100 PROGRESS IN MICROSCOPY 



The image A' (all the transparent details of which are perfectly 

 visible owing to interference) is observed through the objective O^ of 

 an ordinary microscope. 



If the instrument is constructed as described above, the path 

 difference between the two rays MAN and MBN is zero provided 

 the object is not present. Both rays are in phase and intensity at its 

 peak. Let us bring in the specimen, i.e. the object: the path difference 

 is no longer zero for the rays having passed through the object A, 

 and the intensity in the image A' is lower than in the remainder of 

 the field. The object is dark, to a greater or lesser degree, on a bright 

 ground. In fact, both plates L^ and Lo are not strictly parallel-sided 

 plates but very-small-angled prisms. If the slopes lie in the same 

 direction, edges of the prisms being parallel, phenomena occur as 

 in the foregoing. The field is uniform. The plate L^ is shifted by means 

 of the screw V and the path MACN in relation to path MBN is adjust- 

 able at will. If, through motion of Li, the ray MBN passes through the 

 plate Li in an area where it is thicker than L^, the path MBN is longer 

 than that of MACN and the path difference between the two is ad- 

 justable as mentioned above. Let us assume this path difference to 

 equate an odd number of times A/2: both rays MBN and MCN are 

 in opposition. The field of view is dark except in the area where the 

 object A lies, as the rays having passed through the latter are no longer 

 in opposition with rays such as MBN. The typical image exhibits 

 a dark ground. Instead of adjusting the path difference so that the 

 paths MEN and MCN (without object) are exactly in opposition, 

 let us bring in between them a slight additional path. Phenomena occur 

 as if it were a typical phase-contrast image. Actuation of the screw V 

 will vary contrast at will. If the sides of both plates L^ and L, slope 

 in opposite directions, the path-difference varies continuously from 

 one end of the field to the other. Straight, parallel and equidistant 

 fringes are developed. Actuation of the screw V shifts all these fringes 

 and the path is varied. It should be noted that, owing to the feasibility 

 of adjusting path difference to very low values the instrument may 

 be illuminated in while light. 



When describing Fig. 3.4 no other reflections but those discussed 

 above were taken into consideration. In the plate [..,, for instance, 

 there are other reflections besides those at C and A^; many reflections 

 lake place but the relevant images are not in the same plane as image 

 A' on which the microscope objective Oy is focused. These images 

 all have a dark centre caused by the diaphragm cfTect of the opaque 



