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A NEW OBJECT GLASS BY ZEISS, AND A NEW 

 METHOD OF ILLUMINATION. 



By Edward M. Nelson, F.R.M.S. 

 {Read March 24///, 1914.) 



Figs. 1-3. 



As Object Glass upon an entirely new plan has been brought 

 out by the firm of Carl Zeiss. This lens has not yet been 

 catalogued, but as it will undoubtedly effect a considerable 

 change in the construction and use of microscope objectives a 

 short account of it may prove of interest to the Club. 



The object glass is a short tube oil- immersion 1 of "9 1ST. A. 

 Upon taking it out of its black box the first thing that will be 

 noticed is that it is nickeled all over, and the next is that the 

 front lens is set in a push tube, and not screwed up as usual ; 

 these two new departures from the usual type are also found in 

 the oil-immersion T Vth recently issued by this firm. 



In very early times objectives were made on this plan. Both 

 Ross and Smith, before 1840, used to screw the front lens to a 

 tube, which was pushed on to another holding the back lenses ; 

 this tube was then rotated until the best point was found, when 

 a small screw was put in at the side to keep the tube in that 

 position. 



This form of construction has gone on continuously to the 

 present day, especially in the cheaper series of objectives, while 

 the more expensive ones, including oil-immersions, have had the 

 cells holding the lenses screwed into their proper positions. But 

 this type of objective, so far as I am aware, for an oil-immersion 

 is quite new, as also is an oil-immersion with a N.A. of less than 

 1'0. Now with regard to the performance of this lens, the 

 corrections are very perfect ; although no fluorite is used in its 

 construction it is very nearly apochromatic, and shows a consider- 

 able advance over semi-apochromatism, for only a slight trace of 

 outstanding blue can be seen. 



The defining power of this objective is quite remarkable, for it 

 surpasses all object glasses of similar aperture I have seen. 



On a M oiler's Probe-platte of 60 diatoms all are resolved except 



Journ. Q. M. C, Series II. No. 75. 26 



