PHOTOMICROGRAPHY AND TECHNICAL MICROSCOPY 



787 



slightly more than the depth of penetration of the system, optical sectioning was 

 obtained. 



In developing the art of optical sectioning, experimental malignant tumor material 

 was used. At first this material was fixed and stained, but soon it was discovered that 

 staining of tissues was unnecessary and that better results were secured with unstained 

 material. Differentiation of structure resulted by selective absorption of the ultra- 

 violet light. The fact that organic material manifested selective absorption paved 

 the way to the study of living material and the elimination of changes in the specimen 



Fig. 16. — A three-dimensional picture of manganese steel, water toughened and drawn at 

 750°C. (Magnification, 3500 X.) 



(artifacts) incident to fixation, staining, and mounting. Thus it became possible to 

 photograph living cells in isotonic salt solutions most of which readily transmit ultra- 

 violet light. Further experimental work demonstrated that most living cells were 

 not affected by light of wavelength of 2750 or 2573 A. of intensity and duration of 

 exposure ordinarily required in ultraviolet microscopj\ The rule, however, is not 

 imiversal in its application, as some few living organisms are almost immediately 

 disintegrated and others soon shrivel and die. Most cells are immune to long 

 exposures at 2750 and 2573 A., but at shorter wavelengths such as those of the 2300 A 

 group and for still shorter wavelengths, the lethal action is verj^ rapid. 



