ABBE'S CONSTRUCTION OF FIRST HOMOGENEOUS OBJECTIVE 29 



follows that the correction collar, though still a refinement and aid 

 in the attainment of the finest critical images, would be a necessity 

 no more. 



The desirability of the construction of a combination of lenses 

 which would satisfy these conditions was urged by Mr. Stephenson 

 upon Professor Abbe, and he secured the profound knowledge, 

 which, as a mathematical optician he possessed, for the complete and 

 practical solution of the problems involved, and the production of a 

 remarkable series of lenses, marking a distinct epoch in the progress 

 of theoretical and practical optics. 



He had, in fact, as we have hinted, already approached the con- 

 sideration of the subject from another point of view, believing that 

 petrographic work the study of thin sections of mineral substances 

 could be far more efficiently accomplished by the use of homo- 

 geneous lenses. But in the new aspect in which the problem was 

 presented by Mr. Stephenson it carried with it new interest to Abbe, 

 not only as promising to largely dispense with the ' correction collar,' 

 but also io greatly enlarge the ' numerical aperture,' and therefore 

 secure a greater resolving power in the objective. 



One of the difficulties was to find a suitable fluid to meet the 

 necessities as to refraction and dispersion. But after a long series 

 of experiments Professor Abbe found that oil of cedar wood so 

 nearly corresponds with crown glass in these respects that it served 

 the purpose w^ell. 



The result of Abbe's calculations based on Mr. Stephenson's sug- 

 gestion was the construction by Carl Zeiss of a T Vth with a N.A. 1 

 of 1*25 of fine quality, and still higher promise, and subsequently 

 of a ^th and a T Vth in. objective of a like character. 



It may be well to note that Amici suggested the use of oil 

 instead of water prior to 1850, and Mr. Wenham again revived 

 the suggestion in 1870. 2 But neither of these is in even a remote 

 sense an anticipation of the ' homogeneous system ' of lenses as we 

 now understand it. The 'oil immersion' in both instances was an 

 expedient. The principle 011 which the construction carried out by 

 Professor Abbe depended was the ' optical ' principle that a medium of 

 high refractive power gives an aperture greatly in excess of the 

 maximum (180) of a dry lens ; while Abbe's explanation, propounded 

 in 1874, of the important bearing which the diffraction pencils have 

 on the formation of the microscopic image makes the resolving 

 power of the object-glass dependent upon the diffraction pencils that 

 are taken up by it. 



All this was unknown or unadmitted by those who had previously 

 suggested oil as an immersion medium, which leaves the homogeneous 

 system as now employed wholly dependent upon the principles 

 enunciated by Abbe, arising from the practical suggestion of Stephen- 

 son and resulting in the beautiful object-glasses of Abbe and Zeiss, 

 although it is best just to remember that Tolles always maintained 

 that his immersion objectives had a greater aperture than 180 air 



1 The meaning of this expression will be found on p. 49, but the whole of Chap. II. 

 must be carefully read. 



2 Monthly Micro. Jonrn. vol. iii. p. 303. 



