AND MODERN PHYSICS. 101 



rod und 334 of green ; each set of discs has a dirty 

 yellow tinge. 



Now, in this experiment, black is not a colour ; 

 practically no light reaches the eye from a dead 

 black. We have, however, to fill up the circumference 

 of the top in some way which will not aft'ect the 

 impression on the retina arising from the mixture 

 of the blue and yellow; this we can do by using 

 the black disc. 



Thus we have shown that G6'6 parts of red and 

 334 parts of green produce the same chromatic effect 

 as 29'1 of yellow and 24*1 of blue. Similarly in this 

 manner a match can be arranged between any four 

 colours and black, the black being necessary to 

 complete the circumference of the discs. 



Thus using A, B, C, I) to denote the various 

 colours, a, />, c, d the amounts of each colour, taken, 

 we can get a series of results expressed as follows: 

 a parts of A together with b parts of B match c parts 

 of C together with d parts of 1); or we may. write this 

 as an equation thus: 



a A + b B = c C + r/ D, 



where the + stands for " combined with," and the = 

 for " matches in tint", 



We may also write the above 



d I) = a A + 6 B - c C, 



or d parts of D can 1x5 matched by a pwper combina- 

 tion of colours A, B, C. The sign shows that in 

 order to make the match we have to combine the 

 colour C with D; the combination then matches 

 a mixture of A and B. 



