INFRA-RED AND ULTRA-VIOLET MICROSCOPY 



231 



ratio and long working distance (Fig, 9.3), designed for ultra-violet 

 microspectroscopy. 



The rays issuing from the object A are reflected by the flat mirror /;?! 

 on to which they revert after being reflected by the concave mirror M. 

 Next, they are reflected once more by the convex mirror m.^ and end 



I ',;,v,,,',, tur 



A p 



Fig. 9.3. Thornburg reflecting objective. 



in the image A'. The residual spherical aberration can be corrected 

 by altering the shape of the surface nii . The N.A. of this objective 

 is 0-77 (dry front lens), its occlusion ratio 034 and magnifying 

 power X 100. 



Objectives which are both reflecting and refracting have also been 

 devised. Such arrangements comprise one or two mirrors added to 

 refracting elements which provide correction of aberrations without 

 having recourse to non-spherical surfaces. They usually comprise 

 a fairly high number of air-glass surfaces which may promote origi- 

 nation of stray light but, against this, they have a low occlusion 

 ratio and an extensive field. Using a quartz meniscus one face 

 of which is aluminized, B. K. Johnson evolved a high N.A. objective 

 (Fig. 9.4). The rays from the object A pass through the lens A (fused 

 quartz) and are reflected on the 45°-slanted plate G. After passing 

 through the lens 4, they are reflected from its aluminized back to end 

 in image A'. The back of G should be coated in order to prevent 

 a double-image. As shown in Fig. 9.4, the N.A. of the objective 

 is 0-84 which can be increased to 1-27 by adding a meniscus lens 

 above /j. When the objective is focused for a spectral line in visible 



16 



