98 HERMANN VON HELMHOLTZ 



discusses two propositions that were of great importance in 

 the development of the theory of knowledge, that, on the 

 one hand, all is not light that is perceived as light (a dictum 

 enforced by Johannes Muller), while on the other, there 

 is also light to which we are not sensible, i. e. invisible light, 

 such as the chemical rays, which exert a chemical action 

 beyond the visible spectrum. It is highly probable, he says, 

 that light-rays and heat-rays are identical within the luminous 

 portion of the spectrum, although the intensest heat lies beyond 

 the red end, so that radiant heat and light may be regarded 

 as identical ; the reason that luminosity is confined to so 

 small a group of the long series of vibrations appears from 

 Brucke's theory that the transparent media of the eye admit 

 these only to the retina, while all the rest are excluded. 

 From the fact that sensibility to light and heat do not exactly 

 correspond in their limits, Johannes Muller had previously 

 concluded that the specific character of luminous sensation 

 is conditioned by the specific activity of the optic nerve, 

 which, excite it as you will, can only yield the one sensation 

 of light. The radiation which we term now light, now radiant 

 heat, impinges on two different kinds of nerve end-organs, 

 in the eye and in the skin, and the disparity in quality of 

 the sensation is due not to the nature of the object sensed, 

 but to the kind of nervous apparatus that is thrown into 

 activity. 



From this simple and obvious truth, Helmholtz developed 

 his entire theory of knowledge. Which colour combinations 

 appear the same, depends only upon the physiological law 

 of their composition ; equality of colour arising from different 

 mixtures of coloured light has only a subjective value, and the 

 groups of isochromic combinations of colour correspond with 

 no objective relations, independent of the nature of the seeing 

 eye. But if this be true for colour as a property of light, it 

 must necessarily be true for colour as a property of bodies 

 also. A body that only gives out orange light must have 

 a different internal structure from a body that gives out only 

 red and yellow, or a third which gives red, orange, and 

 yellow. Yet the colour of these three bodies during white 

 illumination must be the same ; the similarity has no 

 objective, but merely a subjective value. At the end of the 



