432 



TYLER AND PREISENDORFER 



[chap. 8 



have undoubtedly come close to yielding the attenuation coefficient as defined 

 in the section on theory, others have not. Rather than describe specific instru- 

 ments we will put down here the essential optical characteristics of an a-meter. 

 The basic principle of an a-meter is contained in the equation 



T = NrINo = e-^^. 



PROJECT ION 

 LENS 



6 5 VOLT FIELD 



LAMP FILAME NT STOP 



CONDENSING 



PLEXI GLAS 

 WINDOW 



5.05 diameter 

 beam 



About 200 mm 



PLEX IGL AS 

 W IN DOW 



8 08 

 diameter 



(a) 



All dimensions in millimeters 



EXIT PUPI L 

 STOP 



7 83 

 diameter 



filament 2 708 



diameter image diameter 



field stop 



About 170 mm 



711 diameter 

 tfoter image 



PHOTOCELL 



Scattering angle max = I 

 All dimensions in millimeters 



(b) 



Fig. 12. Optical system for measuring a, the total attenuation coefficient, (a) Details of 

 the projection system ; (b) details of the detection system. 



Since transmittance {T) is obtained from the ratio of an air reading to a 

 water reading it is important to utilize a beam of light which is not adversely 

 affected by a change in index of refraction along its length. A suitable optical 

 system is shown in Fig. 12. 



In this system the beam is cylindrically restricted to a constant cross-section 

 by imaging the field stop at the photocell with a magnification of one between 

 the aperture stop and the image of the field stop. A change in index of refraction 



