OPTICAL THEORY OF LIGHT MICROSCOPE 



mains cannot be overlooked. The colloidal microscope, and the second (or eyepiece) 

 magnetite method has long been established stage results in the very large "virtual inl- 

 and has become an important adjunct as a age" shown near the bottom. Fig. 1 also is 

 microscopical method applied to magnetism, helpful in showing how the mirror directs 

 This method gives strong delineation of the the entering light upward toward the con- 

 domain patterns with a minimum of speci- denser, which in turn concentrates the light 

 men preparation and outlay for specialized into a brightly illuminated spot on the ob- 

 equipment. On the other hand, the polarized ject. 

 light method has the advantage that it 

 yields instantaneously to field intensity or Resolving Power 



directional changes, and employs no surface The statement that the magnification of 



additives. Also, it may be possible to use the light microscope ranges from about 10 X 



this method to study specimens at elevated to 2500 X raises the question as to why it 



temperatures. It is believed that with fur- cannot be extended beyond 2500 X. Actu- 



ther development of the polarized light ally it is quite possible to do this, but very 



method a more intensive study of the geo- little is gained by doing so, due to the limit 



metrical configuration of the magnetic do- in resolving power imposed by the finite size 



main will be possible. of light waves. Thus resolving power is in 



^ ^ ^ the final analysis the fundamental quality 



F. Gordon Foster , • , • .• i ^■ ■^. ^.u 



which imposes a practical upper limit on the 



.^.^.■-....^ ...^r^^^^^^-o r, .-...^■...— r, magnifying power of the microscope. 



MEASURING MICROSCOPES. See ENGINEER- ^ ^. .^ ^ , \^ ,.,.^ 



iM^ xAirDncrriDcc a to Resolving power is a measure of the ability 



ING MICROSCOPES, p. 439. » ^, . ^■ .■ ■ -u c ^ ^ -i 



of the microscope to distinguish hue detail. 



OPTICAL THEORY OF THE LIGHT It is, in quantitative terms, the distance be- 



MICROSCOPE tween two points in the object which can 



,, ._ . just be detected as being separated and not 



Maenmcation *. , 



single, ihe resolving power oi a microscope 



The light microscope is an optical instru- depends generally on the design of the ob- 



ment which produces highly enlarged images jective. An objective capable of utilizing a 



of very small objects. The microscope large angular cone of light coming from the 



achieves its magnifying power in two stages, specimen will have better resolving power 



The objective performs the first stage of the than an objective limited to a smaller cone 



magnification, forming a magnified image of of light. This is demonstrated in Fig. 2, 



the object near the top of the microscope where the same specimen area has been pho- 



tube. This image is further enlarged by the tographed with two different objectives of 



eyepiece, which acts as a magnifier. Objec- quite different designs as indicated below 



tives range in power from about 2X to each photomicrograph. 



100 X. Eyepieces range in power from about The quantity A^ sin U in Fig. 2 is called 



5X to 25 X. Thus the total magnification, the Numerical Aperture (N.A.) of an ob- 



being the product of objective and eyepiece jective. By definition: 

 magnification, ranges from about 10 X to 

 2500 X. 



Fig. 1 is a ray diagram of a typical micro- where N is the lowest refractive index be- 



scope, and illustrates the two stages neces- tween the specimen and the objective, and U 



sary to achieve the final magnification. The is the half-angle of the cone of light as shown 



first (or objective) stage results in the "pri- in Fig. 2. The objective shown at the right 



mary image", shown near the top of the has a higher N.A. and has the superior re- 



445 



NA = AT sin U 



