120 On the Sensibility of the Eye to Variations of Wave-length. 



difficulty was found in making the measurements in the yellow, where the 

 luminosity of the spectrum is nearly a maximum, and hence a slight move- 

 ment of the slit causes no appreciable change in the brightness. In the 

 red and green, however, a movement of the slit causes a much more 

 apparent change in brightness than of hue. The method adopted to make 

 the settings was to move the slit till no change in brightness obtained by 

 opening or closing the slit would cause the two patches of colour to look 

 the same. 



Table. 



Wave-length 

 of colour used. 



Amount of white added 

 (100 = luminosity of coloured 

 light). 



Change in wave-length which 



produced a clearly 

 observable difference in hue. 



pp. 





m- 



589 







1 -55 





10 



1-55 

 1-6 





20 

 40 



1-8 





80 



2-05 





160 

 320 



2- 5 



3- 95 



632 







5-8 





10 



5-8 





20 



6-3 





40 



7-7 





80 



9-6 





160 



15-0 



527 







6-1 





10 



6-0 

 6-3 





20 

 40 



7- 5 



8- 9 





80 

 160 



13 -7 



It will be observed that in the case of yellow (D) light, there is no 

 observable effect on the minimum change in wave-length required to produce 

 an observable change in hue produced by small additions of white light. 

 Even when the luminosity of the added white is three times the luminosity 

 of the coloured, the minimum change in wave-length observable is 

 decidedly less than the extent of the monochromatic patch. 



In the case of the red also, the addition of white does not increase the 

 sensitiveness of the eye to changes in wave-length. With the green, however, 

 a slight increase of sensitiveness appears to be produced by the addition of a 

 small quantity of white. The effect, however, is very small, and, owing to 

 difficulties due to change in luminosity referred to above, is hardly measurable. 

 By allowing the white to only fall in half each of the patches, and noticing 



