414 ON THE THEORY OF COMPOUND COLOURS. 



results arrived at in this paper, the ordinutes being made to represent the 

 intensities of each of the three elements of colour, as calculated from the 

 experiments. 



The most complete series of experiments on the mixture of the colours of 

 the spectrum, is that of Professor Helmholtz*, of Konigsberg. By using two 

 slits at right angles to one another, he formed two pure spectra, the fixed 

 lines of which were seen crossing one another when viewed in the ordinary 

 way by means of a telescope. The colours of these spectra were thus combined 

 in every possible way, and the effect of the combination of any two could be 

 seen separately by drawing the eye back from the eye-piece of the telescope, 

 when the compound colour was seen by itself at the eye-hole. The proportion 

 of the components was altered by turning the combined slits round in their 

 own plane. 



One result of these experiments was, that a colour, chromatically identical 

 with white, could be formed by combining yellow with indigo. M. Helmholtz 

 was not then able to produce white with any other pair of simple colours, and 

 considered that three simple colours were required in general to produce white, 

 one from each of the three portions into which the spectrum is divided by 

 the yellow and indigo. 



Professor Grassmannf shewed that Newton's theory of compound colours 

 implies that there are an infinite number of pairs of complementary colours in 

 the spectrum, and pointed out the means of finding them. He also shewed 

 how colours may be represented by lines, and combined by the method of the 

 parallelogram. 



In a second memoirj, M. Helmholtz describes his method of ascertaining 

 these pairs of complementary colours. He formed a pure spectrum by means 

 of a slit, a prism, and a lens ; and in this spectrum he placed an apparatus 

 having two parallel slits which were capable of adjustment both in position 

 and breadth, so as to let through any two portions of the spectrum, in any 

 proportions. Behind this slit, these rays were united in an image of the prism, 

 which was received on paper. By arranging the slits, the colour of this image 

 may be reduced to white, and made identical with that of paper illuminated with 

 white light. The wave-lengths of the component colours were then measured by 

 observing the angle of diffraction through a grating. It was found that the 



* PoggendorfTs Annalen, Band LXXXVIL (Philosophical Magazine, 1852, December). 



t Ibid. Band LXXXIX. (Philosophical Magazine, 1854, April). J Ibid. Band xciv. 



