Micro: 3 



In addition to the penetrating properties of X-rays, the desiyn of tho 

 instruments permit a very large depth of field. Thus, it is possible to 

 make stcreophotomicrographs even at the highest magnification. Such 

 stereographs enable one to see the true shape and distribution of the 

 various structures. The specimen is examined in the atmosphere, that is, 

 it does not have to be placed in a highly evacuated chamber as required 

 in the electron microscope. Nematodes, small roots, and leaves would not 

 be too thick to be observed vrithout sectioning, thus eliminating the need 

 for specimen preparation for microsectioning or in situ staining. View- 

 ing of the image, as with the electron microscope, is done on a fluores- 

 cent screen. For greater resolution and for records, photomicrographs 

 are easily made. 



The idea of developing an X-ray microscope and an appreciation of its 

 potentialities for new ways in microscopy dates back to soon after 

 Roentgen discovered X-rays. In fact. Roentgen tried to develop such an 

 instrument. His research and that of numbers of others, until quite 

 recent times, failed in finding how to appreciably focus X-rays. 



There are now three types of X-ray microscopes evolved in recent years. 

 In the sin^ilest type, the contact X-ray microscope , the specimen is placed 

 in close contact with a fine grain panchromatic film or plate and exposed 

 to X-rays. The resulting image on the film is developed and enlarged 

 optically. This is a sinple, easy method quite similar to making a dental 

 X-ray, but presently limited to a resolution of about 1 ji and magnifica- 

 tion up to ^00 diameters. In the second type of X-ray microscope, the 

 reflection X-ray microscope , mirror systems analogous to optical mirrors 

 have been developed. (No refracting lenses analogous to those of the 

 light microscope can be used with X-rays.) Resolutions to about 1 }i have 

 been attained vd.th these reflecting systems. The third type of X-ray 

 microscope, the projection X-ray microscope , has so many advantages that 

 production models have been developed in three countries. Resolution of 

 0.1 p and magnifications of 2000 diameters have been attained, and as 

 mentioned, the theoretical limits have not been met. 



General Electric X-ray microscopes are commercially available in this 

 country and represent the projection X-ray system. The problem of focus- 

 ing the X-rays has been overcome in an ingenious way. First a stream of 

 electrons is focused by means of electrostatic lenses to form a single 

 spot of less than one micron in diameter on to a thin beryllium target 

 window coated with tungsten. The result is an equally tiny spot-source 

 of X-rays. A specimen placed near this X-ray source casts an enlarged 

 image on the fluorescent screen or photographic plate. Thus, one part 

 of the instrument is like an electron microscope, but instead of enlarging 

 the electron beam within the highly evacuated tube, it reduces it to a 

 very small spot image. The other portion of the instrument lies outside 

 the target end of the electron beam tube and is simply a shadow projection 

 microscope utilizing the X-rays that emanate from the tiny spot-source at 

 the end of the sealed and evacuated electron "demagnifying tube." 



VJhether or not the specimen would be killed by the X-rays depends on the 



