348 Professor Tyndall [March 19, 



or other, the observed passage and intensification, Goethe held as 

 firmly, and enunciated as confidently, as if his analysis of the pheno- 

 mena 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 

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

 2)roduced the penumbral surfaces, whose aid Goethe invoked, 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 com- 

 pared 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 comple- 

 mentary light, or yellow. Both effects are immediately 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 turbidity 

 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 trans- 

 mitted light which reaches the eye being therefore complementary to 

 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, 

 incapable 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 bor- 

 dering 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, 

 no matter what the substance may be of which they are composed, 

 shed forth a pure firmamental bine; and that from them w^e can 

 manufacture in the laboratory artificial skies which display all the 

 phenomena, both of colour and polarization, of the real firmament. 



With regard to the production of the green of the spectrum by 

 the overlapping of yellow and blue, Goethe, like a multitude of 

 others, confounded the mixture of blue and yellow lights with that of 

 blue and yellow pigments. This was an error shared by the world at 

 large. But in Goethe's own day, Wiinsch of Leipzig, who is ridiculed 



