GOETHE'S FARBENLEHRE. 223 



of light passing through the prism, which, according to Newton, is the 

 indispensable condition requisite for the production of a pure spectrum, 

 as an impure and complicated mode of illustrating the phenomenon. 

 The elementary fact is, according to Goethe, obtained when we oper- 

 ate with a wide rectangle the edges only of which are colored, while 

 the center remains white. His experiments with the parchment had 

 made him acquainted with the passage of yellow into red as he multi- 

 plied his layers ; but how this passage 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 inten- 

 sification, Goethe held as firmly, and enunciated 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 transpar- 

 ent medium, of minute particles having a refractive index different 

 from that of the medium. But the act of reflection which produced 

 the penumbral surfaces, whose aid Goethe invoked, did not charge 

 them with such disci-ete 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 imme- 

 diately deducible from the principles of the undulatory 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 tur- 

 bidity scatter more copiously the small waves of the spectrum than the 

 large ones. Light scattered by such particles will therefore always 

 contain a preponderance of the waves which produce the sensation of 

 blue. During its transmission through the turbid medium the white 

 light is more and more robbed of its blue constituents, the transmitted 

 light which reaches the eye being therefore complementary to the blue. 



Some of you are, no doubt, aware that it is possible to take matter 

 in the gaseous condition, when its smallest parts are molecules, inca- 

 pable of being either seen themselves or of scattering any sensible 

 portion of light which impinges on them ; that it is possible to shake 

 these molecules asunder by special light-waves, so that their liberated 

 constituents shall coalesce anew and form, not molecules, but particles ; 

 that it is possible to cause these particles to grow, from a size border- 

 ing on the atomic, to a size which enables them to copiously scatter 

 light. Some of you are aware that in the early stages of their growth, 

 when they are still beyond the grasp of the microscope, such particles, 



