'O TEE MICROSCOPE. 



what less than that of a dry objective of 180*, and 

 that the aperture of the oil immersion exceeds thau 

 of the latter by 30% 



The advantage of immersion, in comparison with 

 dry objectives, is also at once apparent. Instead of 

 consisting merely in a diminution of the loss of light 

 by reflection or increased working distance, it is seen 

 that a wide-angled immersion objective has a larger 

 aperture than a dry objective of maximum angle, so 

 that for any of the purposes for which aperture is 

 essential an immersion must necessarily be preferred to 

 a dry objective. 



That pencils of identical angular extension but in 

 different media are different physically, will cease to 

 appear in any way paradoxical if we recall the simple 

 optical fact that rays, which in air are spread out 

 over the whole hemisphere, are in a medium of higher 

 refractive index such as oil compressed into a cone of 

 82 round the perpendicular, i.e., twice the critical 

 angle. A cone exceeding twice the critical angle of 

 the medium will therefore embrace a surplus of rays 

 which do not exist even in the hemisphere when the 

 object is in air. 



The whole aperture question, notwithstanding the in- 

 numerable perplexities with which it has hitherto been 

 surrounded, is in reality completely solved by these 

 two simple considerations : First, that " aperture " is 

 to be applied in its ordinary meaning as representing 

 the greater or less capacity of the objective for 

 receiving and transmitting rays ; and second, that 

 when so applied the aperture of an objective is 

 determined by the ratio between its opening and its 

 focal length ; the objective that utilizes the larger 

 back lens (or opening) relatively to its focal length 

 having necessarily the larger aperture. It would 

 hardly, therefore, serve any useful purpose if we were 

 here to discuss the various erroneous ideas that gave 

 rise to the contention that 180 in air must be the 

 maximum aperture. Amongst these was the sugges- 

 tion that the larger emergent beams of immersion 

 objectives were due to the fact that the immersion 



