VISIBLE AND U.V. ABSORPTION HISTOSPECTROSCOPY 121 



are coaipensated by casting a portion of the radiation on photocell 

 Lc by means of the semireflecting glass (D). The current generated 

 in this photocell is amplified and made to oppose that from photo- 

 rell L. 



The galvanometer is employed as a null-point instrument and the 

 amount of radiation striking the photocell L is controlled by the 

 rotating sector (K) . The rotating sector with its motor is mounted on 

 a slide, adjustable by a rack and pinion arrangement. The radiation 

 passing the sector can be controlled over a greater range (0-50%) 

 by the use of two discs, each with a 90° segment removed, which are 

 made to rotate in opposite directions. Kortiim (1934) has described 

 a simple sector wdiich operates on a similar principle. Norberg finally 

 employed a sector patterned after the Askania-Werke (Berlin) 

 model, which enables adjustment and reading while the sector is in 

 action, and the accuracy obtained in the absorption is 0.025%. 



The amplifying circuit for the photocell current is shown in Figure 

 13. It is a modification of that described by Custers (1933) and it 

 utilizes two Philips 4060 electrometer tubes. The apparatus can be 

 used with either the single photocell (L, Fig. 12), employing com- 

 pensation with the potentiometer, or with both photocells (L and 

 Lc) . The galvanometer used by Norberg was a Zernike C (Kipp and 

 Zoonen) instrument having a tension-sensitivity of 10,000 scale divi- 

 sions per volt and a stability level of 3 X lO^^-'' amp. By means of the 

 Ayrton shunt ( V) (Fig. 13) the sensitivity can be reduced by 0.1 and 

 0.01. The galvanometer is shown at iV; the potentiometer for com- 

 pensation where only one photocell is used at U; Re resistance 

 = 1.2 X 10^^ ohms; R© leakage resistance = 1.1 X 10^*^ ohms. 



Other resistances are marked in ohms. The apparatus must be 

 mounted within a Faraday cage to avoid electrostatic disturbances 

 and unsoldered contacts must have large frictional contact surfaces. 



Manipulations 



The filament current in the amplifier is turned on 30 min. before 

 measurements are made, and the tungsten band lamp is also turned 

 on long enough in advance to attain steady illumination. The sample 

 slide on the microscope stage is adjusted so that the image of the 

 exit slit of the monchromator ajipears in the middle of the sample 



