INTERFERENCE MICROSCOPY 



oa 



I 



^H4H fH-H-l + 



Birefringent 

 Plate 



Component 

 parallel to~^' 

 optic axis 



Original vibration 



Component perpendicular 

 to optic axis 



H t < M 



o.a\ 



Biretnngent 

 'Plate 



Fig. 2. Action of birefringent plates on polarized light, (a) Optic axis parallel to face of plate. The 

 incident polarized light has a component vibrating parallel to the plane of the paper, indicated by the 

 dashes, and a perpendicular component indicated by the dots. Before entering the birefringent plate 

 they are in phase. On entering the plate the parallel component moves at a higher velocity, so that on 

 emerging the parallel component is ahead of the perpendicular component, (b) The resolution of the 

 incident plane polarized vibration into two components which occurs at the entrance face of the plate, 

 as seen by an observer viewing the oncoming light, (c) Optic axis inclined with respect to the surface. 

 In this case not only is there a phase difference introduced between the two components, but one com- 

 ponent is refracted differently than the other. Crystals of calcite (Iceland spar) found in nature exhibit 

 this effect, which results in a double image of everything viewed through the crystals. 



2c. The two beams emerging from such a 

 plate are not capable of interfering even if 

 they are brought into coincidence, because 

 their planes of polarization are mutually per- 

 pendicular. They can be brought to the same 

 plane of polarization by means of an ana- 

 lyzer with its transmission axis at 45° to 

 the plane of vibration of each beam. It can 

 also be done by means of various compensa- 

 tors which permit the precise determination 

 of the phase difference between the two com- 

 ponents. 



Another birefringent device frequently 

 used is a half -wave plate. As shown in Fig. 3 

 it retards one component of polarized light 

 by one-half wavelength relative to the other 

 component. The net effect is to rotate the 

 plane of polarization from azimuth — « to 



The action of a quarter-wave plate is 



shown in Fig. 3b. If the incident plane- 

 polarized light is oriented at 45° to the optic 

 axis, the two components formed at the first 

 surface are equal in magnitude. When they 

 emerge, 90° out of phase, the resultant vibra- 

 tion is circularly polarized. 



Principles of Commercially Available 

 Interference IVIicroscopes 



IVIultiple-beani. The multiple-beam sys- 

 tem can be set up with as little equipment 

 as a standard microscope, a slide and cover 

 glass with partially reflecting coatings, and 

 a pinhole at the first focal plane of the con- 

 denser (2, 3, 4). A typical system (5) is a 0.5 

 mm diameter pinhole at the focal point of a 

 25 mm objective. Monochromatic light, such 

 as the green radiation of a mercury arc 

 isolated by a suitable filter, is used to illumi- 

 nate the pinhole. Light is partially trans- 



422 



