AND MODERN PHVSICH. 103 



white source bo placed at the other end of the appara- 

 tus, so that R is illuminated with white light, only red 

 rays will reach S. Similarly, if another slit bo placed 

 in the green at (, and this be illuminated by white 

 light, only the green rays will reach S, while from 

 a third slit V in the violet, violet light only can 

 arrive at S. Thus by opening the three slits at V, 

 (J and R simultaneously, and looking through S, the 

 retina receives the impression of the three different 

 colours. The amount of light of each colour will 

 depend on the breadth to which the corresponding 

 slit is opened, and the relative intensities of the three 

 different components can be compared by comparing 

 the breadths of the three slits. Any other colour 

 which is allowed by some suitable contrivance to 

 .enter the eye simultaneously can now be matched, 

 provided the red, green and violet are primary 

 colours. 



By means of experiments with the colour box 

 Maxwell showed conclusively that a match could be 

 obtained between any four colours ; the experiments 

 could not be carried out in quite the simple manner 

 suggested by the above description of the principle of 

 the box. An account of the method will be found in 

 Maxwell's own paper. It consisted in matching a 

 standard white by various combinations of other 

 colours. 



The main object of his research, however, was 

 to examine the chromatic properties of the different 

 parts of the spectrum, and to determine the form 

 of the curve which ought to replace the circle in 

 Newton's diagram of colour. 



