60 GOETHE'S "FARBENLEHRE." 



The elementary fact is, according to Goethe, obtained 

 when we operate with a wide rectangle the edges only 

 of which are coloured by refraction, while the centre 

 remains white. His experiments with the parchment 

 had made him acquainted with the passage of yellow 

 into red as he multiplied his layers; but how this pas- 

 sage occurs in the spectrum he does not explain. That, 

 however, his hazy surfaces his virtual turbid media 

 produced, in some way or other, the observed passage 

 and intensification, Goethe held as firmly, and enun- 

 ciated as confidently, as if his analysis of the phenomena 

 had been complete. 



The fact is, that between double images and turbid 

 media there is no kinship whatever. Turbidity is due 

 to the diffusion, in a transparent medium, of minute 

 particles having a refractive index different from that 

 of the medium. But the act of reflection, which pro- 

 duced the penumbral surfaces, whose aid Goethe in- 

 voked, did not charge them with such discrete particles. 

 On various former occasions I have tried to set forth 

 the principles on which the chromatic action of turbid 

 media depends. When such media are to be seen blue, 

 the light scattered by the diffused particles, and that 

 only, ought to reach the eye. This feeble light may 

 be compared to a faint whisper which is easily rendered 

 inaudible by a louder noise. The scattered light of the 

 particles is accordingly overpowered, when a stronger 

 light comes, not from the particles, but from a bright 

 surface behind them. Here the light reaches the eye, 

 minus that scattered by the particles. It is therefore 

 the complementary light, or yellow. Both effects are 

 immediately deducible from the principles of the un- 

 dulatory theory. As a stone in water throws back a 

 larger fraction of a ripple than of a larger wave, so do 

 the excessively minute particles which produce the 



