1C6 



NUMERICAL APERTURE. 



water immersion. That is, the angular aperture ought to be 164^ 

 for a dry lens, or 96" for a water lens. Now for the power. Say 

 the eye-piece magnifies 10 times, and you want it resolved 100 

 lines to the inch, divide by 1,000, and you get 



=99 or — inch will answer. 



/ 10 



I trust the above will give sufficient of the information 



required. I have dwelt at length on refraction, as it seems to me 



that it is there that so much misconception has crept in : it being 



so difficult to see why different angles in different media may mean 



one and the same thing. Diffraction I have only mentioned, as 



it would lengthen this article inordinately, and has also nothing to 



say to the measurement of aperture. 



Table of Natui^al Sines to every 5''. 



