PHENOMENA OF ALL MICROSCOPES 17 



discussion, the main effect of including the light from the remaining 

 points in the condenser diaphragm is simply to produce a brighter image 

 of the specimen. 



The optical path of a given thickness of a uniform medium is equal to 

 the product of the thickness and the refractive index of the medium. 



The most important application of the phase microscope is to improve 

 the visil)ility of particles that differ from their surrounding medium 

 only l)y a small amount in optical path. We shall suppose throughout 

 this section that neither the particle nor its surround absorbs light and 

 that the optical path difference between the particle and the surround 

 is a small fraction of a wavelength, for e.xample X/20. In order to 

 distinguish more clearly between the behavior of an ordinary microscope 

 and a phase microscope, this section will be subdivided into five parts. 

 The first part, 3.1, will explain why the contrast in the image is poor in 

 the ordinary microscope when the particle differs from the surround 

 only in refractive index. The second part, 3.2, will show how the con- 

 trast in the image of such particles can be improved by a suitable choice 

 of diffraction plate in the phase microscope. 



3.1. Phenomena of all microscopes 



The following diffraction phenomena occur in all microscopes and, on 

 the basis of the simplified theory, lead to the conclusion that particles 

 differing from their surround only in optical path should not be visible 

 in the ordinary microscope. 



If the condenser diaphragm is placed near the first focal plane of the 

 substage condenser, the light wave radiated from a point C in the opening 

 of the iris emerges from the substage condenser as a substantially plane 

 wave so that the rays from C are parallel upon passing through the 

 object specimen as in Fig. II. 3. The object particle forms an obstacle 

 in the path of the incident wave. Conseciuently, the incident wave 

 does not pass without interruption through the object plane but is 

 diffracted. A portion of the diff'racted incident wave continues on its 

 original course undeviated, but the remaining portion of the diffracted 

 wave is deviated away from the direction of propagation at incidence. 

 We may therefore regard the incident wave as broken into two related 

 waves by diffraction at the object. The undeviated portion of the in- 

 cident wave is called the undeviated wave, whereas the deviated portion 

 is called the deviated wave. 



The rays belonging to the undeviated wave are drawn as solid lines 

 in Fig. II. 3. These rays pass through a small neighborhood about the 

 point C in the second focal plane of the objective. The point C in the 

 condenser diaphragm is imaged about the point C. After passing 



