AS PERCEIVED BY THE EYE. 293 



increased, the disc appears perfectly uniform in colour. From these experiments 

 it appears, that the apparent mixture of colours is not due to a mechanical super- 

 position of vibrations, or to any mutual action of the mixed rays, but to some 

 cause residing in the constitution of the apparatus of vision. 



(7-) Presentation of the Colours to be mixed one to each Eye. 



This method is said not to succeed with some people ; but 1 have always 

 found that the mixture of colours was perfect, although it was difficult to con- 

 ceive the objects seen by the two eyes as identical. In using the spectacles, of 

 which one eye is green and the other red, I have found, when looking at an ar- 

 rangement of green and red papers, that some looked metallic and others trans- 

 parent. This arises from the very different relations of brightness of the two 

 colours as seen by each eye through the spectacles, which suggests the false con- 

 clusion, that these differences are the result of reflection from a polished surface, 

 or of light transmitted through a clear one. 



Note II. 



Results of Experiments with Mr Hay's Papers, at Cambridge, November 1854. 



The mean of ten observations made by six observers gave — 



•449 It + -299 G + -2 52 B = -224 W+776Bk .... (l). 



•696R + -304G=181B+327 Y + -492Bk .... (2). 



These two equations served to determine the positions of white and yellow in 

 diagram No. 2. The coefficient of W is 4-447, and that of yellow 2506. 



From these data we may deduce three other equations, either by calculation, 

 or by measurement on the diagram (No. 2). 



Eliminating green from the equations, we find — 



•565B+-435 Y = -307R+-304 W + -389Bk .... (3). 



The mean of three observations by three different observers, gives — 



•573 B + -477 Y=-313 R + -297 W + -390 Bk 

 Errors of calculation, --008 B + -008 Y--006 R+007 W--001 Bk 



The point on the diagram to which this equation corresponds is the intersection 

 of the lines BY and RW, and the resultant tint is a pinkish-gray. 

 Eliminating red from the equations, we obtain — 



Calculation, . -533 B + -150 G -f 317 Y=-337 W + -663 Bk j 



By 10 observations, -537 B + -146 G + -317 Y= -337 W + -663 Bk > . . . . (4). 



Errors, . . --004 + -004 _ _ __ J 



Eliminating blue, 660 R + -340 G=-218 Y + -108 W + -682 Bk \ 



By 5 observations, -672 R + -328 G=-224 Y + -094 W + -672 Bk V . . . . (5). 



Errors, . . --012 + -012 --006 + -014 +008 J 



