COLORIMETER FOR MONOCHROMATIC LIGHT 151 



tinuous spectrum, which is focused by the second objective 2 on the 

 horizontal slit S. The filament acts as a primary slit of the monochroma- 

 tor. A selected narrow part of the spectrum, after passing through the 

 slit, is reflected by two rectangular prisms (P), one half to the right 

 and the other half to the left. These two beams are now made parallel 

 by the objectives O3 and O4. Identical beams therefore pass through 

 absorption cells C\ and C 2 . The transmitted beams are focused by 

 means of objectives. O5 and 06 on the vacuum thermocouples I and II. 

 The thermocouples are connected in opposition through the resistances 

 to the galvanometer. The resistances are adjusted so that the galva- 

 nometer reads zero. Now if a semitransparent solution is introduced in 

 absorption cell C\ and the solvent into C 2 , the current generated by 

 the thermocouple I decreases and the galvanometer deflects. The 

 equilibrium is restored by reducing the current in circuit II in the follow- 

 ing manner. Each thermocouple is shunted with a 50-ohm resistance. 

 The shunt of II is a variable-resistance box R 2 connected in such a way 

 that a known fraction of the drop in potential across R 2 can be removed 

 from the galvanometer circuit. This fraction is adjusted so as to balance 

 the current through the galvanometer to read zero. The resistance 

 between the two keys K\ and K 2 then indicates the percentage of the 

 drop in potential which has been made inoperative. This number is 

 thus equal to the percentage extinction caused by the absorbing solute 

 for the wavelength under examination. The accuracy obtainable is 

 claimed to be of the order of 0.01 per cent. 



The sensitivity of such an instrument is dependent largely on the 

 spectral region used. For the shorter wavelengths, the radiant energy 

 emitted by an incandescent lamp burning at its normal voltage is small. 

 The dispersive power of the Amici prism, however, is higher in the blue 

 than in the red. Since the slit width per 100 A is smaller in the red than 

 in the blue, a wider slit may be used in the blue than in the red end 

 of the spectrum. 



To test the applicability of Beer's law to any solution, a preliminary 

 test (Fig. IV-11) should be made to show that the logarithm of the 

 transmissivity for a given wavelength (log 7\) is proportional to C. 

 For this test a cell of known thickness is filled with the solution, and a 

 spectral absorption analysis is obtained. If a sufficiently thin cell is 

 chosen, most wavelengths available will be appreciably transmitted. On 

 coordinate paper having a logarithmic scale ruled along the vertical 

 axis and a uniform scale of concentration on the horizontal axis, as in 

 Fig. IV-11, a plot of log 7\ is made for a given wavelength with change 

 in concentration. Unity on the log scale is chosen as 100 per cent 

 transmission. Straight lines are drawn through the data. Any varia- 



