The shutter of the eyepiece may be partially closed so that only 

 a narrow spectral range is visible; this allows the eye to concentrate 

 on the matching of two small fields, each of which appears uniform 

 in color. The instrument is provided with a screw drum, calibrated 

 in wave lengths, by means of which the prism may be rotated in 

 such a manner as to bring light of any desired wave length into 

 the center of the field of vision. 



irism 



divided circle for 

 pnotometrit adjusW-nt 



-wave-length drum 

 ior rotation of pnsm 



tye p. 



Diagram of 



Spectrophotometer 



water cell- 



photomaUr Lox 



.dy« eeD 



Parallel Yearns from 

 common light source 



Fig. 2. Diagramatic section thru a spectrophotometer. 



The photometer circle may be calibrated in various ways. In 

 the general examination of dyes it is convenient to obtain the data 

 in the terms of a factor known as the Bunsen extinction coefficient 

 (E), and to employ a circle from which such values may be read 

 directly. 



In measuring the complete visible absorption of a dye, a series 

 of measurements is made over the portion of the spectrum in 

 which any appreciable absorption may be noted. This is done by 

 setting the drum at some definite wave length and observing 

 whether both beams of light reaching the eye are of the same in- 

 tensity; if not, the photometer circle is turned until the beam 



27 



