478 JAMES CLERK MAXWELL. 



Y, the prism, as seen by an eye at E, will be uniformly 

 illuminated with green light ; and if white light be admitted 

 at X and Y simultaneously, the colour seen at E will be a 

 compound of red and green, the proportions depending on 

 the breadth of the slits and the intensity of the light which 

 enters them. The third slit Z enables us to add a third 

 colour, and thus to combine any three kinds of light in any 

 given proportions, so that an eye at E shall see the face of 

 the prism at P uniformly illuminated with the colour result- 

 ing from the combination of the three. The position of 

 these three rays in the spectrum is found by admitting the 

 light at E, and comparing the position of the slits with the 

 position of the principal fixed lines. 



At the same time, another portion of the light from the 

 source (generally a sheet of white paper) enters the instru- 

 ment at BC, is reflected at the mirror M, passes through 

 the lens L, is reflected at the mirror M', passes close to the 

 edge of the prism P, and is reflected along with the coloured 

 light at e, to the eye-slit at E. 



In this way the compound colour is compared with a 

 constant white light in optical juxtaposition with it. The 

 mirror M is made of silvered glass, that at M' is made of 

 glass roughened and blackened at the back, to reduce the 

 intensity of the constant light to a convenient value for the 

 experiments. By adjusting the slits properly the two 

 portions of the field may be made equal both in colour and 

 brightness, so that the edge of the prism becomes almost 

 invisible. When light enters at E, the instrument gives a 

 spectrum in which Fraunhofer's lines are very distinct, and 

 the length of the spectrum between Fraunhofer's lines A and 

 H is 3 '6 inches. The outside measure of the box is 3 feet 

 6 inches, by 11 inches, by 4 inches, and it can be carried 

 about, and set up in any position, without readjustment. 



In making an observation the centres of the slits X Y 

 Z are placed opposite the divisions of the scale corresponding 

 to the colours to be mixed, the breadth of each slit being 

 varied till the resultant light cannot be distinguished from 

 the white light reflected from M and M'. If the mixture 



