148 



PROGRESS IN MICROSCOPY 



being reflected in the semi-reflective plate G (the vertical illumi- 

 nator is not shown) and, passing through the objective Oi, the 

 ray enters the system B^, B.., B^ and, next, is reflected by the 

 convex side and splits in two as it impinges on MM' . One of these 

 rays is reflected on the object P and the other on the mirror R. 

 Both rays are superimposed anew as they impinge on the convex side. 

 The reflected beam reverts to the objective O^. As the image P' is not 

 exactly in the centre of curvature of the convex side, some spherical 

 aberration is developed which may be corrected by giving a suitable 

 thickness to the block B^. Thickness variations of approximately 

 30 A have been detected by the Dyson layout. Since the object is 

 examined through reflection, optical paths are twice as long. There- 

 fore, a 30 A optical path variation in the object correlates a 60 A 

 variation as detected in the instrument. This remark applies equally 

 well to any reflected-light instrument. 



Fran^on's layout (Fig. 4.29.) 



No mention need be made regarding the interference eyepiece 

 described in Chapter III as merely setting this device in a reflecting 

 microscope evinces interferential images of reflective objects. Fig- 



'■/'■.■,////. 



777. 



P 

 (a) 



(b) 



Fig. 4.29. Frangon interference microscope. 



ure 4.29 shows the principle of a diflerent and simple polarizing 

 interference device. Where low-magnifying power is involved, the 

 observation device consists of a mere magnifying glass O as shown 



