25G Transactions of the Society. 



XIV. — On Stephenson s System of Homogeneous Immersion for 

 Microscope Objectives.* 



By Professor E. Abbe, of Jena, Hon.F.E.^I.S. 



{Read 12th March, 1879.) 



The inventor of the Immersion method, Amici, with whose name so 

 many important improvements in the IMicroscope are connected, 

 attempted to use other fluids than water for the immersion medium. 

 Amongst others he tried the highly refractive oil of aniseed, pro- 

 bably from the idea that the advantage obtained by replacing the 

 stratum of air by a more refractive medium would increase with the 

 increase in the refractive indices of the media employed. More 

 recently others have used glycerine, and the well-known American 

 optician Spencer has, according to report, produced objectives by 

 this means of excellent quality. 



The theoretical analysis of the immersion principle shows, that 

 in several respects far more favourable results can be attained with 

 a highly refracting substance than with water : it proves, however, 

 at the same time, that the advantage to be expected is by no means 

 proportional with the progressive increase in the refractive index; 

 on the contrary, there is a maximum beyond which the results 

 become less favourable. When the cover-glass and the front lens 

 are of crown-glass, which is generally the case, this maximum 

 is reached when the immersion fluid has the same refractive index 

 as crown glass. A connection, which is optically homogeneous, is 

 then established between the preparation and the objective, which 

 eliminates all refraction in front of the first spherical surface of the 

 optical system. Not only is the loss of light by reflection obviated, 

 a loss which is experienced at every surface separating different 

 optical media when the incident rays are oblique, but what is still 

 more important, a very considerable amount of spherical aberration 

 is at the same time prevented which otherwise would have to be 

 corrected in the upper portion of the objective, but which must 

 leave a residuum. Apart therefore from other advantages, such 

 a method of " homogeneous immersion " gives promise at all events 

 of a more perfect elimination of spherical aberration, and conse- 

 quently more favourable conditions for what is called " definition " of 

 the objective, than water immersion. It also possesses the further 

 advantage, which is by no means inconsiderable, of getting rid of 

 the disturbing influence of the cover-glass and doing away entirely 

 with the otherwise indispensable correction. For where the inter- 

 vening medium is equal in its refraction and dispersion to the 

 cover-glass, it is immaterial, as regards the optical effect, whether a 

 * Translated by E. Woodall, Esq., F.R.M.S. 



