PHOTOMICROGRAPHY AND TECHNICAL MICROSCOPY 783 



completely replaced by finer ones. From the 400 paper, the specimen goes to a hori- 

 zontal lap wheel covered with broadcloth and charged with 600 alundum powder where 

 the polishing continues until the scratches are removed. Final polishing is carried out 

 on another lap wheel covered with a very fine broadcloth and charged with a mag- 

 nesium oxide paste. The paste is made by mixing magnesium oxide with water or 

 with a mixture of water and glycerin. Magnesium oxide forms carbonates readily 

 when in contact with air and water, and because of the difficulty encountered in secur- 

 ing uniformly good powder it is advisable to prepare the powder by air floatation. 



It is not within the scope of these brief notes to give complete details for preparing 

 specimens, but the successful photography of the specimen depends to a large extent 

 on the care with which the surface has been prepared. For that reason it has been 

 deemed advisable to give some indication of recommended procedure. More detailed 



Fig. 13. — Bench and horizontal laps for polishing metal specimens. 



accounts and modifications of the procedure will be found in the literature, to which 

 reference should be made. 



The Ultraviolet Microscope. Development of the Ultraviolet Microscope. — About 

 1900 Kohler of the Zeiss Works developed a microscope which had about twice the 

 resolving power of other systems. Its superior resolving power was due to the use of 

 ultraviolet light. Prior to that time onlj^ visible light had been employed to illuminate 

 microscopic preparations. To take advantage of the greater resolving power through 

 the use of shorter wavelengths, Kohler devised an apparatus which operated at a 

 wavelength of 2750 A. Since glass is opaque to wavelengths in this region of the 

 spectrum, all optical parts were made of quartz. The system is monochromatic 

 because optical materials of suitable refractive and dispersive powers are not available 

 with which to achromatize objectives for a range of wavelengths in the ultraviolet 

 region. Chromatic aberrations are eliminated by confining the system to a single 

 wavelength of light. Spherical aberrations are corrected by virtue of the curves to 

 which the lens elements are ground. Monochromatic light is secured bj'^ means of a 

 suitable monochromator. 



