RESOLVING POWER 



63 



gives an approximation to the depth of focus of an 

 objective. 



LIMITING RESOLUTION 



The highest resolution as yet obtained is achieved by 

 the use of ultra-violet light of a wave length of 2750 or 

 2800 A.U. But the special properties of these rays, 

 involving the use of quartz lenses instead of glass, which 

 is opaque to ultra-violet rays, make the process so 

 expensive, that a description of the apparatus, which is 

 made by Messrs. Zeiss, is not given in this book. 



Brightness of Image, This property requires no further 

 explanation, but it is of importance, as it governs ex- 

 posure. The superiority in this respect of apochromatic 

 and fluorite lenses over achromats is very marked, 

 and is a great point in their favour, particularly with 

 high powers which need a very good light or long 

 exposure. 



Penetration is the power of an objective to bring points 

 in various planes to a sufficiently exact focus at the same 

 time, and is of importance when photographing an 

 object of any thickness. It decreases as the square of the 

 equivalent focal length, the numerical apertures being 

 the same. Thus, a 4 mm. objective has four times 

 the depth of focus of a 2 mm. of the same numerical 

 aperture. 



