428 ON THE THEORY OF COMPOUND COLOURS. 



The following Table contains the means of four sets of observations by the 

 same observer (K.) : 



TABLE IV. (K.) 



44-3 (20) + 31 -0(44) + 27-7 (68) = W. 

 16-1 (28) + 25-6 (44) + 30-6 (68) = W. 

 220 (32) + 12-1 (44) + 30-6 (68) = W. 

 6-4 (24) + 25-2 (36) + 31-3 (68) = W. 

 15-3 (24) + 26 -0(40) + 30 -7 (68) = W. 

 19-8 (24) + 350 (46) + 30-2 (68) = W. 

 21 -2 (24) + 41 -4 (48) + 27O (68) = W. 

 220 (24) + 620 (52) + 130 (68) = W. 

 21-7 (24) + 10-4 (44)+ 61 -7(56) = W. 

 20-5 (24) + 23-7 (44) + 40-5 (60) = W. 

 19-7 (24) + 30-3 (44) + 33-7 (64) = W. 

 180 (24) + 31 -2 (44) + 32-3 (72) = W. 

 17-5 (24) + 30-7 (44) + 440 (76) = W. 

 1 8 -3 (24) + 33 -2 (44) + 63 7 (80) = W. 



VIII. Determination of the Average Error in Observations of different kinds. 



In order to estimate the degree of accuracy of these observations, I have 

 taken the differences between the values of the three standard colours as 

 originally observed, and their means as given by the above Table. The sum 

 of all the errors of the red (24) from the means, was 31'1, and the number 

 of observations was 42, which gives the average error 74. 



The sum of errors in green (44) was 48 '0, and the number of observa- 

 tions 31, giving a mean error 1'55. 



The sum of the errors in blue (68) was 46'9, and the number of observa- 

 tions 35, giving a mean error T16. 



It appears therefore that in the observations generally, the average error 

 does not exceed T5 ; and therefore the results, if confirmed by several obser- 

 vations, may safely be trusted to that degree of accuracy. 



The equation between the three standard colours was repeatedly observed, 

 in order to detect any alteration in the character of the light, or any other 

 change. of condition which would prevent the observations from being comparable 

 with one another ; and also because this equation is used in the reduction of 



