INTERFERENCE MICROSCOPY IN TRANSMITTED LIGHT 



115 



similar to those observed in the Wollaston prism. The difference is 

 in the location of such fringes. In the case of the Wollaston prism, 

 they are perceived within the latter as if they were painted. Savart's 

 polariscope requires on-infinity observation. Changing the polariscope 

 direction in relation to the eye, the fringes move as a whole in ac- 

 cordance with the motion imparted to the instrument. 



These preliminary notes are intended to make clear the working 

 principle of the interference eyepiece. Referring to Fig. 3.24, Savart's 

 polariscope S is placed between the field lens Oo and the eyepiece 



n P 



Fig. 3.24. Principle of the interference eyepiece. 



lens O3. Neither the polarizer ahead of S nor the analyser next after 

 it are shown in Fig. 3.24. The specimen is at P and the wave surface 

 I is deformed after passing through the transparent object. The 

 objective Oi images the specimen {P') close to the field lens O,. The 

 Savart polariscope S splits the wave incoming at P' in two waves 

 whose arrangement is that shown in Fig. 3.14. As explained in § 5, 

 the transparent object is perceived by its slopes. In order to have 

 the image surrounded by a uniform field, as was the case with the 

 Wollaston prism, the illuminating beam must be fonned on a fringe 

 of the Savart polariscope. We have already seen that these fringes 

 are on infinity; they should therefore be found, in Fig. 3.24, at the 

 image focal-point F^^ of the objective (in which case they are virtual). 

 They could actually be observed at F^,^ provided the direction of light 

 were altered, e.g. substituting the source for the eye. 



Hence, to have a uniform field, a narrow slit must be placed in the 

 condenser focal-plane (not shown in Fig. 3.24). The specimen P is 

 parallel-beam illuminated and the narrow slit imaged on one of the 

 Savart polariscope fringes at Fq^. SHghtly tihing the polariscope the 

 fringes are shown seriatim at F^,,: the path difference is varied and 

 the image seen in many colouis. To obviate using a slit and allowing 

 the condenser to have a normal aperture, a compensating device is 

 provided ahead of the condenser. Figure 3.25 shows the final layout 



