IMKKI KKENCE MICROSCOPY 



>■/. 



2 Rote 



Ci^ystal plc3l"es cut at| 

 T -45° with optic axes 



Fig. 5. Lebedev's microscope. 



V^oMQs1"on pnism 



Objective 



Con<deriser-> 

 Wollas+On pnism 



Fig. 6. Smith's shearing interference micro- 

 scope. 



astigmatic. A similar principle has been used 

 by Smith (6) for a high-power instrument, 

 replacing the lower plate by a plate of ciuartz 

 of increased thickness, the half-wave plate 

 being no longer required. This is of impor- 

 tance in high-power instruments where very 

 large convergence angles are employed. The 

 amount of shear is restricted by the thickness 

 of the plate which can be employed, so both 

 images appear in the field. In consec}uence 

 only a limited area of the edge of a large 

 specimen can be examined without overlap 

 and loss of explicitness. On the other hand, 

 the limited shear makes for increased ease of 

 adjustment. 



A critique of this and the Dyson instru- 

 ment in Figure 5 has been published by 

 Davies (7). 



Another type of shearing microscope has 

 been described by Smith (3), (Figure 6), 

 using two Wollaston prisms in conjugate 

 planes as shown. This avoids the astigma- 

 tism of one of the images, but this time the 

 shear is limited by aberrations of the con- 



jugate planes. Tl is interesting to notice that 

 the lower Wollaston may be dispensed with 

 if the condenser apertvu'e is reduc^ed to a 

 nai-row slit parallel with the apex edges of the 

 prism, but this will of course give rise to 

 some deterioration of the image (luality. 



The shear is produced in an instrument 

 due to Frangon (8) by means of a Savart 

 plate just below the eyepiece, using slit 

 illumination. 



Shearing microscopes of low power have 

 considerable advantages of simplicity and 

 stability, making them perhaps the best 

 instruments for the important field of the 

 measurement of thin films, as described later. 



Interference Microscopes for Opaque 

 Objects 



The distinctions between the three classes 

 of instruments for opaque objects are if 

 anything of greater significance than in the 

 case for transparent objects, for path differ- 

 ences produced by reflection are usually 

 greater than those produced by transmission. 



Instruments of Class I. The Linnik 

 instrument described above is typical of this 

 class, and the discussion of this instrument as 

 applied to transparent objects applies here 

 also. 



It is possible to construct microscopes of 

 this class in which both beams traverse the 

 same objective. An instrument due to Dyson 

 (9) (Figure 7) uses a small reference flat 

 separated axially from the specimen, with a 

 splitting plane midway between them. The 



Objective 



Small silvered spot 



•Half s.ilvered surface 



r777r,'rr7T7 



Fig. 7. Dyson's interference microscope for 

 opaque objects. 



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