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LXXXVI1I. The Absorption of Light by the Goldberg Wedge. 

 By V. C. Toy, 3I.Sc, A. Inst. P., British Photographic 

 Research Association, and J . C. Ghosh, D.Sc, University 

 < 'ollege, London * . 



[Plate X VIE.] 



MEASUREMENTS of light absorption are now fre- 

 quently made by means of a thin wedge of lamp-black 

 in gelatine contained between inclined glass plates. This 

 wedge was devised by Goldberg f in 1910, and is now 

 available in several commercial forms. It is usually assumed 

 to be neutral, i. e., to have a constant extinction coefficient 

 for all wave-lengths. As this constancy does not appear to 

 have been verified over any considerable range, an investi- 

 gation of its possible variation was made. The " gradation " 

 of the wedge, ?'. e. the increase of density (in the photo- 

 graphic sense) % per unit length in a direction perpendicular 

 to the isopaques, is its most important characteristic, and the 

 variation of this gradation with wave-length was investi- 

 gated. For, while this variation is directly proportional 

 to the variation of the extinction coefficient, it cannot be 

 directly calculated from the latter, which is therefore of 

 lesser practical importance. 



For the details of the method used, the reader is referred 

 to a previous paper §. This method is independent of the 

 Schwarzschild effect ||, so that greater accuracy can be 

 obtained than in methods previously employed. The accuracy 

 of the method allows the variation of the gradation with the 

 wave-length to be followed very closely. 



Fi<r. 1. 



Fig. 1 shows dia grammatically the arrangement of the 

 apparatus. The source of light M is a quartz mercury lamp 



* Communicated bv Prof. A. W. Porter, F.R.S. 



t B. J. P. lvii.p. 648 (1910). 



\ The density is defined as— logioT, where T is the photometric 



Intensitv of Light Transmitted 

 transparency, t. e. - T - — ' ° , . 



1 • Intensity ot Light Incident. 



§ Proc. Roy. Soc. xcvii. p. 181 (1920). 

 I| Photo. Jonrn. lvi. p. 11 (1916). 



