ULTRAVIOLET MICROSCOPY 



Fig. 7. An object consisting of a coarse grid, 

 one rectangle of which measures 1.43 mm by 1.72 

 mm, within which there is a fine mesh with 4 lines 

 per mm, photographed through a pinhole whose 

 aperture was chosen to produce the optimum reso- 

 lution for the wavelength and distances involved. 

 Only the coarse grid is resolved. Compare this with 

 Fig. 8. 



be w/2 since all the zones have equal areas. 

 For our zone plate, n/2 = 19. The actual 

 exposure time used for the pinhole was 40 

 times greater than that required for the 

 zone plate. Since the densities of the plates 

 were compared only by eye the experimental 

 values do not seem out of line. 



Next we compared the resolution of our 

 zone plate with that of a lens of similar focal 

 length, of aperture equivalent to that of the 

 zone plate. For both the lens and the zone 

 plate / = 10.2 cm and D = 0.26 cm. Fig. 9 

 (right) shows the image of a screen with 4 

 lines per mm photographed with the lens, 

 and Fig. 9 (left) shows the same screen 

 photographed with the zone plate. Adja- 

 cent centers of the object screen subtended 

 an angle equal to 2.6 d^.i^ ■ Other constants 

 were p = 47 cm, g = 13 cm, and X = 4358 

 A. The lens had a simple plano-convex form. 

 If a highly corrected camera lens had been 



chosen the comparison might have favored 

 the lens, but since it was stopped down to 

 //1 8 and the field covered was not very 

 large, it was not subjected to undue demands 



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Fig. 8. The object of Fig. 7 photographed 

 through a zone plate, with all other factors fixed 

 except the exposure. The exposure time required 

 for the pinhole picture was about 40 times greater 

 than that used for the zone plate picture. 



Fig. 9. Comparison of a lens and a zone plate 

 as image forming devices near the limit of resolu- 

 tion. On the left is a picture of a fine mesh screen 

 taken with a zone plate. On the right is a picture of 

 the same screen taken with a plano-convex lens of 

 similar aperture and focal length. All other factors 

 were kept the same except the exposure which was 

 20 times greater for the zone plate. The wavelength 

 used was 4358 A. If the exposure comparison had 

 been made at 1000 A the results would have been 

 much more favorable for the zone plate. 



558 



