244 



PROGRESS IN MICROSCOPY 



reflective plate Af^ splits the incident beam in two parts, one of which 

 passes through the microscope, the other next to it serving to illuminate 

 the neutral wedge W. 



The imaged wedge on the photographic plate, is developed at the 

 same time and, hence, under the same conditions as the image of 

 the specimen. 



The upper portion of Fig. 10.3 shows the aspect of the photographic 

 plate, including the wedge image W. The lower portion of the figure 



Fig. 10.2. Double beam microspectrophotometric device (P. M. B. Walker). 



shows the microdensitometer-analysed negative. This analysis is 

 effected as follows: absorption of the image W is analysed at any 

 random point, the incident and the transmitted intensity being denoted 

 by /o and / respectively. At such point, the transmittance is defined 

 by the ratio T = ///q. In actual practice, the optical density cL i.e. the 

 common logarithm of the inverse ratio l/T, is frequently used, then: 



log 



log 



T 



(10.1) 



