364 E. M. NELSON ON A NEW OBJECT GLASS BY ZEISS, 



the two specimens of Amphipleura pellacida. The next most 

 difficult diatom to the Amphipleura is the Nitzschia curvula, and 

 as this diatom counts 89 thousand per inch it shows what this new 

 lens can do with oblique light and a stop, the illuminant being 

 an ordinary microscope paraffin lamp with a \ in. wick. With 

 axial light, without any stop, the Brazilian Lindheimeri is dotted. 

 On M oiler's Typen-platte, with 400 forms, the Nitzschia curvula 

 (there called the JV. sigmatella) is very thin and difficult, and the 

 lens fails to resolve it, but it easily resolves all the others on that 

 line except the Homoeocladia Martiniana, which is more difficult 

 than A. pellucida. It resolves the N. crassinervis on that plate 

 quite easily, and it will just show the striae on the Grammatophora 

 oceanica, which counts 88 thousand to the inch. This diatom is 

 probably the G. subtilissima ; anyhow, it is very much finer than 

 the diatom of the same name on the Probe-platte. 



The image given by this new lens of the Poclura scale is very 

 fine indeed. Undoubtedly in this new objective we have a lens 

 of great beauty and power. An important question arises as to 

 the influence this lens will have upon our battery of objectives. 



In former times a 2 in., 1 in., a | in., a | in. and ^ in. or 

 yg- in. represented a full battery, but now we may have a 

 battery consisting of only a | in. and an oil-immersion T V in. 

 Here the gap is very wide, and the new lens will fill it very 

 satisfactorily. 



This lens will, to a certain extent, supersede the oil-immersion 

 T V in. in medical schools and colleges. It is sufficiently powerful 

 to do all that is wanted in practical study, but necessarily in 

 research work a T V in. of wider aperture is required. For a 

 student it will be especially valuable, for it has of course more 

 working distance and a larger field than a T V in. 



Another very important point is that, because of its great 

 working distance, it does not pick up by capillary attraction an 

 unfixed cover-glass. This is a source of great trouble when 

 working with a - in. 



Zeiss supply a funnel for reducing the aperture of this objec- 

 tive, so that a dark ground may be obtained with an ordinary 

 dry condenser and a stop. Of course with an oil-immersion 

 condenser no funnel is required. 



Henceforth, for research work, a perfect battery will consist of 

 a 2 in., 1 in., | in., ^ in., this iin., and a T V in. oil-immersion. 



