174 E. M. NELSON ON A NEW EYE-PIECE. 



This is in illustration of what I have frequently urged, viz., that 

 every microscopist should see aberration by making images of a 

 lamp flame on a piece of paper with the field lens of one of his low 

 eye-pieces, turning first the convex and then the plane side to the 

 focal point on the paper. 



Another very instructive experiment is the reading of a micro- 

 photograph of printed matter with a plano-convex lens, reversing 

 the lens and noticing the effect. p IG q 



With the convex side to the photograph the 

 printed matter is quite misty, and the field is 

 fairly flat, but with the plane side the field is very 

 much curved, and the print in the centre of the 

 field becomes black, the fog going off. I there- 

 fore thought that if I reversed the eye-lens of 

 the Huyghenian eye-piece (see Fig. 6) I should 

 get increased definition at the expense of flatness of field. Nor 

 was I disappointed, for I found on trial that I got material increase 

 of sharpness in the centre of the field, the margin being quite out 

 of focus. 



This out of focus part I stopped out by making the diaphragm 

 smaller, so reducing the size of the field from 16in. to 8in., which 

 leaves it still larger than that given by the compensating ones, and 

 quite large enough for all practical purposes. 



Therefore, if anyone is desirous of improving the definition of 

 his eye-pieces, all that is necessary is the inversion of the eye-lens 

 and the contraction of the diaphragm. Of course it is only advis- 

 able to treat eye-pieces of less than lin. focus in this manner, as a 

 large flat field is of paramount inrportance with the lower eye- 

 pieces, and the loss of definition is not so apparent. The best 

 result I have obtained with this plan is by achromatizing the eye- 

 lens. The one which I have here to-night is the best eye-piece I 

 have ever seen. It is of 12 power, and gives sharper pictures than 

 one of Zeiss' compensating series of the same power, which was 

 carefully selected from several sets. This eye-piece works perfectly 

 with the 24 mm. and 3 mm. Zeiss apochromatic objectives. You 

 will notice there are only two surfaces to work in the simple form 

 and four in the achromatic form, against seven in the compensating 

 eye-piece. 



