246 PROGRESS IN MICROSCOPY 



and the actual densities (d) (of the neutral wedge) as abscissae. The 

 area, i.e. B' is a vacant area and is going to be investigated by com- 

 paring it with the absorption of the detail being examined. The curve(l) 

 in Fig. 10.3 denotes the photographic density (d^^) for B'. Re-plotting 

 this value on the curve in Fig. 10.4 denotes the actual density d^^ at B'. 

 Likewise the photographic density (d^^) of the detail A is obtained 

 (Fig. 10.3), to which the actual density d^^ is correlated (Fig. 10.4). 

 In relation to B' the density d^^^^ of the detail A' is: 



^A'B' = ^A'-ds" (10.2) 



According to (10.1) 



^ A' ^ B' 



may be written and, provided T^,^' be the transmittance of A' in 

 relation to B' , then: 



T^^n' = ~ ^L-B' = log ~^ = log -— . (10.3) 



J 5' ^ A' ^ A'B' 



The equations applicable to microspectrophotometric determina- 

 tions are (10.2) and (10.3). The foregoing measurements evince 

 T^'B' and c/^.^j/ for a single wave-length. Measurements are to be 

 resumed for the other wave-lengths of the spectrum and the curve, 

 showing the T^^^' or c/^-^^ variations versus the wave-length, are to 

 be plotted. When the specimen and the object itself are considered, 

 spectroscopists substitute the word ''density" for "absorbance": the 

 word "density" is solely applied to measurements carried out with 

 the photographic plate. The latter may be replaced by a photo- 

 electric cell (Fig. 10.5). The source 5", fed by the stabilized power- 

 supply unit Fj, illuminates the monochromator whose outlet sUt F 

 acts as source of monochromatic light. The microscope consists of 

 the condenser C and the objective Oi, the latter imaging the specimen P 

 on the screen E. A small aperture, made in the screen £", isolates 

 the imaged portion to be photo-analysed. The photo-multiplier R, 

 fed by the stabilized power-supply unit V., is set at the bac'K of T, 

 the lens /. imaging Ox on R. The image A' of the investigated detail A 

 overspreads the aperture T and the measuring instrument shows 

 a deflection D^>. Shifting the specimen evinces a deflection D^/ 

 for the area B' . The transmittance of A' in relation to B' is 

 T^,,i, =^ D^'IDji>, whence the density d^'^^. 



