THE CAUSES OF DIFFERENTIAL DYEING 107 



one-half of the hght that is shone on to its surface. Make a pile 

 of four such sections, one on top of the other, and illuminate 

 them from below. The second from the bottom receives half the 

 original light, and sends up half of what it receives to the next; 

 this halves the light again, and so does the uppermost section. 

 Thus only one-sixteenth of the light gets through. But in these 

 four sections together there is the same amount of gelatine — and 

 therefore the same amount of dye — as in a single section cut from 

 the stronger gel. The latter, for the same reason, lets through 

 only one-sixteenth of the incident light, though the amount of 

 dye is only four times as great as in the section that let through 

 one-half. For this reason we are apt to form an exaggerated 

 opinion of the difference in the amount of dye taken up by 

 objects in microscopical preparations. 



When dividing cells are coloured with a cationic (basic) dye, 

 each metaphase chromosome takes up far more of it than an 

 equal volume of cytoplasm does. This results partly from the 

 fact that nucleoprotein is much more acidic (basiphil) than the 

 cytoplasm, but partly also from the fact that the chromosomes 

 are denser. Their density is indicated by the fact that anionic 

 {acid) dyes also colour chromosomes more deeply than the cyto- 

 plasm; indeed, they are used to colour chromatin in several 

 familiar techniques. For instance, acid fuchsine is used for this 

 purpose in Mallory's method for the differential colouring of 

 collagen. ^^^ Whenever a particular object can be more deeply 

 dyed than the cytoplasm or nuclear sap by both anionic and 

 cationic dyes, it is likely to be denser (though one requires an 

 interference microscope for the actual measurement of density). 

 The converse, however, is not necessarily true: we cannot con- 

 clude that a particular object contains little matter, from the fact 

 that it takes up little or no dye. It may possess few or no acidic or 

 basic groups, capable of attachment to cationic or anionic dyes. 

 Thus triglyceride droplets, though very dense, cannot be dyed. 



