The Microscope. 91 



I would not trust a single result produced by its means, when 

 oblique light in one azimuth is employed, especially with the 

 chromatic flint condenser provided by the firm of Zeiss for its 

 illumination. It is fatal to its truth. We can almost get any 

 desired result with it. It is a very optical Witch of Endor for 

 calling up ghosts and ghostly visions. * * * But now 

 comes the pragmatic question, which we are bound to ask, 

 " What does this objective contribute to the practical work to 

 which, for the attainment of the highest results, the microscope 

 must be applied ? " 



I say at once, for the amateur and the lover of splendid im- 

 ages the objective may be a delight. But I have pointed out 

 before that even immersion objectives, though they have a 

 great, have nevertheless a ver}^ limited, use in strict biological 

 inquires of a certain kind. This is true of water ; it is doubly 

 true of oil. If we are examining minute life under a limited 

 cover, the fluid above, between the lens and the top of the cover 

 glass, will ultimatel}^, in following the travels of the living 

 creature, be caused to mingle with the fluid between the cover 

 and the slip, and so destroy the work. But in spite of this, 

 immersion, and especially homogeneous immersion, objectives 

 have an enormous value for experiments in control and com- 

 parison. But with the new lens of this great aperture, not only 

 have we to use flint covers, specially and expensively ground, 

 and flint slips, but of course we have to employ a dense mount- 

 ing medium absolutely fatal to all organic tissues. Flinty and 

 carbonaceous animal and vegetable products, however fine, may 

 be examined by its means ; but the cell as such, to say nothing 

 of the living cell and unicellular organisms, can never at pres- 

 ent be subject to its optical analysis. 



Now it must not be supposed that this fact was not fully 

 known to its accomplished makers when they devised and sent 

 it out ; that would be an error. But in our inquiry as to the 

 influence it will exert upon the special work of the microscope 

 in unravelling the structure and deportment of animal and 

 human tissues it is a great factor. In spite of the splendid re- 

 sult attained by it, as biologists we gain nothing. We are 

 where we were, and studies of cells and cell life must be made 

 with dry and immersion apochromatics of N. A. 1.4, or at most 

 1.5. With this fact before us it will be well for us to remember 



