ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 131 



Origin of Homogeneous Immersion. — In describing a new 



diatom * {Navicula synedriformis), the Abbe Castracane mentioned 

 that he had made use of a homogeneous-immersion objective of Zeiss, 

 " the principle of the construction of which is due to the celebrated 

 professor G. B. Amici, but the realization to Professor Abbe." 



On this statement Professor Abbe writes to us as follows : — 



" My sincere estimation of the prominent merits of Amici — whom 

 I consider to be the very father of modern microscopical optics — need 

 not prevent my pointing out that it is incorrect to ascribe the homo- 

 geneous-immersion method to him. 



Amici, it is conceded, first applied oil immersion, but the use of 

 oil, by itself, does not constitute homogeneous immersion. 



Amici did not aim, and indeed at that time could not have aimed, 

 at the specific advantage of an immersion fluid being as near as 

 possible, in refractive and dispersive powers, to the crown glass. 

 Some of his oil lenses require, for good correction, a liquid of con- 

 siderably less and others a liquid of considerably higher refraction. 

 From what is known of Amici's oil lenses it is clear that he availed 

 himself of the different refractive powers of various oils and mixtures 

 of oils for obtaining the best correction of his lenses after he had 

 finished them, but did not direct his work to any definite refractive 

 index of immersion fluid prescribed previously, except perhaps in 

 favour of water immersion. This is so natural that it would be 

 unintelligible if Amici had proceeded otherwise ; he could not aim at 

 the peculiar optical benefits attendant upon the index 1 • 50 in com- 

 parison with 1 • 45 or 1 • 55, because it would have been utterly 

 impossible to utilize it practically at that period. This requires a 

 refinement of technical art which was not attained by the manufac- 

 turers of immersion objectives until a much later time. 



The essential fact, in the principle of homogeneous immersion, is 

 the increase of optical performance obtained from the total suppres- 

 sion of spherical aberration in front of an objective, and it was 

 Mr. J. W. 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 afibrd very 

 favourable conditions for further increase of aperture, and suggested 

 that the matter should receive an exhaustive theoretical and practical 

 investigation by Mr. Zeiss and myself. 



This suggestion, which had not been previously made (though it 

 is very self-evident now, as is always the case after a thing has been 

 done), is the true origin of the homogeneous-immersion method, and 

 the basis of the superior performance of objectives of this kind." 



The Essence of Homogeneous Immersion. — In a subsequent note 

 Professor Abbe gives the following further explanation of what is the 

 essence of homogeneous immersion from the optical point of view : — 



" The peculiar performance of the non-achromatic, approximately 

 hemispherical, front lens which is always used for wide-angled 

 systems (the invention of which by Amici is in my opinion the very 



* Accad. Pontif, de' Nuovi Lincei, xxxiii. Sess. II., 25 Gennaio, 1880. 



K 2 



