232 



PROGRESS IN MICROSCOPY 



light, it is equally well focused for a spectral line in the ultra-violet. 

 Furthermore, the spherical aberration of the arrangement is well 

 corrected both in visible and ultra-violet light. Lastly, owing to the 



Fig. 9.4. Johnson reflecting objective. 



45"-slanted plate G, the light-beams are not occluded. B. K. Johnson's 

 system acts as a spherical mirror whose aberrations are corrected by 

 a correcting lens. 



Bouwers has also used correcting meniscus lenses the study of 

 which was carried out by D. D. Maksutov (Fig. 9.5). The light from 



Fjg. 9.5. Reflecting objective with correcting lens L (D. D. Maksutovj. 



the object A is first reflected by the mirrors /;; and M, then 

 passes through the correcting lens L (meniscus) and ends in the 

 image A' . 



D. S. Grey has described Schwarzschild-type objectives comprising 

 many refracting elements. The objective shown in Fig. 9.6 is cor- 

 rected from 2200 A up to the lower infra-red range, it has a N. A. 1 • I 

 and an occlusion ratio of approximately 26. The refracting elements 

 between the object and the mirror m enable one to increase the N.A. 

 while those between M and m increase the magnifying power and 

 provide improved correction. 



