114 



PROGRESS IN MICROSCOPY 



(Fig. 3.22). The axis of the second plate does not he in the plane 

 of Fig. 3.22. In fact, the axis is 45° from the plane of Fig. 3.22. 

 In the first plate, the incident ray splits in two: the ordinary ray Oi 

 and the extraordinary ray E^. As the second plate is 90° from the 

 first, the ordinary ray of the latter becomes extraordinary in the second 

 and vice versa. The dotted fine shows that the ray OiEo. emerging 



y" 



■E,o. 



0,E2 



Fig. 3.22. Savart plate. 



from the second plate, is not in the plane of the figure but parallel 

 to the ray EiO^ which does lie in said plane. Figures 3.23(a) and (b) 

 readily show the rays' paths. Figure 3.23(a) shows the rays' trace in 

 a front plane after passing through the first plate. The ordinary and 

 extraordinary rays are at O^ and Ei, respectively. Figure 3.23(b) shows 

 the rays' path in a front plane after passing through the second plate. 

 The ray O.. extends the ray Ei but the ray E.. is shifted horizontally. 



(a) (b) 



Fig. 3.23. Ray paths in the Savart plate. 



In the horizontal plane, the £"0 shift in relation to Oi is the same as 

 that of £1 in relation to Oi in the vertical plane. When the incident 

 ray is normal in relation to Savart's polariscope the outgoing rays 

 Ei^O. and O^Eo are in phase. Slightly tilting the polariscope varies 

 the path difterence which is thus readily adjustable. Let us place 

 Savart's polariscope between two polarizers and observe through this 

 system a remote uniformly-illuminated object, such as the sky. If the 

 eye is not accommodated, a series of virtually straight, parallel and 

 equidistant fringes are perceived. If the polarizers are crossed, there 

 are, on each side of the dark central fringe, colour fringes which are 



