430 ON THE THEORY OF COMPOUND COLOURS. 



in another colour. Now the hue of the resultant depends on the ratios of the 

 components, while its brightness depends on their sum. Since, therefore, the 

 difference of two colours is always more accurately observed than their sum, 

 variations of colour are more easily detected than variations in brightness, and 

 the eye appears to be a more accurate judge of the identity of colour of the 

 two parts of the field than of their equal illumination. The same conclusion may 

 be drawn from the value of the mean error of the sum of the three standards, 

 which is 2 '6 7, while the square root of the sum of the squares of the errors 

 is 176. 



X. Reduction of the Observations. 



By eliminating W from the equations of page 428 by means of the standard 

 equation, we obtain equations involving each of the fourteen selected colours of 

 the spectrum, along with the three standard colours; and by transposing the 

 selected colour to one side of the equation, we obtain its value in terms of 

 the three standards. If any of the terms of these equations are negative, the 

 equation has no physical interpretation as it stands, but by transposing the 

 negative term to the other side it becomes positive, and then the equation may 

 be verified. 



The following Table contains the values of the fourteen selected tints in 

 terms of the standards. To avoid repetition, the symbols of the standard colours 

 are placed at the head of each column. 



TABLE VI. 



