24 Microscopic Histochemistry 



that the time at which a tissue detail first shows up in black 

 should not be missed, since estimation of further increase in 

 color development beyond the level of black is impossible. 

 Nothing can be blacker than black. 



The specific enzymatic activity (activity /ml of tissue) of 

 a structural detail ( granule, fiber, brush border, etc. ) appear- 

 ing in a uniform shade can be calculated on the basis of the 



800 



30 40 50 60 70 80 90 100 IIO 



Thickness of Lines, Micro 



Fig. 2 



principle that the colorimetric density of a homogeneous 

 layer is proportional to the absolute amount of colored ma- 

 terial per unit of projection area. Therefore, the specific ac- 

 tivity of any detail will be identical with that of the mark 

 showing up in the same shade, multiplied by H/h, where 

 H stands for the thickness of the fluid layer of the mark and 

 h for the thickness of the tissue section, provided that the 

 structure occupies the entire thickness of the section. The 

 value for H can be obtained from the empirical curve given 

 in Figure 2, after measuring the average width of the mark 



