POLARISED LIGHT 363 



vibration in phase), the elliptical orbit, and the circle. The 

 other plan is to arrange in front of the condensers a small 

 pendulum apparatus made for the purpose, drawing the figures 

 in white on a slide of glass covered with a film of blue 

 printer's ink. This is projected in action on the screen, so 

 that the eye follows the tracing. Other methods with pen- 

 dulums are plentiful, but cannot be called projections. 



Having thus shown that rectangular vibrations in the 

 same phase compound into one plane vibration at an angle of 

 45, and that difference in phase of half a vibration results in 

 a plane vibration at right angles to this (which in polarisa- 

 tion reverses all chromatic phenomena), also that vibrations 

 differing a quarter- vibration in phase result in a circular orbit, 

 and intermediate differences of phase in elliptical orbits ; that 

 all this is so in the phenomena of polarised light is simply 

 demonstrated. The fourth band in Fox's wedge ( 205) giving 

 half a wave difference of phase in the two rays, and the 

 second band and sixth band a quarter-wave difference, the 

 matter can be tested, large films of similar thickness being 

 examined in the stage. The half- wave film does reverse the 

 phenomena, changing the bright field to dark, and the dark 

 to light, and causing the complementary colour in any design 

 when it is placed in the stage. The quarter- wave plate also 

 restores light to the dark field, but in a different manner. 

 The analyser cannot now quench the light in any position. 

 The field is always evenly illuminated [experiment], and so far 

 the field might appear to be one of common light. If an aper- 

 ture be placed in the stage with the film, and the double-image 

 prism employed as an analyser [experiment] it will be seen that 

 the two images are always alike in brilliancy. Nevertheless, 

 by placing any selenite or other preparation in the stage 

 after the quarter-wave [experiment], it will be seen that the 

 light is polarised somehow, for the design shows colours, with 

 tints simply differing by a quarter of that one of Newton'a 

 ' orders ' to which it belongs. 



