8 The Microscope 



the right has a very high dispersion, it will pull the colors proportionately 

 closer together even though it does not bend the light so much. Hence all 

 the colors will come together at the same point and produce a colorless 

 or "achromatic" image. Moreover, this image will be relatively free of 

 spherical aberration since the thick edge of the negative lens balances 

 the thick center of the positive one. 



Unfortunately all this is a theoretical dream. Glasses of very high 

 refractive index and very low dispersion— or vice versa— do not exist. All 

 lens design is a compromise, sometimes involving as many as six kinds of 

 glass, each bending or separating or compressing light in varying amounts. 

 Lens designers early learned to incorporate fluorite as a substitute for 

 one of the glasses, and during World War II many synthetic nonsilica 

 glasses were developed. But the perfect lens is still in the future. De- 

 signers effect the best possible compromise between reality and theory in 

 the light of the specific requirements of a microscope objective. These 

 requirements must next be examined. 



REQUIREMENTS OF A MICROSCOPE OBJECTIVE 



Resolution. The most important part of a microscope is the objective. 

 Every other part of the instrument is designed to help the objective pro- 

 duce the best possible image. The best image is not the largest— it is the 

 clearest. There is no purpose in looking at an object through the micro- 

 scope unless we arrive at a better understanding of its structure. Mere 

 size is no aid to understanding. A simple black dot the size of a pin's 

 head is just as understandable as a simple black dot an inch in diameter. 

 What we want to know from the microscope is whether the pinhead- 

 sized dot is a simple dot or whether its smallness conceals a pattern. 

 The ability of the microscope to reveal this pattern is known as "resolu- 

 tion," and resolution is therefore the most eagerly sought characteristic 

 of a lens. It is obvious that a lens with chromatic and spherical distor- 

 tions will not resolve satisfactorily, but there is more to resolution than 

 the correction of aberrations. 



This elementary book is no place to present involved mathematical 

 arguments about the cause and nature of resolution. There is, however, 

 a very simple analogy that will serve to introduce the subject. Suppose 

 that you have in your hands a sheet of material and that you do not 

 know whether it is woven from fine fibers or rolled from a structureless 

 plastic. You would immediately try to pull your hands sideways to spread 

 any fibers that might be present. Nothing would be gained by plucking 

 the sheet toward you. It is the same with lenses. If they are to resolve 

 closely spaced lines and dots, they must spread out the image. They do 

 this by spreading light rays— and the wider the spread, the greater the 



