ULTRAVIOLET MICKOSCOPY 



Fig. 4. The layout of the apparatus on the optical bench. The light source consisted of a tungsten 

 filament lamp and a condensing lens system. For the experiments at 2537 A the source was a 4-watt 

 General Electric germicidal lamp, No. GT4/1. The filters are described in the text. The zone plate was 

 mounted at the open end of a bellows extension. The camera was an Exakta VX Ila. Eastman Kodak 

 Microfile Contrast Copy film was used throughout, processed in D 19. Object and image distances are 

 labeled p and q, respectively. 



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Fig. 5. Pictures of a mesh with 4 lines per mm 

 taken with the zone plate at three different wave- 

 lengths. Working out from the center we see pic- 

 tures taken at 6700 A, 4358 A, and 2537 A, in sizes 

 proportional to the original image sizes obtained 

 with a fixed object-to-zone plate distance of 47 cm. 

 The shorter wavelength results not only in a longer 

 focal length and hence a larger image size but also 

 in improved resolution. See also Fig. 6. 



relied on the strength of a spectral line, as 

 in the case of the 2537 A source, to produce 

 approximately monochromatic radiation. 

 The object distance, p, and the image dis- 

 tance, q, (Fig. 4) were chosen to exhibit the 

 improvement in resolution that accompanies 

 the use of shorter wavelengths. We operated 

 in a region close to the limit of resolution. 

 The parameters were purposely chosen to 



exhibit the improvement in resolution from 

 red light to ultraviolet. For example, in 

 Fig. 5, three pictures of the same object 

 mesh made with the zone plate are shown 

 superimposed on one another to exhibit the 

 ratio of the sizes in which the original pic- 

 tures appeared when taken with red (6700 

 A), blue (4358 A) and ultraviolet (2537 A) 

 light. The increase in image size results from 

 the fact that the focal length of the zone 

 plate increases as the wavelength decreases, 

 while the object distance, p, remains fixed 

 at 47 cm. This increase in image size alone, 

 can bring about an improvement in signal- 

 to-background ratio, but notice that an 

 improvement in resolution has also taken 

 place. The ultraviolet picture is not only the 

 largest; it is the best resolved because it was 

 made at the shortest wavelength. 



To exhibit this improvement in resolution 

 apart from the change in magnification, the 

 pictures of Fig. 5 were enlarged photo- 

 graphically so that the distance between 

 squares remained constant. The improved 

 resolution becomes clearer. In Fig. 6 (left), 

 p equaled 47 cm and q was adjusted to 7.2 

 cm, a value that produced the best focus 

 experimentally for a wavelength of 6700 A. 

 A gelatin filter was used with a pass band 

 of about 400 A. The object mesh had 4 lines 

 per mm. The angle subtended at the zone 

 plate by two successive centers of open mesh 



556 



