INTERFERENCE MICROSCOPY 



The spherical mirror together with the With it visual settings arc made by matching 



upper surface of plate B is essentially a unit- the luminance in the specimen or surround 



power relay system which increases the work- with that of a reference spot superimposed 



ing distance of the microscope objective. An in the field. 



image of the object is formed by this relay The above methods measure only the 

 system below the objective and is magnified fi'action of a wavelength of optical path. If 

 in the usual fashion by the ol)jective and the OPD is greater than one wavelength, or 

 eyepiece. if it is not certain whether the path differ- 

 As can be seen by comparing the solid ence is positive or negative, the use of white 

 and dashed paths, the distance between the light and the fringe field can usually settle 

 reference and object beam depends on the the question. 



angle, 7, of the incident ray. Thus when a The advantages and disadvantages of this 



full cone of light is incident from the con- microscope have been discussed by Hale 



denser, the reference beams are passing (18). 



through various portions of the slide sur- The AO-Baker Interference Microscope. The 



rounding the specimen. AO-Baker interference microscope (19, 20) 



In order to eliminate poor contrast and utilizes a birefringent plate located above the 



errors due to nonuniformities in the slide condenser as a beam-splitter. Plane polarized 



and coverglass, immersion fluid is used in light incident on the birefringent plate is 



the three places shown in Fig. 7. divided into two mutually perpendicular 



For quantitative measurements, several polarized beams, as described earlier. The 



methods are available. If plates A and B have extraordinary beam, consisting of vibrations 



their wedges aligned in opposite directions in the plane of the optic axis, is the object 



the field is uniform, except for the specimen, beam. 



As plate A is moved laterally by means of In the shearing system, shown in Fig. 8a, 



the micrometer screw the field goes through the optic axis is in the plane of the paper but 



maxima and minima of luminance. The mi- inclined at an angle of about 45° to the sur- 



crometer dial is turned to obtain minimum faces of the plate. This causes the object 



luminance in the specimen and then in the beam to be refracted to the left, or sheared, 



background immediately adjacent to the as shown. The reference beam, the ordinary 



specimen. The difference in micrometer beam, passes through the plate as ordinary 



readings is proportional to the optical path light passes through a glass plate, 



difference between the specimen and the On passing through the half-wave plate 



surround. If the plates A and B have their both beams have their planes of polarization 



wedges aligned other than exactly opposite, rotated by 90°. This allows the beams to be 



the field is crossed by fringes. Their spacing reunited by a second birefringent plate 



depends on the relatively angle between the placed before the objective. It is identical 



wedge directions, and they move as plate A in thickness and orientation with the first 



is adjusted laterally. The micrometer dial plate. As shown in Fig. 8a one beam has 



is turned until a dark fringe is located first passed tlirough the object while the other 



in the specimen and then in the background has passed through an adjacent area of the 



immediately adjacent to the specimen. slide. After being reunited by the second bi- 



Another method of measuring is to photo- refringent plate these beams travel together 



graph the field with uniformly spaced fringes, through the objective and to the image. In 



Photographic photometry can then be used general there is a phase difference between 



(17). the beams. It represents the optical path 



A photometer eyepiece is also available, encountered by the object beams minus that 



426 



