Manchester Memoirs, Vol. Iviii. (1914), No. 15. 9 



Twenty-three spectra of exactly five seconds exposure 

 are spaced across a plate, leaving room for another 

 exposure between each. The gaps are then filled with 

 spectra transmitted through varying lengths of the solu- 

 tion for various exposures. Thus : 



Standard Spectrum. Transmitted Spectrum 



(Through 2 mm. of Solution). 

 Sees. Sees. 



5 90 



5 60 



5 40 



5 30- etc. 



After development, those points are noted at which the 

 transmitted and standard spectrum have the same 

 intensity. By applying the Schwartzschild formula for 

 the intensity of the photographic image, and Beer's law, 

 the extinction coefficient for various wave lengths can be 

 arrived at. For instance, in the above example, the 

 transmitted and standard beam will be of equal intensity 

 at a certain point, X, for the exposure 90 sees, and the 

 length 2 mm. The two laws can now be applied, in the 



forms — 



/ / 1 V> 



7r(\r 



(Schwartzschild's formula for the densit)- of the photo- 

 graphic image where / and /, are intensities / and /i times 

 of exposure, and ;/ n number depending on the make of 

 plate, '9 in the Wratten panchromatic plate), and 



/. = /. 10-^'^ 

 (Beer's law where A is the intensity of the transmitted 

 beam, / that of the original beam, /c a constant, and d 

 the thickness of layer traversed). 



Combining the two formulae and substituting the 

 numerical values, we have 



A=- logy = 5-65. 



