PHOTOMICROGRAPHY AND TECHNICAL MICROSCOPY 797 



are not easy to remove, presumably because they become electrified by brushing and 

 merely move about. However, by carefully brushing the plates in one direction with 

 a camel's-hair brush, they can be made dustfree. It is best to avoid the use of all 

 plates which are poorly packed. 



Preparation of Specimens. — For ultraviolet microscopy specimens are prepared 

 somewhat differently than for visible-light systems. Since ordinary glass is wholly 

 opaque to ultraviolet light of short wavelengths, special quartz slides and cover slips 

 must be used. Some of the special ultraviolet-transmitting glasses may serve but 

 these lack the full transmitting qualities of quartz and consequently reduce the effi- 

 ciency, lengthen the exposure, and make the problem of focusing more difficult. 

 Slides and cover slips of these materials are relatively cheap compared with quartz 

 slides and covers, but, since the latter may be cleaned and used repeatedly, there is 

 little point in working with the glass slides. The quartz slides measure 26 by 30 mm., 

 and the cover slips are 12 mm. in diameter. The slides are placed in a small metal 

 holder of the same size as an ordinary glass slide, and thus are convenient to handle. 



Usually, for examination with visible-light microscopes, tissue is fixed, sectioned, 

 stained, and mounted on glass slides. Many of the biological fixatives contain sub- 

 stances which are totally opaque to ultraviolet light or by combination with the tissue 

 render the latter opaque. Other fixatives, such as formaldehyde, are satisfactory, but 

 they often have serious limitations for cytological studies. Practically all common 

 stains absorb ultraviolet light. Differentiation of structure will result through 

 selective absorption of the ultraviolet light. 



For ultraviolet microscopy of tissue the best practice, as previouslj^ pointed out, 

 is to use living material mounted in a suitable isotonic salt solution. The cover slip 

 is sealed with a mixture of vaseline and olive oil applied by a small camel's-hair brush 

 while revolving the slide on a turn table. This sealing medium has no effect on the 

 preparation, whereas lacquer and other similar mediums may infiltrate the mounting 

 medium and have harmful effects. 



Other mounting mediums, such as glycerin, mineral oil, olive oil, castor oil, water, 

 and some of the synthetic resins and gums, may be used for special purposes with 

 technological preparations. Most of the natural gums, balsams, etc., are opaque to 

 ultraviolet light and therefore not suitable for use. 



Bibliography 



Periodicals: 



KoHLEE, A.: Microphotographic Examinations with Ultraviolet Light, Z. Wis. Mikroskopie, 21, 129-165 



and 273-304 (1904). 

 KoHLEB, A., and M. Von Rohh: A Microphotographic Arrangement for Ultraviolet Light, Zeit. 



Instrumentenk. 24, 341 (1904). 

 Lucas, F. F.: Photomicrography and Technical Microscopy, Bell System Tech. J., 3 (No. 1) (1924). 



: High Power Metallography, J. Franklin Inst., 201, February (1926). 



: An Introduction to Ultra Violet Metallography, Mining and Met., June (1926). 



— : Microtome Methods for the Preparation of Soft Metals for Microscopic Examination, Mining 



and Met, February (1927). 

 : Observations on the Microstructure of the Path of Fatigue Failure in Specimen of Armco Iron, 



Trans. Am. Soc. Steel Treating, 11, April (1927). 

 : A R6sum6 of the Development and Application of High Power Metallography and the Ultra 



Violet Microscope, Proc. Intern. Congr. Testing Materials (Amsterdam, Holland), September (1927). 

 : Photomicrography and Its Application to Mechanical Engineering, Mech. Eng., 50, March 



(1928). 

 : Further Observations on the Microstructure of Martensite, Trans. Am. Soc. Steel Treating, 15, 



February (1929). 

 : Structure and Nature of Troostite, Bell System Tech. J., 9, January (1930); also in Proc. World 



Eng. Congr. Tokyo. 



: The Architecture of Living Cells, Proc. Nat. Acad. Sci. U. S., 16, September (1930). 



: On the Art of Metallography, Mining and Met., May (1931). 



