WILLIAM R. AMBERSON 



529 



Windows 1 and 15 carry no filter screens at all, their transmission 

 values being taken as unity. Transmission values for the other 

 windows are expressed as percentages of these two. In practically 

 every experiment the densities produced at 1 and at 15 check well 

 within the known average error of the film. 



The whole calibration apparatus is enclosed in a box Hned with 

 black felt. All metal parts are painted a flat black, and every pre- 

 caution has been taken to secure complete absorption of all light 



TABLE I. 



Transmissions of Calibration Fillers. 



which is not directly incident upon the film through the fifteen 

 openings. There remains only the possibihty of cross reflection 

 between the portions of the film exposed through the filters. Calcu- 

 lation has shown that this could not possibly increase the light incident 

 upon other filters by more than 0.5 per cent, and this factor has been 

 neglected. 



The determination of the transmission values of filters 2 to 14 

 requires a brief consideration. The light which falls upon such a 

 screen as is formed by a developed density in a photographic emulsion 

 is transmitted in two ways. A certain percentage passes through by 



