130 



QUANTITATIVE STUDIES 



will not be resolved, and the transmission of light will be prac- 

 tically nil. If d is of the order of A/3 or A/4, the percentage of 

 transmission is unpredictable. The great danger therefore lies 

 in this region, since the chequers cannot be resolved when d is of 

 this order of A/3 or A/4. Thus, in a specimen in which the ab- 

 sorbing material is aggregated in this way, errors of hundreds of 



Fig. 8. To illustrate the absorption of light by a chequerboard distribution 

 of an absorbing substance. The areas containing the absorbing substance 



will be resolvable if d > X/2. 



percent could arise in calculating the concentration of absorbing 

 substances from extinctions. 



Probably the most effective way of surmounting this latter 

 difficulty is to study a specimen at several different regions of 

 the spectrum. 



Errors due to Fixation and Diffusion 



Displacement of substances in the cell by diffusion may occur 

 at the time of fixation or at a later stage of treatment. As was 

 indicated in Chapter 2, the only reliable method of fixation for 

 tissues is freeze-drying. Diffusion, however, may occur even 

 within the medium used for mounting a section, or during the 

 various stages of a cytochemical reaction. A method of study 



