1836.] Composition of White Light. 3t 



of rays supplied by the breadth of light which falls on the 

 face of the prism. The fringes are formed of light which 

 wants some' one or more of the colours necessary to consti- 

 tute white light. Now, whatever number of colours may 

 be included in the spectrum, only two of those which make 

 their appearance on the fringes, can be simple colours ; the 

 one which, by the refraction of the prism, is lifted above 

 the rest in consequence of its superior refrangibility ; and 

 the one which is left below all the others in consequence of 

 its inferior refrangibility ; the former of these is the violet, 

 and the latter the red. The blue under the violet, and the 

 yellow above the red, both adjoining the central white, 

 must necessarily be compound colours; for, the violet 

 which appears at the upper edge extends to the central 

 white, and therefore, must be superposed so as to form a 

 constituent part of the blue which lies between them, and 

 the red which appears at the lower edge must, for the same 

 reasons, enter into the composition of the yellow. 



Blue and yellow, then, being thus discovered to be com- 

 pound colours, and it being also discovered that violet enters 

 into the composition of the former, and red into the compo- 

 sition of the latter ; the appearance of the spectrum when, 

 upon its further developement, the green makes its appear- 

 ance, is easily accounted for ; the violet and red, the two 

 extreme colours of the spectrum, being in this stage of 

 its progress quite separated, leave the green (which was 

 before superposed by both these colours in the central white) 

 a distinct colour ; and as the violet and red, by continuing 

 to increase the distance of the screen, recede from each 

 other, the green becomes more and more developed, until 

 it acquires its full breadth, when the blue and yellow, in 

 both of which it must have been a constituent principle, 

 disappear. 



Red, green, and violet, then, are evidently the only primi- 

 tive colours. Red and green form yellow; in larger propor- 

 tions of the former orange ; violet, and green form blue ; 

 in larger proportions of the former, dark-blue and indigo : 

 Red and violet form crimson ; in larger proportions of the 

 former, pink; and in larger proportions of the latter, 

 purple. 



No shade of colour can be formed of more than two 

 colours ; for, the moment the third is added, its comple- 



