THEORY AND TECIIMQl ES 



is under test. The light reflected from T 

 strikes a fully silvered spot, S, which is lo- 

 cated at the same optical distance from T 

 as is the surface A . The two components are 



mage 



Fig. 6. The Mirau system for surface examina- 

 tion. R: Partiality reflecting mirror, O: objective, 

 T: glass plate with semi-reflecting surface, *S; fully 

 silvered spot, A: surface under test, X and Y are, 

 respectively, the points at which the beam divides 

 and recombines. 



recombined at Y and travel through the ob- 

 jective together. 



These two-beam s3^stems are somewhat 

 easier to use than the multiple-beam meth- 

 ods. However, the two-beam precision is 

 lower, being the order of Ko fniige, or 0.025 

 micron. 



Two-beam Interference Micro.scopes 

 for Kxaniinalion of Transparent Ob- 

 jects. The Dyson Interferometer Microscope. 

 The Dyson interferometer microscope (Hi), 

 manufactured by Cooke, Troughton, and 

 Simms, Ltd., utilizes semireflecting coatings 

 for beam-splitters. As shown in Fig. 7 the 

 upper surface of plate A transmits part of 

 the incident beam, which then proceeds 

 through the object. The reflected portion of 

 the incident beam is again reflected by the 

 lower surface of plate A. This reference beam 

 then passes through an area of the slide 

 away from the specimen. 



Plate B serves to reunite the object and 

 reference beams. The fully silvered spot on 

 the upper surface prevents rays having small 

 values of y from reaching the objective. The 

 reason is that for these rays the reference 

 beam is so close to the object beam that 

 both may pass through the specimen. 



Slide, specimen 

 and cover glass 



Spherical surface 

 Fully silvered 



Immersion liquid 



-^Ct^^-^ Micrometer screw 



Condenser 



Semi-reflec+ing surface 

 Fully reflecting surface 



Fig. 7. The Dyson interferometer microscope. Plates A and B are very slight wedges, and the wedge 

 directions can be aligned either in the same or opposite directions to give a fringe field or uniform field, 

 respectively. Plate A is movable transversely for measurements. 



425 



