32 COLOUR VISION 



III, 581yu./x ; IV, 635'5/x/A : and of the corresponding minima between 

 I and II, 458 ^ ; II and III, 533 /*/z ; III and IV, 627 



8-0 

 Pf- 



60 



IF 



v \ / \ I 



w 



<K)Q 50 500 50 600 50 700 



Fig. 5. Curve of discrimination sensibility for hues of the interference spectrum of the arc 

 light. Abscissae, wave-lengths of the lights ; ordinates, differences of wave-length 

 (6X) capable of being discriminated. (Steindler.) 



These results are of very considerable theoretical importance, and 

 will be referred to again later. 



The number of discriminable hues in the spectrum can be calculated 

 from these data 1 . If A and X + Sx are the wave-lengths of two 

 just discriminable monochromatic lights, then &X varies with A and 

 may be regarded as a function of A. The reciprocal value of Sx 

 therefore gives the number of hues which can be discriminated in 

 an interval of the spectrum in which A varies according to the same 

 chosen unit. The total number of distinguishable spectral hues is there- 

 fore equal to the integral I ^- . dX over the whole visible spectrum. 



Uhthoffs values for SX, are given above. Since from 655 p,^ to the red 

 end and from 430 yti/z to the violet end there are no changes in hue, 



- is here = 0, and the integration can be performed graphically by 



drawing the curve and measuring the area enclosed between it and the 

 axis of abscissae. From these calculations Konig concluded that 

 Uhthoff could discriminate 165 hues in the spectrum. 

 1 Konig, Gesammclte Abhandlungen, p. 307. 



