MICKOSCOPY. 



By Professor HAMILTON L. SMITH, 



Hobart College, Geneva, N. Y. 



IMPROVEMENTS IN MICROSCOPICAL OBJECTIVES AND 

 MICROSCOPICAL APPARATUS. 



The New Oil-immersion Object-glass of Carl Zeiss. 



Dr. Dallinger publishes a letter in Nature highly commen- 

 datory of this objective. The spherical and chromatic aber- 

 rations are beautifully corrected, and all the most crucial 

 tests readily mastered. He states that he has not been able 

 to do more with it than with the new formula one-eighth of 

 Powell and Lealand, but that the same results are accom- 

 plished much more readily, as there is no correction to be 

 brought into operation by the German glass, which has sim- 

 ply to be brought into focus. A drawback upon the use of 

 this objective is the fact that the oil is a solvent of most 

 varnishes and gums used in the mounting and finishing 

 slides, except shellac-varnish ; and he further remarks that 

 immersion objectives are of very limited service in observa- 

 tions continuously conducted upon minute living organisms 

 in fluid. We may gladly call in their aid in determination 

 of a delicate change of form, or in the more perfect detec- 

 tion and definition of an obscure point of structure ; but for 

 steady and constant work we are bound to avoid them ; for 

 the fluid under the delicate cover is in danger every moment 

 of being "flooded" by coming into contact with the water 

 (or other fluid) on the top of the cover and between it and 

 the lens ; because the movements of the organism have to be 

 counteracted by the movements of the mechanical stage, in 

 order to keep any form that may be studied in view con- 

 stantly. Since, then, there are these difficulties in the use of 

 immersion-lenses in biological investigations, Mr. Dallinger 

 expresses the hope that the English, the Continental, and the 

 American opticians will not abandon their efforts for the still 



