Techniques of AnAtomy — roofe And lesher 337 



cancer of various origins and other diseases. Human patients, in a 

 few centers of America, are also being studied intensively. 



PHASE MICROSCOPY 



Phase microscopy is used to observe materials that are too trans- 

 parent for ordinary bright-field study. Specimens usually include 

 such objects as living organisms, unstained tissue, slightly pigmented 

 or faded preparations, emulsions, and plastics. 



The American Optical Co. offers the following brief explanation 

 of the theory of phase microscopy : 



In the phase microscope a diffraction plate or coating is added within the 

 objective and an annular diaphragm below the condenser of the bright-field 

 microscope, thereby converting slight and invisible alterations of light passing 

 through the specimen into images which may be seen and photographed. The 

 annulus in the condenser controls the illumination on the diffraction plate or 

 coating, where the light from the specimen and its surround is selectively 

 modified so that it will recombine into an image of adequate visibility. The 

 range of phase objectives makes possible several degrees of contrast, with or 

 without reversing of bright and dark details, to provide best visibility, to empha- 

 size detail, and to insure that no detail is missed in the microscopic examination 

 of such materials. All objectives have incorporated into them the latest develop- 

 ments in this relatively new field. 



THE ELECTRON MICROSCOPE 



Tlie electron microscope has so far only limited application in 

 biology. This limitation is brought about by the fact that the material 

 studied has to be in very highly desiccated condition and the thin- 

 ness of the material is of an extremely small order. One cannot 

 observe material that is 2 or 3 microns in thickness. It has to be 0.2 

 or 0.3 /x, in thickness in order that the electron microscope can bring 

 out in detail structures that could not otherwise be seen. Its greatest 

 use eventually, and probably even now, is that it can deal with large 

 protein molecules within certain fibrillae of protoplasmic strands. 

 However, it may be that new inventions or new techniques in electron 

 microscopy will render this instrument more useful to biology. The 

 instrument does not use ordinary light source, but depends upon 

 X-rays to bring out the objects observed. Instead of glass or quartz 

 lenses, the lenses of the electron microscope are large electromagnets 

 which may constrict or diffuse the X-rays as an ordinary microscope 

 does light. 



TISSUE CULTURE 



Tissue culture has given us considerable advantage in studying 

 living systems. Under this technique, cells or tissues may be removed 

 from the living body and kept alive in a medium adequately supplied 



