Cata-dioptric Immersion Illuminator. By J. W. Stephenson. 211 



(1 • 652) its effect is to diminish, and in some cases to neutralize, 

 the refraction at the junction of the flint- with the crown-glass lens, 

 so that with some media it is achromatic when certain apertures — 

 which are determinable by the stops — are employed. 



This brings us to the question, whether, after all, spherical and 

 chromatic aberrations are obnoxious in condensers as far as defini- 

 tion is concerned. On this point there is a conflict of opinion, but 

 some at least of the greatest optical authorities certainly hold that, 

 in this respect, these aberrations are and must be unobjectionable. 



I may add, that the well-known property of the ellipse of accu- 

 rately reflecting to one of its foci, rays diverging from the other, 

 has induced me to experiment in that direction, and there is now 

 on the table such a lens made for me by Mr. Anderson. The 

 major axis of the ellipsoid, of which this is a small segment, is 

 12 '6 in. in length and the distance between the foci 12 in. 

 Hence it follows that its focal length is * 3 in. 



It is no doubt practically impossible to construct a perfect 

 ellipsoid, but perfection of form is obviously of less importance in 

 the case of an illuminator in which a sharp image of the illuminant 

 is not required and a comparatively small portion of the lens is in 

 action at one time, than it is in an instrument in which the pro- 

 duction of a perfect image is indispensable. 



That which I now produce is as far as range of aperture is 

 concerned, absolutely identical with the cata-dioptric lens just 

 described, and if accurately made would be free from both spherical 

 and chromatic aberrations on any object mounted in a medium 

 optically identical with the glass of which it is made. 



I had hoped and believed that long ere this I should have been 

 able to produce an objective constructed on the principle of the 

 Catoptric Immersion Illuminator described by me in January 1879, 

 with an aperture at least as great as that of the illuminator now 

 described ; but the difficulty of effecting a perfect correction of the 

 spherical aberration appears to be insurmountable. Had such an 

 objective been made, it would not have been suitable for general 

 use ; its principal aim would have been to increase to the utmost 

 possible extent the resolving power of the Microscope ; this can 

 only be effected by using rays of the utmost obliquity, and to this 

 its action would necessarily have been confined. This subject is 

 one of the greatest interest, and one must not despair of seeing 

 stni further progress in the aperture of our objectives. 



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