2 INTRODUCTION TO PHASE MICROSCOPY 



other properties to reveal themselves as brightness or color contrasts. 



The improvement of microscopes since their invention about 350 years 

 ago has proceeded along two general lines, both of which have been 

 necessary. First, there has been improvement of the instrument itself 

 in order to make it a better and more convenient image-forming appa- 

 ratus. Thus more effective forms of illumination have been developed, 

 such as lamps and condensers. The microscope objectives and eyepieces 

 have been improved by optical designers to provide sharper images and 

 improved resolving power. Microscope designers of the past and present 

 have been notably successful in these aims, and they are still attempting 

 to make the microscope a more nearly perfect optical instrument for 

 gaining the nearest possible approach to "point-for-point" imagery, that 

 is, to obtain the image that most nearly reproduces the brightness and 

 color differentiations of the object specimen with greatest fidelity. We 

 have microscopes which, at least o\'er the central portion of the field of 

 view, achieve the full resolving power considered attainable from theory 

 based on the wavelength of light. 



The above-mentioned line of progress does not depend on the specimen 

 and its peculiar optical properties. The second approach in the improve- 

 ment of the microscope is to consider the specimen as an essential part 

 of the optical system and, as it were, to build the microscope around the 

 specimen. To be sure, this results not in simplification of the microscope 

 as an instrument but rather in greater diversification. 



Let us consider what optical properties of the specimen are to be 

 "seen" in order that profitable information regarding its details may be 

 acquired. The ordinary, conventional microscope is without rival for 

 observing areas of light absorption and color differences in specimens. 

 The darkfield microscope is designed to reveal particles by virtue of 

 their property of scattering light. Details that polarize the light or have 

 birefringence are revealed with the polarization microscope. If a speci- 

 men fluoresces under the action of radiation a fluorescence microscope 

 is used. But a common property of all these microscopes is the forma- 

 tion of an image in terms of brightness or color contrasts which the eye 

 is capable of observing or which can be converted into an observable 

 image. 



Until the invention of the phase microscope no very useful means were 

 available for observing differences in optical path in a specimen. Optical 

 path is the linear path of light through a transmitting medium multi- 

 plied by the index of refraction of the medium. Refractive index in a 

 microscopic specimen depends upon the specimen's physical and chemi- 

 cal properties. Optical path differences may arise from differences in 

 either refractive index or thickness or a combination of both. The 



