iSi Dr. Seebeck on the Heat of the Prismatic Spectrum. 



as little as any other eiFect ; it is, on the contrary, to be no- 

 ticed, that even the eye is variously active in these polar influ- 

 ences. 



If we operate with colourless light, we see the effects on 

 bodies increase and decrease in proportion as the intensity of 

 the light increases and decreases. But every thing is different 

 when the light has assumed a definite colour; then the intensity 

 of light alone no longer decides. Glasses and coloured liquids, 

 which allow an equal quantity of light to pass through them, 

 and which are of equal intensity of colour, may act in quite an 

 opposite manner when coloured lights which belong to the op- 

 posite halves of colours are compared with one another. And 

 this is a proof that the contrast in colours discovered by 

 Goethe*, and which on account of its analogy with the polar 

 power of the magnet, &c., he has denonominated polar, is not 

 merely external, but that in this contrast the change which 

 is manifested in the light is of the most essential kind. 



The law, that the effect of light increases and decreases in 

 the direct ratio of its intensity, will only hold good with co- 

 loured transparent bodies so long as the colours are of the 

 same kind, and do not differ from one another very considera- 

 bly. 1 say, not considerably ; for the colours change, and pass 

 into others as they become deeper (yellow turns into red, blue 

 into violet), which in the comparison of colours of the same 

 half must not be overlooked. 



From all this we may easily learn that the maximum of any 

 effect can only take place in a definite degree of colour and a 

 proportionate degree of intensity of light ; and that therefore 

 intensity of light and colour must be in a definite ratio, corre- 

 sponding with the effect intended, if the highest degree of effect 

 is to be attained. 



If we consider this, several apparent contradictions will be 

 reconciled, and we shall not be surprised at finding colours of 

 the same half differing in a certain degree in effect, and, on the 

 other hand, colours of opposite sides sometimes acting in the 

 same manner. This last circumstance, for example, may oc- 

 cur with leuchtsteine, which may become equally bright under 

 glasses of a pale yellow, or of a dark blue verging upon violet. 

 This circumstance, so tar from contradicting the principle of 

 polar contrast, will again tend to prove it. The explanation 

 of it will be found in the experiments communicated by me in 

 Goethe's Farbenlehre, vol. ii. p. 705, &c.; to which I now par- 

 ticularly refer, as they may tend to call the public attention a 



* Vide his Beilrage zur Opikk, p. 46, No. 15. Weimar 1791 ; and his 

 Farbenlehre. Tubingen 1816. 



little 



