116 MICROSCOPIC TECHNIQUES 



a photomicrographic camera (X). Every object measured must be 

 photographed in order to estabhsh its exact position and dimensions. 

 The photocell is connected to a string electrometer and both of these 

 instruments are well shielded and also protected from moisture by- 

 means of phosphorus pentoxide. Various photocells are used for 

 different wavelengths; gas cells are usually employed. For the 

 shortest ultraviolet, cadmium; for medium ultraviolet (260-350 m/x), 

 sodium; for long ultraviolet and visible (350-550 nifi) , potassium; 

 and for wavelengths over 550 m/x, potassium-cesium cells are used. 

 The telescope (0) is placed in front of the photocell to control the 

 optical centering of the system ; this centering must be very exact. 



It is necessary to compensate for variations in the source of 

 intensity of the radiation, and for this purpose a quartz plate (G) 

 is interposed in the optical path in order to reflect a small percentage 

 of the radiation on a photocell ( V) . Readings of the changes in the 

 photocell current can be used to correct the readings of the electrom- 

 eter (S) . (T and U are leak resistance and four-step potentiometer, 

 respectively.) 



Measurements are made by taking the deflection of the electrome- 

 ter with the object in position and in focus, and then moving the 

 object away so that a clear space on the slide lies in the optical 

 axis. The opening in the rotating sector is reduced until the amount 

 of radiation striking the photocell is the same as before, i.e., the 

 same electrometer deflection is produced. The absorption in the 

 object will then be equal to the decrease effected by the sector, and 

 extinction coefficients may be calculated. 



Gersh and Baker Modification. A somewhat simplified set-up, 

 with American-made instruments, is employed by these investiga- 

 tors, as may be seen in the diagram of their apparatus (Fig. 10) . 



The source of radiation is a Daniels and Heidt (1932) type of 

 medium pressure mercury arc in a quartz capillary tube which is 

 mounted about 1 cm. from a quartz window in a large copper box. 

 The lamp is water cooled, and since the rate of cooling affects the 

 radiation output, the water line is equipped with a pressure regulator. 

 The lamp consumes 500-700 watts from a 220 volt D.C. line; a 

 ballast resistance is placed in series with the lamp. 



The entrance opening of the monochromator is a circular hole of 

 about 0.8 mm. diameter in a thin sheet of copper fixed 0.5-1.0 mm. 

 in front of the arc, in the water bath. The two equilateral quartz 



