CHAPTER I 



INTRODUCTION TO 

 PHASE MICROSCOPY 



1. SEEING MICROSCOPIC PARTICLES 



The microscope is commonly described as an instrument used for 

 seeing small objects. This definition leaves more unsaid than said. 

 What do we mean by seeing small objects? The human eye sees because 

 of two properties of the light entering the eye from the objects seen. 

 The eye recognizes only differences in brightness and differences in color. 

 Visual images are based on these two properties and no other. Differ- 

 ences in brightness of different objects or their component parts give 

 rise to brightness contrast; differences in color cause color contrast. 

 Almost all our knowledge of observed scientific phenomena is based on 

 our ability to see. No matter how simple or how complicated is the 

 physical apparatus used in making our observations, the final presenta- 

 tion of results must be in a form that can be seen. These results may 

 be scale readings, graphs in one or more colors, or direct images as in the 

 microscope. But they all appear to the human eye as brightness or 

 color contrast in the objects viewed. Brightness is a function of the 

 amplitude of the vibrations of light waves proceeding from the objects 

 viewed. Hence, brightness contrast might be called amplitude contrast. 

 Color depends on the wavelength of the light entering the eye. Physi- 

 cally, then, the eye is responsive only to the amplitudes and wavelengths 

 of light. As will be brought out later we, as microscopists, might be 

 tempted under certain conditions to wish that the eye were sensitive to 

 some of the other physical properties of light. 



The microscope, more broadly, can be considered an instrument that 

 utilizes the action of a specimen on the probing illumination supplied to 

 it for the purpose of our gaining knowledge of the specimen through 

 visual impressions. We are interested in several properties of the speci- 

 men, such as, for example, size, shape, and density of its details. If the 

 only properties in which we were interested were light absorption or 

 selective absorption, giving rise to brightness and color contrasts, the 

 microscopical problem would remain simple. But we are interested in 

 other properties, and we must by some instrumental means cause these 



