340 PHYSIOLOGICAL PHYSICS. [Chap. xxvi. 



placed, it will appear of a brilliant red; but as the red 

 ribbon is moved through the spectrum it loses its 

 brilliancy, till, when quite beyond the red, it appears 

 black. Yellow ribbons, in the same way, are yellow 

 only in that part of the spectrum, in other parts colour 

 is absent. It thus appears that bodies are coloured in 

 one way or another according as they behave towards 

 white light. If they transmit the red rays they appear 

 red, if they transmit yellow rays they appear yellow, and 

 so on. If they transmit more than one colour of rays 

 their apparent colour will be a blend. In fact, coloured 

 bodies may be regarded as splitting white light up 

 into its elements, as absorbing some of these ele- 

 ments and as transmitting or reflecting others. Ac- 

 cording to the rays they transmit or reflect is the 

 colour they appear to have. If they transmit the 

 rays they are transparent and of the colour of the rays 

 transmitted, if they reflect the rays they are opaque 

 bodies of the particular colour. If a body reflects all 

 the rays it is white ; if it absorbs all it is black. 



Mixture of colours. A body might transmit 

 or reflect not only rays of one simple colour, but rays 

 of several kinds, and in such a case it would appear 

 to have a colour which is a compound of the different 

 rays. The results produced by the admixture of two 

 or more simple colours of a pure kind were carefully 

 worked out by Helmholtz, who used a spectroscope 

 with a V-shaped slit, by means of which he obtained 

 two spectra, which he was able to manipulate so as to 

 superimpose one on the other. The one limb of the 

 slit was placed at right angles to the other limb, so 

 that the violet of one spectrum was superposed on 

 the red of another. Helmholtz' results are given in 

 the following table, where the top line and the side 

 column indicate the superposed colours of the two 

 spectra, and the other columns give the results of the 

 blending of the two. 



