Methods for Improving Spherical Correction. ByProf.Ahhe. 817 



dioptric performance which is attainable by a really perfect correc- 

 tion of spherical and chromatical aberration. 



In consequence of the numerous grave drawbacks which 

 attended objectives of such construction, the result of this trouble- 

 some and expensive experiment was considered by Mr. Zeiss and 

 by myself as a matter of theoretical interest only — a glance at the 

 Microscope of the future. But, as some of the principal difficulties 

 would be considerably diminished in objectives for 

 homogeneous immersion, owing to the omission of 

 the correction setting, I am of opinion, that this 

 method of independent correction under the new con- 

 ditions is still capable of successful application in 

 the improvement of high-power objectives. 



In the meantime, however, another way of elimi- 

 nating the chromatic difference of spherical aberration 

 has suggested itself to me, reverting to the long- 

 established system of allied correction. 



In a recent investigation suggested by the introduc- 

 tion of the homogeneous-immersion system and during 

 the attempts at still further increase of aperture, I 

 touched once more upon the old problem, and observed 

 that a device, which I had discussed in theory eight 

 years ago, had not been then pursued to its legitimate 

 conclusion, and that the residual defect of spherical 

 aberration — though not simultaneously the secondary 

 chromatic difference — could be got rid of by the ordi- 

 nary means within reach of practical optics. 



The general idea of the plan in question is shown 

 by the diagram. 



M N is an objective, and 0* the two con- 

 jugate foci of object and image; M (represented, for 

 sake of simplicity, by a single lens in the diagram) the 

 anterior part of the system considerably MwcZe r-corrected 

 spherically and chromatically ; and N the posterior 

 part, over-corrected in both respects, just sufficient for 

 balancing both aberrations of the anterior part ; N being separated 

 from M by a relatively considerable distance. The continuous lines 

 in the figure indicate the path of an oblique red ray, the dotted lines 

 the path of the corresponding hlue ray, both derived from the same 

 incident ray P. 



The chromatic correction of the entire system being assumed to 

 be perfect for the axis, red and blue rays of small obliquity starting 

 from will meet the axis at the same point 0* ; and supposing 

 the system to be corrected spherically for the red, the red oblique 

 ray will be collected to the same point 0*. These two conditions 

 can obviously be fulfilled in every system ; but then in the ordinary 



VOL. II. 3 I 



