334 



POLARISATION. 



portion to the force of the lateral pressure in proportion, conse- 

 quently, to the excentricity of the ellipse. If the apparatus admits 

 of a gradual increase of pressure, the resulting colours form a series, 

 which coincides with that of Newton's rings. The same series 

 would also be produced if we were to gradually increase the thick- 

 ness of the refracting medium by superposing a greater number of 

 equally strong but slightly less compressed plates of glass; for it is 

 evident that the difference of path which the lowest of the plates 

 would produce becomes greater by a certain amount through 

 each succeeding one. The action of a wedge is to be similarly 

 explained ; the gradually increasing thickness produces the colours 

 in their natural sequence. 



Inasmuch as the crystalline media act in regard to these rela- 

 tions analogously to compressed glass, we arrive at the general law 

 that the interference colour rises and falls with the excentricity of the 

 effective ellipse, and with the power of the refracting medium. 



Since the colours of the higher orders vary merely in red and 

 green tints, which become more and more faint the higher they 

 rise, and consequently graduate less distinctly from each other, the 

 first three orders are of paramount importance for microscopical 

 observation, and therefore call for a more detailed examination. 

 For this reason we have enumerated in our table the undistinguish- 

 able shades of the lower colours with somewhat greater complete- 

 ness than is usual in the text-books of Physics. In the second 

 column we have given the complementary colours for each order, 

 which correspond to Newton's rings in transmitted light. 



