162 PROGRESS IN MICROSCOPY 



over 14 millimetres. A standard objective may be converted into 

 a long working-distance objective by using the Dyson layout (Fig. 4.41). 

 The object P lies in the centre of the curvature A of the concave 

 mirror M. The object-originated rays pass through the semi-reflective 

 plate m and are reflected back by the mirror M on to the plate m and 

 end at A' . Therefore, A is imaged at A\ i.e. the object, which may 

 now be examined by a powerful objective with short working-distance. 

 The working distance of the Dyson layout is approximately 13 mil- 

 limeters. 



