66 PHOTOMICROGRAPHY 



As an example of the variation of working distance 

 with N.A., three lenses of approximately the same focal 

 length made by Zeiss may be instanced : 



The 4 mm. apochromat has a N.A. of 0-95 and a working 

 distance of 0-2 mm. 



The 4-3 mm. achromat has a N.A. of 0-85 and a working 

 distance of 0-4 mm. 



The 4-2 mm. achromat has a N.A. of 0-65 and a working 

 distance of 0-6 mm. 



N.A. and the Power of Objectives. Although an 

 objective is judged by the definition it gives, which 

 depends on the perfection of its corrections, on its resolv- 

 ing power, and on the brightness of the image formed, 

 a lens of high N.A. is not always the most satis- 

 factory. Penetration, flatness of field, and working dis- 

 tance must all be taken into account, and the special 

 requirements considered ; and for many purposes it is 

 best to make a compromise and select one with a 

 moderate N.A. 



N.A. of Low- Power Objectives. In low-power work the 

 chief considerations are usually flatness of field and 

 penetration. Resolution may generally be neglected, 

 so a low N.A. is all that is required. 



N.A. of Medium- Power Objectives. -The selection of an 

 aperture suitable to the work in hand is of considerable 

 importance with medium-power objectives. To take the 

 Zeiss 4 mm. objectives given above, as examples, the 

 resolving power of the apochromat and of the achromat 

 of N.A. 0-85 are as great as can be desired with dry 

 lenses of each class, but their penetration and working 

 distance are notably less than those of the achromat 

 of N.A. 0-65, and it is well to sacrifice some of the 

 resolving power and be content with a N.A. of 0-65 when 

 working distance or penetration is important. 



N.A. of High- Power Objectives. The maximum resolu- 

 tion possible must be achieved by high-power objectives, 



