RESOLVING POWER 



153 



object, the greater the resolving power, hence, the value 

 of the oil-immersion system the refractive index of air 

 being 1-0, of water 1-33, and of oil 1-515 ; (2) the shorter 

 the wave-length of the light employed in illumination, 

 the greater the resolution of a given lens. Hence, if 

 monochromatic light towards the violet end of the 

 spectrum be used instead of ordinary white light, resolu- 

 tion is increased. Photographically, rays far beyond the 



ir 





FIG. 25. Diagram to illustrate the course of rays of light through an 

 objective. 



visible spectrum, ultra-violet rays, may be used, and 

 resolution can be almost doubled thereby. But the 

 result can only be registered on a photographic plate, 

 as such rays are invisible to the eye, and special optical 

 systems of quartz, which alone is transparent to the ultra- 

 violet rays, must be employed ; and (3) within certain 

 limits, the greater the resolving power, the greater must 

 be the magnifying power of the lens in order to utilise it. 

 Hence, for the lower-power lenses it is no use having too 

 great a N.A. Conversely, a certain magnifying power 

 having been reached, it is no use increasing the magnifica- 



