System of Homogeneous Immersion. By Prof. E. Abbe. 257 



thicker layer of glass and a corresponding thinner layer of the fluid, 

 or vice versa, is inserted between the object and the objective. 



The idea of realizing the various advantages of such a kind of 

 immersion, by constructing objectives on this system, had for some 

 time presented itself to my mind, but I thought that there was not 

 much to be expected, as regards the scientific usefulness of such 

 objectives, as I believed their use would be limited on account of 

 the necessity of using oil or some other inconvenient material 

 as the immersion fluid. It appeared to me that, except perhaps 

 for the examination of diatoms, scarcely any other scientific sphere 

 remained than petrographic research, which would afibrd scope for 

 realizing the optical advantages of such objectives. 



The matter assumed, however, subsequently a different shape 

 in consequence of a suggestion made by Mr. John Ware Stephenson 

 (the Treasurer of the Eoyal Microscopical Society of London), who 

 independently discovered the principle of Homogeneous Immersion,* 

 but by whom, in addition to its other advantages, special attention 

 was drawn to the doing away with the cover-glass correction, and 

 to the possible enlargement of the angular aperture, with conse- 

 quent increase in the resolving power of the objective. This idea 

 of Mr. Stephenson, which made the matter one of universal scien- 

 tific interest, was at once followed out, the calculations being made 

 by me, and the technical execution by Mr. Zeiss, and resulted in 

 the production of a series of objectives on this system which in 

 several respects are manifestly superior to the ordinary water-im- 

 mersion objectives. Having now been used by a number of micro- 

 scopists, it has been found, that although the nature of the peculiar 

 immersion fluid will naturally much restrict the employment of 

 such objectives, it does not present any obstacle to their use in 

 various widely different spheres of microscopic research ; and in 

 particular, biology furnishes many problems to which the new 

 lenses may render useful service. 



Since the construction, about a year ago, of the first objectives 

 on this system, the focus being ^" nominal (more exactly 2 ■ 6 mm. 

 equivalent focus), and all of them calculated for the long tubes of 

 the English Microscopes, some have been made of y^" (1 '8 mm.), 

 which give suflScient magnifying power, even with the shorter 

 tubes of the continental instruments ; and quite recently a third 

 series, i\" nominal (1"2 mm. focus) has been produced, by which, 

 especially in histological observations, great amplification can be 

 obtained with low eye-pieces. 



The angular aperture of all these objectives is about 114° in 

 the immersion fluid for which they are adapted, the index of refrac- 

 tion being taken in round numbers as := 1 '50. 



* J. W. Stephenson " On a Large-angled Immersion Objective ^vitilout 

 Adjustment Collar," &c. — This journal, i. (1878) 51. 



VOL. II. S 



