attention as to strength, time it is allow- 

 ed to act, etc., and these details necessarily bring us to a 

 consideration of the methods employed. 



There are two distinct methods, all the various modifi- 

 cations being grouped under one or the other : — (1) The 

 Progressive (2) The Regressive. The progressive method 

 consists in staining just so long as to bring out the ele- 

 ments required, and stopping its action before the other 

 elements are affected sufficiently to destroy differentiation. 



The regressive consists in allowing the whole of the 

 preparation to become stained, and then washing out by 

 means of alcohol or acid the stain from those elements, 

 which give it up most readily ; of this method we have 



