35* OPTICAL PROJECTION 



transmitted, according as the analyser pile or the separate 

 pile vary in position by 90. 



In all these demonstrations of the fundamental phenomena 

 of plane polarisation, the polariscope itself has not been 

 employed ; the optical front has sufficed, with the pieces of 

 apparatus described. These are equivalent to a polariscope, 

 and form a convenient method for also illustrating of what that 

 instrument essentially consists, and its various possible forms. 



202. Polarised and Common Light. The best illustra 

 tion of the probable nature of the transverse vibrations of 

 common and polarised light, will be found in the actual pro- 

 jection on the screen, by any of the apparatus described in 

 Chapter XVII., of Lissajous' unison figure in a state of transi- 

 tion, i.e. passing from planes though ellipses into circles, and so 

 back to planes at right angles to the former. Such a transi- 

 tional orbit will represent the probable nature of the vibrations 

 of common light. If, with the reed apparatus, the orbit then 

 be ' steadied ' at any one form, that will be a vivid illustration 

 of either plane, elliptic, or circular polarisation. 



203. Interference Colours. At this stage the polariscope 

 will be arranged for use, and the analyser turned to that 

 position which gives extinction, or what is called the * dark 

 field.' If now the rotating tourmaline be placed in the stage, 

 in two positions the field is still dark ; but directly the tour- 

 maline is placed at all obliquely to the polarised plane, the 

 vibrations of the polarised beam are ' resolved ' into others 

 obliquely inclined to them, so that the tourmaline appears a 

 light image on the field. No other phenomena appear, because, 

 of the two rectangular vibrations into which the original plane 

 is resolved, one is absorbed by the tourmaline. 



But if a thin slice of a non-absorbent crystal like selenite 

 or mica be placed in the stage, mounted so that its polarising 

 planes are at an angle of 45 with the original polarised plane 

 and that of the analyser, both the re-polarised beams get 

 through, just as in the Huygens experiment, only not appre- 



