408 



blackened glass, so as to be seen by the observer in immediate contact 

 with the compound light which enters the slits and is refracted by 

 the prisms. 



Each experiment consists in altering the breadth of the slits till 

 the two lights seen by the observer agree both in colour and brightness, 

 the eye being allowed time to rest before making any final decision. 

 In this way the relative places of sixteen kinds of light were found by 

 two observers. Both agree in finding the positions of the colours to 

 lie very close to two sides of a triangle, the extreme colours of the 

 spectrum forming doubtful fragments of the third side. They differ, 

 however, in the intensity with which certain colours affect them, 

 especially the greenish blue near the line F, which to one observer is 

 remarkably feeble, both when seen singly, and when part of a mixture; 

 while to the other, though less intense than the colours in the 

 neighbourhood, it is still sufficiently powerful to act its part in com- 

 binations. One result of this is, that a combination of this colour 

 with red may be made, which appears red to the first observer and 

 green to the second, though both have normal eyes as far as ordinary 

 colours are concerned ; and this blindness of the first has reference 

 only to rays of a definite refrangibility, other rays near them, though 

 similar in colour, not being deficient in intensity. For an account of 

 this peculiarity of the author's eye, see the Report of the British 

 Association for 1856, p. 12. 



By the operator attending to the proper illumination of the paper 

 by the sun, and the observer taking care of his eyes, and completing 

 an observation only when they are fresh, very good results can be 

 obtained. The compound colour is then seen in contact with the 

 white reflected light, and is not distinguishable from it, either in hue 

 or brilliancy ; and the average difference of the observed breadth of a 

 slit from the mean of the observations does not exceed ^ of the 

 breadth of the slit if the observer is careful. It is found, however, 

 that the errors in the value of the sum of the three slits are greater 

 than they would have been by theory, if the errors of each were 

 independent ; and if the sums and differences of the breadth of two 

 slits be taken, the errors of the sums are always found greater than 

 those of the differences. This indicates that the human eye has a 

 more accurate perception of differences of hue than of differences of 

 illumination. 



