SENSATIONS IN TERMS OF LUMINOSITY. 267 



(X.) Method of Determining tlie Composition of the Kay 3 in the Blue, End of the 



Spectrum. 



The most ready method of determining the percentage composition of the colours 

 which lie between b and the violet, is as follows : a slit is placed in position to allow 

 a blue of a natural wave-length to pass, and a second slit is placed at the SSN which 

 corresponds to the yellow, whose composition is already determined. No mixture of 

 blue and this yellow will make a white corresponding to that we have to compare 

 with it, but the slit-holder can be moved towards the red till a match is made, 

 i.e., where the slightly redder yellow is complementary to the blue which passes 

 through the first slit. The shift of the slit-holder from the fixed point is noted, and 

 from it are calculated the new positions of the two slits. From the previous measures 

 made in the red orange and green portions of the spectrum, the percentage composition 

 in red and green sensation-luminosity is known, and the luminosity of the light coming 

 through the slit in the redder yellow is divided proportionally between these two 

 sensations, and we have an equation of the form 



a" (RS) + " (GS) + y" (B) = 100 (W). 



The standard equation (vii.) is used as before, and we get the blue (B) in terms of 

 the two sensations and the violet. 



(XL) Method of Determining the Composition of the Violet. 



The last determination that has to be carried out is the composition of the violet. 

 We already know that up to a point near G it is uniform in colour. But if we place 

 a slit in the blue near the blue lithium line, and another in the red near the red 

 lithium line, and endeavour to match the violet, we shall find that although we get a 

 purple, yet it is too pale ; a third slit is placed in the violet, and by a right-angled 

 prism the beam is diverted and again deflected to fall on da, fig. 2, and the white 

 beam Z is also reflected to fall on the same part of the white surface. The mixture of 

 red and blue falls on ac. White is added to the violet till a match is made. The 

 luminosities of all the colours and the white are compared together, and an equation 

 is formed of this form 



a (V) + b (W) = m (B) + n (R). 



Now, as the red contains no white sensation, that shown on the left-hand side of 

 the equation must be found in the blue, a proportion of blue, green and red going to 

 form it. All the green sensation must be " used up " in forming the white, and only 

 the blue sensation and the red sensation can remain beyond the white. We thus get 



a (V) = n (RS) + [m (B) - b (W)] 

 = n(RS)-f-m'(BS). 

 2 M 2 



