50, ELEMENTS OF APPLIED MICROSCOPY. 
and thus emphasize their outline and bring out the struc-: 
ture more clearly. Sometimes the stain acts upon a 
certain tissue as a whole, picking it out from other tissues: 
surrounding it. More generally, however, the substances: 
used act on a certain part of the individual cell, generally 
the nucleus, making that structure stand out clearly 
from the cytoplasm. 
Staining processes are often complicated, and include, 
not only direct methods in which the stain is allowed to 
act just long enough to affect the desired elements and: 
then washed out, but also indirect methods in which the 
tissue is overstained and the dye then removed by alco-. 
hol or acid from the parts which give it up most readily. 
Small objects may be stained in bulk and sections. 
treated on the slide. 
Of the staining solutions used, hematein and the 
anilin dyes (fuchsin, methylene blue, eosin, safranin, 
Bismarck brown, etc.) are the most generally useful. 
By a proper combination of two of these it is possible to 
stain the cytoplasm of the cell one color and the nuclei 
another. This process of double staining depends of 
course on the greater avidity of the nuclear substance 
for most dyes. A good example for practice in double 
staining is the hematein-eosin combination. The sec- 
tion, attached to the slide with albumin fixative and 
cleared, is passed down through the grades of alcohol to 
30%, and is then placed for one or two minutes in a solu- 
tion of Mayer’s hemalum.* 'The section is then washed 
* Mix 1 gram hematein dissolved in 50 cc. of 90% alcohol with 
50 grams alum dissolved in 1000 cc. water. Cool, settle, and filter 
