CONTRIBUTIONS TO SCIENCE. 481 



are properly selected, the illuminated portion of the screen 

 will appear white or pink. Now place the pieces of glass 

 together, so that the same light passes through both, and the 

 light transmitted will be green. This is explained if we 

 examine with the prism the light transmitted by each glass. 

 That which has passed through the blue glass will show, in 

 addition to blue light, a certain amount of violet and green, 

 while that which has passed through the yellow glass will 

 comprehend, in addition to yellow light, a certain quantity 

 of orange (perhaps red) and green. Now the only light 

 capable of passing through loth glasses is green, and this, 

 therefore, is the colour observed on looking through the two 

 together. Similarly, when blue and yellow pigments are 

 ground together, the light penetrates a little way into 

 the substance of the pigment, is reflected, and emerges, 

 some of its constituents being absorbed by one pigment 

 and some by the other, and the only light which both 

 pigments transmit is green, which is consequently the 

 colour of the emergent light, and therefore the hue of 

 the mixture. 



The set of colour equations given above, and the dia- 

 gram of colours there described, were obtained from the 

 observations of Mrs. Maxwell (K). The diagram obtained 

 from Professor Maxwell's (J's) own observations was slightly 

 different, as before mentioned. 1 The chief differences were 

 that (J) saw more green in the orange and yellow portions 

 of the spectrum than (K), so that in the diagram obtained 

 from his observations these colours appeared nearer the 

 green than in the other, while the colours between green 

 and blue appeared more blue to (J) than to (K), and were 

 therefore placed higher up in his diagram. Thus, when the 

 instrument was adjusted to suit (K), one of the selected 

 colours being (32), (36), or (40), then (J) saw the mixture 

 too green ; but if (48), (52), (56), or (60) were the selected 

 colour when adjusted for (K), it appeared too blue to (J), 

 " showing that there was a real difference in the eyes of 



1 Part I. p. 315 

 2 I 



