188 



PROGRESS IN MICROSCOPY 



Since the slit is no longer imaged on the WoUaston fringes, this image 

 is the more duphcated as the Wollaston is moved farther away from 

 the focal plane of the objective Oi. Figure 7.8 shows the originated 

 beams from the slit-images F' and F" given by the Wollaston. Phe- 



02 



' A 



T \ — ~''~T^:^? 7" 



\ w / 



\ w / 



\ V / 



V 



F' 



\ / 



V 

 f' 



Fig. 7.8. Fringes produced by the slit-images F' and F'\ 



nomena occur as if the image-plane A' were illuminated by the two 

 synchronous sources F' and F". Therefore, veritable Young's fringes 

 are observed in said image plane. 



Since changing over from one fringe to the next causes the path 

 difference to vary, one wave-length /., the two waves i7i and 2*2 

 are arranged in the image A' as shown in Fig. 7.9. The fringes outline 



Fig. 7.9. The two wave surfaces when fringes are developed. 



equal path-difference loci, i.e. loci which are as far apart as the 

 waves i^i and Zo. The fringes involved are, therefore, straight equi- 

 distant and parallel lines at right angles to the plane of Fig. 7.9. 

 In while light and with crossed polarizers, the dark fringe (A = 0) 

 would be developed at A'^. The path difference increases on both sides 

 and colour fringes are perceived. In monochromatic light, the field 

 is streaked with bright and dark fringes. In Fig. 7.10 and following, 



