306 SUMMARY OF CURKENT RESEARCHES RELATING TO 



you take a dry objective and give it a different back lens, and put 

 a drop of water or oil between the front lens and the object, it will 

 have as large an aperture as an immersion objective " ! 



Dry and immersion objectives do undoubtedly differ in their 

 construction, but the same objective may be used at one time as a 

 true dry objective and at another as a true immersion objective 

 without any alteration in its lenses, so that differences of construction 

 do not constitute what logicians would call the " specific differentia- 

 tion " between the two kinds of objectives. When we speak of a dry 

 or air objective, we mean essentially an objective which is used with 

 a film of air intervening at some point hetween the object and the first 

 surface of the objective. (The object, therefore, being either in air 

 or mounted in balsam or other fluid with air above.) While by an 

 immersion objective we mean essentially one in which no film of air 

 so intervenes, but the ichole space between the object and the first surface 

 of the objective is occupied by a substance whose refractive index is 

 greater than air. (A condition which of course implies that the 

 object itself must be immersed in fluid or closely adhering to the 

 cover-glass.) 



As the cardinal point of the angular theory is that a dry objective 

 of 180^ angular aperture (used on an object in air) represents the 

 maximum aperture that is possible, theoretically or practically, it is 

 desirable to appreciate at the outset that it is possible to have a dry 

 objective of angular aperture very closely approaching 180°. When 

 a comparison is made between a wide-angled immersion objective and 

 a dry objective of nearly 180° angular aperture, it is objected* that no 

 such objective can exist, for it is impossible to bring the surface 

 of the lens in close contact with the object, or that even if we could, 

 there " woidd be no working distance and no possible adjustment to 

 suit varying sights." 



But a homogeneous-immersion objective used with an object which 

 is in air and close (but not adhering) to the cover-glass, as shown 



in Fig. 50, is a dry 

 Fig. 50. objective, for a film of 



\ / air is interposed above 



\ / the object. Further, 



\ / by reason of the in- 



tervention of the im- 

 mersion fluid between 

 the front surface of 

 the first lens and the 

 cover-glass, the under 

 side of the cover-glass 

 has become in effect the front surface of the objective ; the object may 

 be close to this front surface, and there is of course full capability of 

 adjustment for different sights by increasing or lessening the distance 

 between the objective and the cover-glass. We have therefore a dry 

 objective of angular aperture closely approximating to 180°, and with 

 very slight spherical aberration in consequence of the exceeding 



* See this Jouiual, iii, (18S0) p. 1090. 



Y' — n/ front lens 



J\^ /) IMMERSION FLUID 

 OBJECT IN AIR \ *" "y^^ j COVER CLASS 

 SLIDE t'" "- "^ "■ ■ J 



