INTERFERENCE MICROSCOPY IN TRANSMITTED LIGHT 121 



on the extraordinary rays. The extraordinary wave is an eUipsoid of 

 revolution about the horizontal axis of the spar plate Q.. Owing to 

 the curvature changes of the extraordinary waves, the extraordinary 

 image exhibits considerable astigmatism: there is virtually no extra- 

 ordinary image left. Nevertheless, as stated in § 5, the extraordinary 

 light provides the uniform ground P'^ in the plane of the ordinary 

 image P'q. The transparent details of the specimen are perceived 

 owing to the interference taking place between P'q and the uniform 

 ground P'^ . The path difference J between Pq and P'j,' may be varied 

 and various colour contrasts observed by means of a compensator, 

 not shown in Fig. 3.31. 



If the instrument comprised only the spar plate Q... diaphragming 

 the condenser would be required. To dispense with this, the quartz 

 plate Q, cut parallel to the axis, is set on the condenser C Provided 

 the thickness of the plates Qi and Qo have a suitable ratio, the unit 

 exhibits on-infinity circular fringes lying in the object focal-plane of 

 the condenser C. To eliminate them and have a uniform pupil, 

 a birefringent element, exhibiting circular fringes localized in its own 

 plane, is set in the focal plane of the condenser C The two fringe 

 systems cancel out and diaphragming is no longer required. 



Several axial image duplication instruments have been devised. 

 In Blaisse's interference microscope, the plate Q. is a spar plate. 

 The plate Q^ is of rutile. Both plates are cut at right angles to 

 the axis. 



Frangon's interference microscope comprises a combination of 

 a half-sphere and two quartz parallel-side plates whose axes are 

 sloping (Fig. 3.32). Employing the half-spheres described previously, 

 thick quartz plates may be substituted for spar. Both plates Qi and Qo 

 are identical and cut slantwise in relation to the axis. The plate Q2 is 

 cemented to the quartz plano-convex lens Q^, cut parallel to the axis. 

 The elements Qo and Q3 make up the half-sphere. The analyser iPo is 

 cemented between Q., and Q^. The latter is set so that its axis is at 

 right angles to the axis of fPo. Under such conditions, the lens Q^ 

 exhibits no birefringence and takes no part in the interference process. 

 Provided the vibration passing through £?2 is directed at right angles 

 to the axis of Q^, the latter's role is merely to complement the half- 

 sphere with glass of the same index as Qy. 



The plate Q.2 gives rise to the axial duplication which is now con- 

 comitant with a sidewise duplication. A' is an ordinary image of A, 

 this interferes with the uniform extraordinary light: the image A' can 



