786 HANDBOOK OF PHOTOGRAPHY 



To visualize the image, a fluorescent screen is employed. This takes the form of a 

 uranium glass wedge on which the image is received. The image is viewed by a small 

 magnifier and is focused in the ordinary way by racking the tube of the microscope 

 upward and downward. In principle this seems rather simple, but in practice it did 

 not work out very well because operators had difficulty in coordinating a system with 

 light which could not be seen. In the first place, the intensity of the light is low, 

 and the image is very small on the uranium glass wedge. Fluorescence in uranium 

 glass is not a surface effect entirely, and this made it difficult to focus the instrument. 



Beck and Barnard of London attempted to get around the focusing trouble by 

 means of an interferometer focusing arrangement which thej^ incorporated in an 

 ultraviolet microscope of their design. Kohler in recent years has developed a more 

 powerful light source and has improved the searcher eyepiece or focusing device, but in 

 principle and in application it is essentially the same as the original. The fact 

 remains that the original apparatus, as available in 1900, was a workable precision 

 system although its successful use was delayed a quarter of a century. 



More powerful illumination for inert material such as finely divided mineral 

 matter, for opaque metallurgical specimens, or for preparations which are quite 

 absorbent at 2750 A. unquestionably wovild be of great benefit. However, organic 

 and living material especially may be affected by exposure to a stronger source of 

 energy. This is something which will have to be determined but the indications are 

 that, while man}'^ living cells may be photographed successfully with the present 

 intensity of light, there is no assurance that disintegration may not occur if the 

 intensity of the light source is much increased. 



In this countr}' Bausch & Lomb have developed a simple ultraviolet system on an 

 entirely different system. They have corrected objectives for two wavelengths, one 

 in the visible region of the spectrum and one in the near ultraviolet. The preparation 

 is focused in the visible light and photographed in the ultraviolet. The source of 

 illumination is a mercury-vapor lamp, and separation of wavelengths is accomplished 

 by filters. Since the wavelength used in the ultraviolet is the 3650 A line, optical 

 glass may be employed instead of quartz. 



The Kohler system, of which the Beck system is essentially but a modification, 

 is without doubt the most powerful microscope ever devised. Its highest power 

 objective has a numerical aperture of 1.25, but owing to the short wavelength of light 

 used its effectiveness is equivalent to an objective with a numerical aperture of at least 

 2.50 when used with visible blue light. However, for many purposes this system has 

 advantages over those using visible light which scarcely can be reckoned in terms of 

 numerical aperture. 



The ultraviolet microscope was intended originally for biological research, but 

 inability to use it successfully all but caused its complete abandonment. About 1925 

 it was redesigned as a metallurgical microscope and was provided with a quartz plate 

 vertical illuminator. At best these illuminators are not efficient, and with the rather 

 weak source of ultraviolet light a,vailable it was practically impossible to do much 

 with the instrument in the field of metallography. The metal specimens absorbed 

 practically all the light and thus very seriously complicated the problems of focusing 

 and photography. 



A great deal of experimental work was done with the equipment in an attempt to 

 improve conditions but without much avail. It was discovered, however, that the 

 system had inappreciable depth of focus, and it was reasoned that, if a transparent 

 biological specimen or one approximately so was substituted for the opaque metallur- 

 gical specimen, the ultraviolet light could be transmitted through the specimen from 

 below in the usual way and that it should then be possible to photograph the specimen 

 on successive planes. By spacing the planes from top to bottom of the specimen 



