Spectrophotometric Analysis 



57 



the Coleman instrument are not given; in principle it is quite 

 similar. In theory, such an instrument merely replaces the eye 

 with a photoelectric cell; but in actual practice the optical system 

 is different. This is due to the fact that the photoelectric cell 

 enables one to measure directly the intensity of the light by means 



Fig. 2. Schematic optical diagram of Beckman model DU spectrophotometer 



Fig. 3. Photograph of an assembled Beckman model DU spectrophotometer 



of a potentiometer. With the eye as the recording agent, two 

 beams of light are necessary, one passing through the unknown, 

 the other through the solvent alone, plus an analyzer with which 

 the intensity of the light can be varied until the two beams of light 

 are equal in apparent strength. With the photoelectric cell, on 



