8 INTRODUCTION TO PHASE MICROSCOPY 



by 3<^ wavelength. Here it can be seen that the amphtude of Z) + S, 

 representing the Hght in the image of the particle, is considerably greater 

 than that of S, the background light, and the particle will appear lighter 

 than its background. But suppose that we now retard the deviated 

 wa\'e, D, by 3^ wavelength (or advance *S' by this amount). The 

 condition shown in Fig. 1.4 is the result. The light in the image of the 

 particle is now S — D, and, as in the previous example, the background 



Fig. 1.3. Increased brightness from additive summation of direct and deviated 



light. 



remains of amplitude S. Now the particle appears darker than the 

 remainder of the field of view. 



Thus, if the phase of the deviated light can be changed with respect 

 to the direct (undeviated) light from a single point in the original source 

 of illumination, we have a means for changing the invisible phase dif- 

 ferences arising in the specimen into amplitude differences in its image, 

 which is thereby rendered visible. Figure 1.5 illustrates how this change 

 is accomplished in a simple example. Starting at the bottom of the 

 drawing, let us consider a beam of light proceeding upward from a single 

 point in the condenser dia'phragm. This point is on the axis of the 

 optical system and is located at or near the lower focal plane of the 

 condenser, from which the beam emerges as a bundle of parallel or nearly 



