516 



Capt. W. de W. Abney. On the Limit of [May 14, 



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~w\\\ be seen that the extinction of the red rays is effected when they 

 are reduced to about 36/100,000 of this standard, whilst the rays near 

 F require a reduction of 5/10,000,000, that is, the sensitiveness of the 

 eye is 700 times greater for the latter colour than the former, and 

 this has a bearing on the extinction of white light of different qualities. 

 It is worthy of remark that the reduction of the rays from about 

 C to the visible limit of the red necessary to cause extinction from 

 the standard luminosity is practically the same, and points to the fact 

 that this part of the spectrum is probably monochromatic ; if admix- 

 ture of any other colour sensation were present, the curve would rise 

 or fall instead of remaining horizontal. The same apparently applies 

 to the violet end of the spectrum, though, owing to the small 

 luminosity, exact measures of it are less certain. The experiments 

 show that the rays having the wave-length of about X 4770 are the 

 last perceived. The shift in the position of maximum resistance to 



