DISCOVERED WITH A NEW ZEISS APOCHROMAT. 201 



Naviculaceae, do not require such protection, the diameter of 

 their main perforations not greatly exceeding that of the coarser 

 secondaries in the capped kinds. Mr. Nelson has recently 

 recorded the discovery of similar capping in the "eye-spot" 

 layer, hence it would seem certain that the protection is very 

 necessary to the well-being, or some function, of the organism. 

 But diatoms guard their secrets well, and assuredly hold many 

 surprises in store for the microscopists of the future, as they 

 have done for those of the past. It appears safe to say that the 

 fascination of the diatom will endure as long as the instrument 

 exists which it has done so much to bring towards perfection. 



The advantages of the new lens lie entirely in the superior 

 correction of its aberrations, so that it will better stand a large 

 working aperture. In spite of its fine corrections there can be 

 no question whatever that there is still ample room for improve- 

 ment in the future, the ideal high-power lens being one which 

 would work at full aperture without turning pale, as a few of 

 the best low-power objectives will do at present. 



It should be clearly understood that the new Zeiss apochromat 

 was manufactured with no intention of being specially employed 

 on the Diatomaceae. As it bears a higher consecutive number 

 than 700, it may be taken as self-evident that comparatively 

 few of its predecessors have been acquired with a view to diatom 

 work, or have been ever used on a diatom. In practice it has 

 been found to yield the most clear and brilliant definition on 

 such widely differing objects as Karyokinesis in the root of 

 Lilium, various bacteria examined under the highest eye-pieces, 

 and the moving bubbles in the fluid cavities of fluorite. My 

 experience is that the whole series of Zeiss apochromatic objec- 

 tives, from the lowest to the highest powers, when used with a 

 large working aperture, afford the truest images of all classes 

 of objects. Their use is general throughout all the principal 

 laboratories of the world for utilitarian purposes of the greatest 

 importance, and their employment on diatoms is by no means 

 widespread, or more than a side issue. The late Dr. Dallinger 

 found it necessary to employ similar lenses for the most difficult 

 points in his original researches on the life-history of the Sapro- 

 phytes. In times past, before lenses of great aperture could be 

 properly corrected, wide-angle glasses were constructed for the 

 express purpose of being employed solely on the Diatomaceae, 



