152 BIOPHYSICALLY ACTIVE LIGHT 



tions from a linear relation show departures from Beer's law. If cor- 

 rections have been made for surface reflections in the cell, the extrap- 

 olated line will pass through transmission factor 1.0 at zero concentra- 

 tion. If this correction has not been made, the common point will be 

 near 0.92 on the " transmission axis " (Luckiesh [1917]) if two surface 

 reflections must be accounted for. Each straight line represents the 

 relation of log 7\ and concentration or depth for the wavelength used. 

 By extending these lines to intersect the concentration axis the spectral 

 characteristic of any depth or concentration may be read from the 

 graph. Some lines are very steep; the larger the absorption coefficient 

 the steeper (greater slope) the transmission curves for a particular wave- 

 length of the incident energy. 



DlCHROMATISM 



It will be seen from Fig. IV-11 that the slopes of the lines labeled 

 4800 A, 5000 A, and 5600 A increase, while those labeled 7100 A and 



o 



7200 A decrease with increase in wavelength. This change indicates 

 that the dye is dichroic. 



This means that the color of a solution is composed of two or more 

 maxima of transparency, and, if the rate of change of these maxima is 

 not the same, dichromatism occurs with change in concentration. 

 Suppose that in a solution the transmission color of the molecule is 

 yellow and the ion blue. The color of the solution with decrease in 

 concentration would vary from yellow through green to blue. Com- 

 parison of sample and standard at concentrations differing to any con- 

 siderable extent would be impossible since a deep column of dilute 

 solution would be blue and a shallow column of a more concentrated 

 solution would be green. 



From the figure it may be seen that methyl green dye in solutions of 

 high concentration or of great depth will be not green but red. This 

 change in color is indicated by the large transmission factor of wave- 



o 



length 7200 A (red) at concentration 3 on the graph as compared with 



o 



the very low transmission of 5000 A (green), indicating that the solution 



o 



has an absorptive band between 5600 and 7100 A. It follows that 

 comparison of sample and standard at concentrations differing by anj r 

 considerable extent would be impossible. 



Duboscq Colorimeter 



The Duboscq colorimeter is designed so that light from an even 

 source of illumination R, Fig. IV-13, is reflected from a fine ground- 

 glass surface. The two glass cups G are inserted in the beam of reflected 



