•JOG UADIATIU.N lllULOGV 



E = extinction, also called optical density, logio 1/7' or logio I d h; 

 X = wave length; 

 E\ = extinction at a given wave length, e.g.. A'b-.o niyu; 

 {E\)2 = extinction for unit amount per unit area; 

 c = concentration; 

 / = length of absorbing pathway; 

 .1 — area of absorbing mass in plane perpendicular to the absorbing 



pathway; 

 V = volume of absorbing mass; 

 A- = specific e.xtinction, e.g., (^^x)^^ cm- 

 Beer's law deals with the relation of light lo.ss to concentration (c) ; 

 Lambert's law deals with the relation of light loss to absorption path (/). 

 A useful simple e(iuation which expresses the fact that extinction is 

 directly proportional to the number of specifically absorbing chromo- 

 phores (k) in the absorbing path, as determined by concentration and 

 thickness is 



E = kcl. 



In most routine (juantitative photometric chemical analj'sis the sample 

 is in a carefully measured cuvette or absorption cell (generally 1 cm 

 thick). The unknown concentration (Cx) may be computed in terms of 

 the extinction of a standard of unit thickness and concentration, e.g., 

 (■^x)i.ncra» where E^ is the extinction of the unknown from the following 

 formula. 



^x % 



('E'x)l.'oc 



In cytological preparations the thickness is always but a few microns. 

 When this thickness has been measured, computation from a standard 

 extinction must take into account the relative thickness. An equation 

 by which, assuming the validity of Beer's law over the concentration 

 range of which the two measurements are the extremes, the percentage 

 concentration in a cytological preparation may be computed from the 

 standard (£/x)L^°e„ is 



10,000 (£:x), 



(^x /c ~ 7/Tn\l% 



h(E,)\, 



cm 



where l^ is the thickness of the cytological structure, in microns. 



Instead of using the .standards ol)tained in the cuvettes of a colorimeter 

 or spectrophotometer, it is more convenient to compute special cyto- 

 logical standards. Most useful is Ea computed as the extinction for 10"'" 

 rfig/M", {E\)li ""~', in which Caspersson has expressed the results of his 

 (juantitative cell analyses. In a thickness of 1 ^ this is equivalent to 10 

 per cent, to 100 mg^cc, or to 10~'" mg/;u^. This standard has been com- 



