INTERFERENCE MICROSCOPY IN TRANSMITTED LIGHT 



109 



the colour is indigo. As previously, sensitivity is improved by giving 

 to zJ a low value, e.g. 004//, that gives rise to the dark grey field. 

 When J = 0, the image is on a dark ground type but it is the object's 

 slopes that are detected. Lebedeff has described the full-image dupli- 

 cation and the differential processes. In the latter he employed several 

 birefringent systems so as to match image duplication with every 

 type of object thus achieving, in all cases, utmost sensitiveness. How- 

 ever, the instrument was not convenient. We were able to generalize 

 the differential process by stating that microscopic specimens exhibit 

 usually rather marked phase-shifting properties. We then suggested 

 to select, once and for all, an image duplication value close to the 

 instrument resolving power. Image duplication is virtually invisible 

 and any object may be examined by the same instrument. As will 

 be seen later, it is then possible to devise highly convenient inter- 

 ference microscopes. 



Type "c" axial image duplication microscopes 



In these instruments, the ordinary image O is far removed from 

 the extraordinary image E (Fig. 3.16), When the ordinary image O 

 is focused by the eyepiece O., owing to faulty focusing, the image E 

 vanishes. There is, of course, some extraordinary hght E' in the plane 



-^ 



Fig. 3.16. Axial image duplication method. 



of the ordinary image. It is well known, too, that slightly defective 

 focusing in microscopy causes the image to vanish while a uniform 

 field emerges. This is what now happens to the extraordinary image. 

 The extraordinary light E\ in the plane of the ordinary image, gives 

 rise to a virtually uniform field: interference is taking place between 

 the ordinary wave and the uniform ground E' originated by the non- 

 focused extraordinary wave. The transparent object is examined 

 without the objectionable duplication. However, this process is not 

 so sensitive as the full image duplication method. As in all other 

 processes, the path difference A between the ordinary wave O and 



