Illumination and Illuminating Apparatus 85 



the use of a dense medium enveloping the object and the 

 front lens of the objective, as we have seen by the descrip- 

 tion of immersion objectives. The oil or other medium 

 employed shortens the wave-length of the light used, 

 whatever its colour, and when we use a light of shorter 

 wave-length than it would have when passing through 

 air, we increase the effective aperture of the objective. 

 Accordingly the resolving power of a lens is increased 

 by shortening the wave-length of the light admitted to 

 it, and this is accomplished in either of two ways (1) 

 By employing blue instead of white light, or (2) by con- 

 verting the lens into an immersion lens, and interposing 

 a layer of oil instead of air between it and the object. For 

 instance, if a microscopic objective were used with white 

 light, and its limit of power to resolve fine structure were 

 50,133 lines per inch with such illumination, its limit would 

 be 54,342 lines per inch with monochromatic blue light 

 (lineF). 



A natural conclusion from these statements would be 

 that the farther towards the violet the monochromatic 

 light were used, the more marked would be the results- 

 obtained ; but although this is correct theoretically, it is- 

 not true practically. Microscopic objectives are corrected 

 for visual purposes for use with the brightest rays of white 

 light, and if the extreme ends of the spectrum were 

 employed the objective not being calculated for these 

 rise would be given to spherical aberration, even in the 

 best objectives, preventing the accomplishment of good 

 work. If a lens were corrected for spherical aberration 

 when used with light from the extreme blue end of the 

 spectrum, under existing conditions of manufacture it would 

 work at its best with the light for which it was designed, 

 and if light lower down in the spectrum were employed, 

 spherical aberration would be apparent. It must be borne 

 in mind that light of extremely short wave-length is sensibly 

 absorbed by glass, also the eyesight is not keen in extreme 

 blue and violet lights, consequently the range of light that 



