MICROSPECTROSCOPY AND MICROSPECTROPHOTOMETRY 



249 



compared with the current I>. generated by R., used as reference. 

 This comparison takes place in the electrical apparatus RC. The 

 absorption changes within the object, at a given wave-length, can 

 thus be determined. The shift of the specimen is linked to the motion 

 of the paper strip of the recorder D which plots the curve /J/o with 

 respect to the position of the point investigated in the object. Motions 

 of both specimen and recorder are provided by the motor MO. The 

 2ear box EG is connected to two selsyn S/i and 5/2, thus enabling 



W 



\ / 



'V A 



Fig. 10.7. Thorell arrangement. 



to adjust ad libitum the ratio of their speeds of rotation. The selsyn 

 51.2 drives the selsyn Sl^ which itself controls the paper-strip motion 

 of the recorder D. The selsyn SJ^ also drives the selsyn 5/4 which, 

 through the device SP (described later), controls the motion of the 

 specimen. Motions of both specimen and recorder therefore take 

 place according to a definite ratio. In these measurements, the wave- 

 length of the light supplied by the monochromator, is constant. 

 The wave-length of the light used can be modified, at will, by con- 

 necting the selsyn 5/i to the selsyn S/5. The recorder may also be 

 substituted for the oscillograph U. The specimen is shifted according 

 to a definite direction by means of a highly sensitive device ensuring 

 a sensitivity attaining 01 /<. The chamber B^ contains some mercury 

 (Fig. 10.9) and is connected through the pipe t to the element 5., 

 the two latter being filled with mercury. The diaphragm of By is 

 connected to the specimen and controls its motion. The unit com- 

 prising /;/. Bo. r, Bi. and the screw V, forming the device SP, is shown 



