173 



On a New Eye-Piece. 



By E. M. Nelson. 

 (Read October 28th, 1887 J 



Having given for some time past a good deal of attention to 

 eye-pieces for telescopes as well as microscopes, and having lately- 

 contrived one which has surpassed any I have yet seen in the 

 sharpness of its defining power, I thought a short account of it 

 would prove of interest to the Club. 



Until lately there have been among microscopists only two 

 kinds of eye-pieces in general use — the Huyghenian and the 

 Kellner, but a little while ago Professor Abbe introduced what 

 are known as compensating eye-pieces. Of these three forms the 

 Kellner may be dismissed by saying that although it possesses one 

 achromatic lens its defining power is undoubtedly bad. The 

 compensating eye-pieces, while being absolutely necessary to some 

 of the apochromatic series of objectives and beneficial to others, 

 improve the definition of ordinary objectives. 



With regard to all my previous work with achromatic singles, 

 doublets, and triplets, &c, I may say that I have not succeeded 

 in producing any combination giving definition equal to the 

 Huyghenian. But the increased definition obtained by the com- 

 pensating series caused me to re-open my investigations. 



The action of the Huyghenian eye-piece is that of an under- 

 corrected lens balancing an over-corrected image ; the strange thing 

 is that the compensating system of eye-pieces, being all over- 

 corrected, should give better results. After going into the matter 

 I came to the conclusion that the Huyghenian system sacrifices 

 definition to flatness of field, this latter quality being one in which 

 the compensating eye-pieces are eminently deficient. Now we 

 know that a single convex lens having the least aberration has its 

 radii 6 : 1, and the outstanding aberration is Sf= — i| XI. 



But the aberration of a plane convex is not much greater if the 

 plane side is to the focus as Sf= — |- L 2 , but if the convex side is 

 towards the focus Sf becomes = — f- y -l, or about four times as much. 



