PHOTOMICROGRAPHY AND TECHNICAL MICROSCOPY 



789 



photographs, distribution curves may be plotted on the basis of particle diameters in 

 fractions of a micron or in small units of mass. In some cases it is surprising how 

 little the older visual-light microscopes have revealed in comparison to the wealth of 

 detail developed with the high power of the ultraviolet microscope. 



Optical System. — The optical diagram of the ultraviolet microscope as arranged for 

 transmitted-light is shown in Fig. 17. The microscope parts are at the left and the 

 light-generating parts at the right. The system is designed to operate on wavelength 

 of 2750 A. (cadmium, 2748) from a high-tension spark generated across cadmium 

 electrodes. The light is spectroscopically decomposed into the slit images of the light 

 source. The slit images are not monochromatic in the strict sense of the word, but 

 each slit image is composed of a small group of lines which the monochromator fails to 

 separate as individuals. For practical purposes, however, the slit images of the 2750 

 and 2573 A. groups are monochromatic. Each group contains a strong line and one 

 or more weak lines. 



The slit images have been analyzed and their true spectroscopic nature determined. 

 Spectroscopic data are given in Table I. The image is photographed with the strong 

 line, before the weaker lines register. Some workers have used a strong magnesium 

 group at 2800 A. because of its greater brilliancy, but the resulting photographic image 

 suffers since a separate image is obtained from each of the strong lines in the group. 

 Photographic negatives taken with this group lack the crispness and brilliancy which 

 are characteristic of the cadmium groups at 2750 and 2573 A. While the optical 

 system is corrected for 2750 A., experience shows that it may be used with equal 

 facility with light at 2573 A. At the lower powers quite good photographs can be 

 secured with the magnesium group at 2800 A., or the cadmium group at 2300 A., but 

 at high powers and when the utmost in resolving power is required, it is necessary to 

 employ monochromatic light or its practical equivalent. For some purposes differen- 

 tiation in structure through selective absorption of the ultraviolet light is more impor- 

 tant than high resolving power, and in such cases the illumination can be adjusted to 



Table I. — Wavelengths Used with Ultraviolet Microscope 



