29 : 8/ Microscopy 551 



10 irut. Accordingly, one might hope to obtain resolutions of the order 

 of 10 rnjjL with X-ray microscopes. However, one cannot use ordinary 

 lenses. Special curved mirrors "illuminated" at grazing incidence can 

 be used to create an X-ray optical system with a theoretical limit of 

 resolution of 7 m/x. Owing to imperfections in construction, alignment 

 and focusing, no X-ray microscope of this type has resolved separations 

 smaller than 500 m^ (as compared to 200 m/x with the bright-field 

 light microscope!). 



Another technique is to use an extremely tiny pin hole to give an X-ray 

 source spreading out from a point. If the specimen is placed close to 

 the pin hole and a photographic film is located many times this distance 

 away, a magnified image (shadow) will be found in the photographic 

 image. This may be further enlarged by ordinary photographic 

 methods in forming the final image. In this fashion, images of quite 

 good quality can be obtained in which separation of the order of 500 m/z 

 can be resolved. 



The principal application of the X-ray microscope has been in micro- 

 spectrophotometric studies. These can be useful in locating atoms of 

 heavier elements such as calcium, phosphorus, iodine, and sulfur 

 within the tissues. For this application, a parallel beam of X rays 

 falls on a sample and is photographed. Repeating this at several wave- 

 lengths and examining the photographs with a light microscope (or 

 with a microdensitometer) allows one to locate these heavier elements. 



8. The Electron Microscope 



All of the special forms of microscopes discussed to this point had limits 

 of resolution larger than, or at best equal to, that of the light microscope. 

 The electron microscope is unique in having a far greater resolving 

 power (that is, far smaller limit of resolution) than any light microscope. 

 The impact of electron microscopy on biology has been particularly pro- 

 nounced. The study of viruses discussed in Chapter 14 is very depen- 

 dent on electron microscopy. In a similar fashion, the cytologists' 

 pictures of cell membranes, of the bacterial surface, and of the sub- 

 cellular structures are fashioned from electron micrographs. At one 

 time, mitochondria were small bodies within cells exhibiting character- 

 istic staining patterns. This view has been replaced by a structure with 

 a double membrane, cristae, and so on, whose appearance in an electron 

 micrograph has a characteristic form. Besides the fields of virology and 

 cytology, numerous others including the form of the visual receptors 

 (see Chapter 7), the chloroplasts (see Chapter 20), and the contractile 

 elements of muscles (see Chapter 8) depend on electron microscopy for 

 their basic structural pictures. 



