X-RAY MICROSCOPY 



object on to a distant fluorescent screen or striiment column is 80 cm (approx. 32 in.) 

 photographic plate with a resolution ap- and so the operator can remain seated while 

 proximately equal to the size of the point inspecting the fluorescent screen, 

 source. The Cosslett-Nixon x-ray microscope Preliminary alignment of the microscope 

 operates on this principle and routinely pro- column is carried out by lateral and axial 

 vides a resolution of the order of a half to adjustments of the two lenses, with the ob- 

 one micron. The best resolution obtained to ject of centering the electron beam upon a 

 date with this type of instrument has been small circular viewing screen which is sub- 

 about 0.1 micron. Since the magnification stituted for the target assembly within the 

 depends on the ratio of the target-object to polepiece of the objective lens. A greatly 

 target-plate distance, both of which are vari- demagnified image of the electron source is 

 able, a high primary magnification of up to first formed and focused to a point upon 

 X200 is obtainable depending on the nature the viewing screen. This screen is then re- 

 of the object under study. placed by the target which consequently 



Projection x-ray microscopy has many ap- emits x-rays from what is virtually a point 

 plications in biology, anatomy, and medi- source, so avoiding the difficult problem of 

 cine. The first biological studies were made focusing x-rays. Further focusing is carried 

 on drosophila (5) and insect flight muscula- out upon a fluorescent screen placed above 

 ture (6) with interesting results. In the vas- the target, with the aid of a small XlO 

 cular field it has been used to study the ocular or magnifier. In actual use the opera- 

 microscopic blood vessels of a variety of or- tor judges the sharpness of the enlarged x-ray 

 gans and tissues, including skin, muscle, image of a test grid placed upon the target 

 bone, teeth, kidney, stomach, intestine, while varying the lens current. A fine silver 

 brain, spinal cord and nerves (7, 8, 9). In mesh grid (1500 mesh/inch) composed of 3 

 the field of histology it has recently been micron bars and 17 micron spaces is used 

 used to examine unstained sections of animal for this purpose, as well as for determining 

 and human tissue (10) and is also being ap- the resolution and calibrating the magnifica- 

 plied to the determination of the dry weight tion. When the grid image is as sharp as pos- 

 (mass) and elementary analysis of cellular sible, the test grid is removed, and the in- 

 structures. strument is ready for experimental purposes. 



Operation of the Instrument. The No further lens adjustments are usually 

 x-ray microscope resembles an inverted elec- necessary during the course of the day, so 

 tron microscope in that it consists of two that attention can be devoted to the experi- 

 variable electro-magnetic lenses, termed the ments in hand. A specimen holder and 

 condenser and objective, which are mounted camera can now be placed over the x-ray 

 vertically above the electron gun. A v-shaped beam, an exposure made, and the plate de- 

 tungsten filament within the gun produces veloped. 



an electron beam which is accelerated at a The instrument has a variable kilovoltage 



selected kilovoltage. The objective lens is (5-30 kv); hence it is possible to select that 



fitted with a special polepiece which produces voltage which gives the optimum penetra- 



a strong magnetic field that focuses the elec- tion and contrast. The choice of the target 



trons onto a thin target of metal foil sup- and the operating kilovoltage, are dictated 



ported by a target assembly within the by the study proposed. The target is inter- 



polepiece. The target gives rise to a beam of changeable, and is simply punched out of a 



x-rays over which a fluorescent screen or thin metal foil, 3 to 12 microns thick. Tvuig- 



specimen holder and camera can be placed sten, gold, silver, and various other elements 



as required. The total height of the in- may be used depending on the characteritsic 



628 



