160 PRACTICAL PHOTO-MICROGRAPHY. 
kind of tissue should have its own stain, and the 
stains must not run beyond the tissues, nor into each 
other. All dyes should be avoided which stain many 
tissues indiscriminately—“ diffuse ” stains ; and we 
should employ stains having special affinities for the 
tissues or bodies to be examined and photographed, so 
that we shall either get none but the bodies or tissues 
under investigation stained, or have the whole prepara¬ 
tion deeply stained first, and the stain afterwards 
removed from all but the special bodies or tissues— 
Maximum Decolorisation. 
The point is to get one tissue visually and photo¬ 
graphically distinct from others. Acid hsematoxylin 
will hardly stain anything but nuclear tissue ; benzo- 
purpurin will stain indiscriminately the whole of the 
tissues in a section if time is allowed, and unless we 
take steps to prevent this diffuse action. And some of 
the basic aniline dyes will stain bacteria, nuclei, tissue 
and all, but they are removed from all except the 
bacteria by suitable—and, in our case, necessary— 
procedure. Moreover, these anilins, if improperly used, 
will stain bacteria in such a way that the outlines of the 
organisms will be “ fuzzy ”—in fact, the staining action 
will extend beyond the bacteria, and give them unsharp 
outlines. Again, by neglect of proper “ fixation ” of a 
tissue, we may get its elements disintegrated, and un¬ 
recognisable, and false in appearance. Lastly, in the 
processes of preparation, we may actually damage and 
destroy our objects ; as when, by over-heating a cover- 
glass preparation, we “ frizzle ” it into a totally mis¬ 
leading shape. 
