TIIKOKY AND TECIIMQLES 



nation is known as a Savart plate. The ray 

 which is extraordinary in the first plate be- 

 comes the ordinary ray in the second plate, 

 and vice-versa. The two rays can then be 

 caused to interfere by bringing their vibra- 

 tions into the same plane by an analyzer 

 oriented at 45° to each. 



The amount of shear is determined by the 

 thickness of the plates and is selected accord- 

 ing to the needs of the user. A small shear 

 shows essentially gradients in optical path 

 in the specimen, as shown in Fig. 9b. It 

 is useful for producing contrast at the edges 

 of objects or at gradients within the speci- 

 men. A large shear (9c) produces a double 

 image and can be used for measurement of 

 the optical path in isolated objects. 



Numerous other uses for the Savart plate 

 and its modifications, as well as the Wollas- 

 ton prism, have been described by Fran^on 

 and Nomarski (26). 



The NIFE Interference Microscope. This 

 system, first described by Johansson and 



Afzelius (27), employs a modification of the 

 Fran(;()ii interferential eyepiece which allows 

 measurements of optical path to be made. 

 As shown in Fig. 10, a glass wedge at the 

 image plane can be rotated in its own plane. 

 Through a hole in the wedge the object to 

 be measured is viewed. With white light 

 illumination, the color of the object will de- 

 pend on the phase difference, 0, between the 

 two wavefronts from the specimen (Fig. 

 9). As the glass wedge is turned the phase 

 difference between the two background 

 wavefronts is changed, becoming a maximum 

 when the direction of the wedge is parallel 

 to the direction of shear. The viewer turns 

 the wedge until the color of the background 

 matches that of the object. An alternate 

 method is to have the image of the specimen 

 fall on the wedge rather than on the hole. 

 Then the wedge is turned until the color in 

 the specimen matches that in the hole. Ingel- 

 stam (28) has discussed the errors in this 



Shearing direction 



Optic axis 



^ J. 



Fig. 9. The Savart plate, (a) Schematic diagram showing the action of a Savart phite. The com- 

 ponent of incident light vibrating in the plane of the paper is sheared to the right by the first plate. 

 The perpendicular component is sheared perpendicular to the plane of the paper by the second plate. 

 The resultant shear is in the direction shown, (b) The differential method. Here the two wavefronts 

 shown have l)een sheared by a small amount, by means of a thin Savart plate. The wavefront jiolarized 

 parallel to the plane of the paper is designated by p, the wuvefront polarized perpendicular by s. (c) 

 The total doubling method. The two wavefronts shown have been sheared by a large amovint , liy means 

 of a thick Savart plate. In order to produce interference in either case b. or c. the incident light must 

 be plane polarized and the two emergent wavefronts must be brought to the same i)lane of polarization, 

 as bj' an analyzer. 



429 



