928 Transactions of the Society . 



XV. — A New Eye-piece. 

 By E. M. Nelson. 



iRead 9th November, 1887.) 



Until quite lately, there have been among Microscopists only two kinds 

 of eye-pieces in general use, viz. the Huyghenian and the Kellner. 

 Eecently, however. Prof. Abbe's compensating eye-pieces have been 

 introduced with beneficial results. Of these three forms the Kellner may 

 be dismissed by saying that although one of its lenses is achromatized, 

 its defining power is undoubtedly considered bad by general consent. 



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 also. 



Having for some time past made a great many experiments with 

 achromatic eye-pieces of doubles, triples, and other forms, I may sum up 

 my results by saying that I had not succeeded in producing any 

 combination whose defining power surpassed that of the Huyghenian. 



When I saw the increase of defining power given by the compensat- 

 ing eye-pieces, I determined to reopen my investigations. 



The theoretical action of the Huyghenian eye-piece requires that an 

 over-corrected image should be received by the field-lens, the over- 

 correction to be of such an extent that the under-corrected field-lens of 

 the eye-piece is not able to neutralize it, but leaves it still over-corrected 

 by an amount equal to the under-correction of the eye-lens. I must say 

 that my surprise was great on obtaining better definition with Prof. 

 Abbe's over-corrected eye-pieces used in conjunction with the supposed 

 over-corrected ordinary achromatic objectives. I concluded, therefore, 

 that by reducing the under-correction of the eye-lens of the Huyghenian 

 eye-piece better definition would be secured. 



We know that in the formula for aberration — 



-^ ' f 2/^2 K^ "•" V / r'J\f r')V 



and where /= principal. focus; y = semi-aperture ; fi = ref. index and 



, / 

 ^, r, = radii ; if we put r = oo , and — r' = -^ we get the aberration for 



a plano-convex lens having its convex side to the focus ; in other words, 



9 y^ 

 the eye-lens of a Huyghenian eye-piece, viz. A/ = — - ^ • 



7 y^ 

 If, however, we invert the lens, A / = - -^ -^ or about 1/4 of what 



it was before. I therefore concluded that by the inversion of the eye- 

 lens there would be an improvement in the definition though a loss in 

 the size of the field. 



In practice I found these conclusions verified. The best results were 

 obtained by achromatizing the eye-lens, i. e. by making it of a biconvex 

 and a plano-concave, with its convex side towards the eye. The aperture 

 in the diaphragm was reduced until the diameter of the field was equal to 

 that of the Abbe compensating eye-piece. 



This eye-piece, with the achromatized eye-lens, gives the sharpest 

 images I have seen. It works perfectly well with the 24 mm. and 

 3 mm. Zeiss apochromatic objectives. 



