System of Homogeneous Immersion. By Prof. E. Abhe. 265 



exceptionally troublesome and delicate task. All these difficulties 

 of technical execution would, however, be considerably diminished 

 if the increase in the angular aperture were to some extent sacri- 

 ficed and we were content with a numerical aperture of 1 ' to 1 • 1, 

 which has hitherto been the ordinary aperture of immersion lenses. 



I must for the present leave undecided the question whether 

 the Stephenson immersion system might not prove of great 

 practical service even under such restrictions. Of course such 

 advantages would be surrendered as arise from the augmented 

 resolving power, since this is essentially determined by the magni- 

 tude of the aperture. But there are surely objects enough in the 

 domain of the microscopist, with respect to which a specially high 

 resolving power is of less moment than the greatest possible per- 

 i'ection of definition ; and the superiority of the homogeneous 

 immersion system on this point, and the great advantage which the 

 elimination of the disturbing effect of the cover -glass involves, 

 would be diminished only to a very limited extent with a reduced 

 angle of aperture. Assuming, therefore, that the nature of the im- 

 mersion fluid admits the frequent use of such lenses, especially in 

 biological researches, it might be desirable to try the system of 

 homogeneous immersion in objectives of more simple construction, 

 which would by their smaller cost be more generally used. 



In the other direction, however, the extent to which the new 

 immersion method will lead us has been by no means exhausted by 

 the new objectives. From the result of the first step it cannot be 

 doubted but that by this system considerably larger apertures of 

 moderately short focal length are still attainable, notwithstanding 

 the increasing difficulties of computation and construction. It 

 being unquestionably a matter of interest to extend the resolving 

 power of the instrument to its extreme limits by any means in our 

 power, even if the unavoidable refinements in such objectives 

 scarcely admit of their frequent application, the attempt has been 

 undertaken in the optical manufactory here. I hope soon to be 

 able to show objectives of 4-3 millimetres focal length, the nu- 

 merical aperture of which is increased to 1 • 35, corresponding to an 

 aperture angle of 128° in a medium with an index of 1 "50. This 

 figure, however, would be the extreme limit which can at present be 

 attained, unless cover-glasses of flint glass are used for the object, 

 and at the same time an immersion fluid of corresponding refrac- 

 tive index is applied. 



