INTERFERENCE MICROSCOPY 



from (^) that either positive or negative 

 vakies of C can be obtained. 



If white hght is used color contrast is also 

 present, which in many cases is useful in 

 resolving the ambiguity which occurs if the 

 optical thickness of a specimen exceeds one 

 wavelength. It also can give rise to images 

 of great beauty. 



Interference Microscopes for Trans- 

 parent Objects 



Instruments of Class I. Some types of 

 instrument designed for opaque objects can 

 be used for transparent objects and fall into 

 this class. One of these is the Linnik (2) 

 microscope which consists of a Michelson 

 interferometer with a microscope objective 

 built into each arm (Figure 1). The trans- 

 parent object is placed on a plane mirror 

 and is imaged by one objective; the other 

 images a plane mirror only. This arrange- 

 ment has some advantages; the optical ar- 

 rangement of the Michelson interferometer 

 allows good control of the background con- 

 ditions, giving the possibility of introducing 

 fringes of controllable width into the field 

 and of varying the background path differ- 

 ence, and the intensity in the two beams 

 can be controlled for good contrast of the 

 fringes. 



Against this, the two beams pass through 

 physically separate optics, which must be 

 selected for equality of aberrations and 

 chromatic dispersion of path difference. 

 Also, as the beams are widely separated, the 

 instrument is very sensitive to vibration. 

 In consequence it must be built into a very 



^SI 



Dooble. f^ocus le.ns 



<> E 



BecD rn 



Spl if te.r jL 



yep 



leoe 



Tnonspanent 

 spe-oirnen. 



Fig. 1. Linnik interference microscope. 



Objective 



Object 



C O ncJ e rt se. r- 



Fig. 2. Smith's interference microscope with 

 double focus lenses. 



heavy and rigid stand, which is necessarily 

 costly, and cannot be used for path-differ- 

 ence measurements of great precision. A 

 further difficulty which arises when using 

 this instrument for transparent specimens is 

 caused by the finite distance between the 

 specimen and its image in the mirror. 



Other instruments in this class will be 

 described under the section on "Instruments 

 for Opaque Objects." 



Instruments of Class II. Instruments 

 of this class have the great advantage that 

 both beams traverse the same optics and 

 are never very widely separated. This confers 

 considerable immunity from the effects of 

 vibration and also eliminates the necessity 

 for matching optics for aberrations and path 

 difference. It is very desirable that splitting 

 and recombination take place in the space 

 between condenser and objective. 



One instrument of this class is the polariz- 

 ing interference microscope of Smith (3) 

 (Figure 2). Double-focus lenses of bire- 

 fringent material are placed in conjugate 

 planes as shown, forming two axially sep- 

 arated images of the light source; the object 

 is placed in one of these, and the two beams 

 are re-combined after passage through the 

 objective. 



Two images of the object are seen, one of 

 which is out of focus and causes an effect 

 similar to the "phase-contrast halo," making 

 a measurement of optical thickness at a 



414 



