98 The Microscope (chap. 9) 



INTERFERENCE MICROSCOPY 



A more sensitive and accurate instrument is the interference micro- 

 scope, which is better adapted to measuring the refractive indices of a 

 specimen. In this microscope, the light splitting and recombining is 

 carried out externally to the specimen. Birefringent plates (doubly 

 refracting) are cemented to the top lens of the condenser and to the 

 front lens of the objective. One set of rays passes through the object, the 

 other set passes through a clear region at one side, and they are then 

 recombined. Any phase differences between them remain constant and 

 can interfere to give light or dark. A refractile object in one beam causes 

 a change in light intensity. 



Phase versus Interference 



Both operate on the same principle — the interference phenomenon 

 which changes phase difference into amplitude difference. 



phase: (1) Light passing through the specimen is made to interfere with 

 light diffracted sideways by it. (2) Only shows up diffracting structures in the 

 specimen; produces a "halo." (3) Apparatus is simple, reasonable in cost, easy 

 to operate and can be added to any conventional microscope. (4) Can be used 

 to study living material, cytoplasm, cell inclusions, nucleus, action of physical 

 and chemical agents on living cells. (5) Adequate for routine examination. 



interference: (1) Light splitting and recombining carried out on outside 

 of specimen and under control of experimenter. (2) No "halo"; variations 

 in the optical path through the object are easily interpreted. Phase change 

 can be measured. (3) Apparatus is expensive and complicated. Requires con- 

 stant checking and adjusting. (4) Can be used on living material to determine 

 dry mass: for example, changes in mass during cell activity; protein distribu- 

 tion, both in cytoplasm and nucleus. 



Polarizing Microscopy 



Closely related to the above types of observation is the use of polarizing 

 attachments. These may be used on most types of microscopes, but for 

 continued use a polarizing microscope is preferable. W'hen a ray of 

 plane-polarized light (vibrating in one plane) falls on the object, it is 

 split into two rays: one obeys laws of refraction and the other passes 

 through the object ^vith a different velocity. After emerging from the 

 object, the two rays are recombined, but because their velocity is 

 different, they will be out of phase. This phase difference is the quantity 

 measured in a polarizing microscope. 



