'22 \ RADIATION HIOLOGY 



inifj;lit otherwise be overlooked, namely, thiit, however striking the baso- 

 philia and ultraviolet absorption of nuclcotide-rich parts of the cell may 

 be. this by no means signifies that they are strongly acidic pools of nearly 

 pun^ nucleic acid. Instead, the al)sorption curves clearly show that pro- 

 tein is usually present in much greater (|uantity than the nucleic acid. 

 This situation must always be recalled in any attempt to interpret chem- 

 ically the results of staining reactions, and it must be the starting point 

 for all speculations concerning the role of nucleoproteins in intracellular 

 physiology (see Pollister, 1952b). 



4. QUANTITATIVE MICROSCOPICAL METHODS 



l-l. VISUAL COMPARISOX 



The (jualitativc c,ytological methods which have just been discussed 

 lead to localization of a substance within a cell by its absorption, "con- 

 trast" in the language of a microscopist, which is detectable visually. 

 Cytologists often speak of the intensity of a stain or color reaction as 

 weak, strong, very dark, etc. Of course these terms imply semicjuantita- 

 tive evaluation of the concentration of the component which is responsible 

 for the color. When two similar objects are side by side in an evenly 

 illuminated microscopic field, or in the two half fields of a comparison eye- 

 piece, visual matching appears to be as accurate as objective photometric 

 measurements. For example, with the comparison eyepiece, Bauer 

 (1932) arranged a series of slides in order of intensity and was thus able to 

 work out the relation of intensity of the Feulgen reaction to time of 

 hydrolysis, which was essentially like that later worked out by Di Stefano 

 (1949) from photoelectric measurements. For objects of the same size 

 proper visual comparison is, then, a rough indication of relative amounts 

 — if two objects are equally dark the}^ may be assumed to have approx- 

 imately the same amount, and if they are different, the darker one may 

 be said to contain more reacting substance. The same conclusions 

 regarding relative concentrations may be drawn of two bodies of eqwdl 

 vertical thickness (equal absorbing path). More often, the (juantitative 

 question which faces a eytologist cannot be answered, even roughly, by 

 visual comparison. For example, one often wishes to know relative 

 amounts in two objects of very different size. It is uncertain to what 

 extent by visual study a microscopist can determine whether two such 

 une(iual objects have the same intensity (a rare condition probably); for 

 the relati\'e sizes of the contrasting surroundings introduce considerable 

 difficulty. If this match could be accurately made, a fairly good estimate 

 of how much more the larger object contained could be computed from 

 the dimensions of the objects. 



The examples cited illustrate the range of visual microscopic com- 

 parison. If, to mention a very common experience, one cytological 



