JEyc-pieces for the Microscope. By E. M. Nelson. 529 



The fixity of tube-length and of magnifying power, which is 

 made so much of in text-books, is a myth, because the actual power 

 of an objective, or eye-piece, has nothing to do with the figures or 

 letters engraved upon it, and every different object requires it own 

 particular tube-length. Moreover, the identities are erroneous ; 

 thus, short-tube eye-pieces, powers 6, 8, 12, and 18, are equivalent 

 to 9^, 12|, 19, and 28^ long tube, and, conversely, 8, 12, 18, 27 

 long tube are equivalent to 5, 7^, 11^, and 17 short tube powers, 

 so it is difficult to see where the enormous advantage which the 

 text-books say arises from the " fudging " of the eye-piece comes in. 



Fig. 86. 



As stated above, I now seldom use compensating eye-pieces, 

 preferring those of the form in Table I., and making the necessary 

 corrections by the use of a rackwork draw-tube.* 



It was pointed out t that vague notions concerning the action ot 

 the Huyghenian eye-piece were prevalent among microscopists on 

 account of the incorrect figures and descriptions which have 



I 



* The first Microscope to possess a rackwork draw-tube was the large one 

 made by Benj. Martiu for George III., now in the Society's cabinet of ancient 

 instruments. This draw-tube was made for the purpose of increasing the 

 magnifying power. The second Microscope to have a rackwork draw-tube was 

 a dissecting Microscope made by Oberhauser for Strauss Durkeim, and for the 

 same reason. 



But the first rackwork draw-tube, for the purpose of lens correction, was 

 made by Messrs. Powell and Lealand to my order. And the first Microscope 

 with a double draw-tube, one for lens correction and the other to enable the 

 instrument to be used with objectives, corrected either for the long or short 

 tube, was subsequently made by Swift to my order in 1884. I regret to have to 

 draw attention to such a matter, but piracy in microscopical devices is especially 

 rampant just now. 



t This Journal, 1900, p. 163. 



