362 Sir D. Brewster on the Undulatory Theory of Light, 



yellow, and green light, and yet it is absolutely immoveable 

 when acted upon by the undulations of violet light, which dif- 

 fer from the others only in their inferior length. 



There are some substances in which the aether will undulate 

 only to violet light ; and there are others in which the aether 

 will undulate only to green light, the body which contains it 

 being absolutely opake to all red and violet rays. 



That very remarkable salt the oxalate of chromium and pot- 

 ash (for fine specimens of which I have been indebted to Dr. 

 William Gregory,) exercises a still more definite action upon 

 light. While a certain thickness of it is absolutely opake to 

 every ray except the red ones, it is also opake to a definite 

 ray in the very middle of the red space ! That is, it is abso- 

 lutely transparent, or its aether freely undulates, to a red ray 

 whose index of refraction, in flint-glass, is 1 # 6272, and also to 

 another red ray whose index is 1*6274; while it is absolutely 

 opake, or its aether will not undulate at all, to a red ray of 

 intermediate refrangibility whose index is 1*6273 ! 



When we consider tha.t green light passes copiously through 

 such a dense substance as a thin film of gold*, and that me- 

 tallic salts of great density afford as free a passage to light as 

 water or even atmospheric air, we cannot ascribe the pre- 

 ceding phaenomena to any mechanical obstruction which the 

 solid particles of bodies oppose to the free, motion of the aether 

 which they contain. But even if we could, by some new as- 

 sumptions, avail ourselves of this principle in the case of dense 

 bodies, it will not be applicable to those strange phaenomena 

 of definite action which I have discovered in the absorptive 

 power of nitrous acid gas. . 



When we transmit light through a very small thickness of 

 this gas, there are no fewer than two thousand different por- 

 tions of the incident beam, which are absolutely stopped by 

 the gas, while other two thousand portions are freely trans- 

 mitted ; and what is equally strange, the same body in the 

 liquid state exercises no such power, but freely transmits all 

 those two thousand portions which the gas stops. The aether in 



* Mr. Potter has remarked" that he cannot, with many opticians, call the 

 translucency of thin metallic leaves transparency" (present volume, p. 278). 

 If he means that the light which such leaves transmit does not pass through 

 the substance of the metal, but through small openings or pores produced by 

 hammering, I beg leave to refer him to an experiment in the Phil. Trans, for 

 1830, p. 136, which, though it was not sufficient to give me a correct mea- 

 sure of the action of gold in changing the plane of polarization, was per- 

 fectly sufficient to show that the green light had its plane of polarization 

 changed, while that which passed through the pores suffered no change : 

 the metallic leaf had therefore the same kind of transparency as all other 

 bodies, varying of course with the colour of the incident light. 



