306 Sir W. de W. Abney. On the Variation in 



Fig. 3. 





































-\ 





































































/ 





















i 





















f 



h 





































i 









H 



1 





a 



! 



Li j 



1 





6°* 



1 I I ! 1 I li II , I 11 X 1 i 



-60 -40 -30 -£0 -10 O 10 20 JO 40 30 



ScaLe of SpecCrum. 



The following table applies to the curve, fig. 3. 



Table III. 



Scale No. 



Sensitiveness. 



Scale No. 



Sensitiveness. 



42 



5 



12 



95 



44 



21 



8 



92 



38 



35 





89 



36 



50 







85-5 



34 



63 



- 4 



82 



32 



74 



- 8 



77-5 



30 



82 



-12 



73 5 



28 



89 



-16 



69 



26 



96 



-20 



64 



24 



99 



-28 



50 



22 



100 



-36 



29 



20 



99 



-42 



13 



16 



97 



-48 







The measurement of the densities on the plates was made by means 

 of an arrangement by which the comparison light was transmitted 

 through a graduated black annulus, whose thickness increased arith- 

 metically with the number of degrees from the zero point. This 

 gave the density measured on a scale of logarithms on a base due to 



