162 Transactions of the Socidij. 



The " Homogeneous Immersion " principle. 



The utility of homogeneous-immersion objectives being estab- 

 lished beyond doubt by practical experience, it is interesting to note 

 that the origin of the principle is very fully recognized by Professor 

 Abbe to be due to our esteemed Fellow Mr. J. W. Stephenson. 



The two essential points in homogeneous immersion are, 1st, 

 thfi increase in aperture obtained by the use of a fluid of high 

 refractive index and, 2nd, the enhanced optical performance arising 

 from the total suppression of spherical aberration in front of the 

 objective. Professor Abbe states that although Amici first applied 

 oil immersion, he failed to recognize the specific advantage of an 

 immersion fluid being as near as possible in refractive and dis- 

 persive powers to the crown glass (i. e. " homogeneous "). He 

 finished his lenses and then sought for oils and mixtures of oils of 

 various refractive powers for obtaining the best correction. " It 

 ■was Mr. Stephenson who, in his first communications with me, 

 expressed the opinion that doing away with the anterior aberration 

 would improve the defining power, and especially would afi'ord very 

 favourable conditions for further increase of aperture." 



The importance of this system will be appreciated when we 

 remember, in regard to the first point (the increase of aperture), 

 that the theoretical resolving power of an objective is thereby raised 

 from 5^)6,400 lines to an inch, which is the maximum of a dry 

 objective, to 14(5,528 the maximum of an oil-immersion objective, 

 the illuminating power being also increased from 1 to 2 • 25 : while 

 as regards the second point, we are able by the homogeneous-im- 

 mersion method to reduce the problem of correcting a very wide- 

 angled objective to the much less difficult one of correcting an 

 objective of moderate air angle. Our lamented President, the 

 Rev. J. B, Reade, declared in 1870 that " the ghost of aberration 

 will never be entirely exorcised even by cold water." But there 

 appears to be good ground for believing that oil has practically 

 accomplished that object. 



During the past year several kinds of fluids for homogeneous 

 immersion have been brought before the Society, such as chloral 

 hydrate and glycerine, iodide of zinc and glycerine, and gum 

 dammar and cedar-oil. Two other vegetable products have also 

 reached us, " tacamaque " and the gum-resin " oliban," or " in- 

 cense," both dissolved in cedar-oil. While the dammar is claimed 

 to be unchangeable, and to be in refractive and dispersive powers 

 very near that ideal of a good immersion medium, " fluid crown 

 glass," there is evidently room for further research in this direction, 

 particularly for a fluid which will not attack the various varnishes 

 in ordinary use. 



Lastly must be noted an important advance in practical manu- 



