T. F. SMITH ON ARACHNOIDISCUS. 251 



belonging to me. The Powell and Lealand's £" fails completely 

 in my hands to show this test, and I can only account for it by 

 supposing that the central part of the glass has been sacrificed to 

 excessive angular aperture. Failing in this test, it also fails to 

 show other objects satisfactorily with central light when the object 

 is composed of two or three layers of structure. 



The Gundlach x y is a very fine lens, and having less aperture 

 shows the new test very well, but not better than the dry -J" I am 

 showing to-night. The measure on this also is its measure on all 

 other objects. 



As an example of the fine detail that can be picked up with this 

 new dry ~ ", and also with the Apochromatic \" of Zeiss of 0'95 

 N.A., I may say that I have seen the flagellum of Spirillum 

 undula with them with central light. Now the diameter of this 

 Spirillum is the -o oWs °^ an i ncn > an( l ^ I ma y take the propor- 

 tion of the flagellum as the one-tenth, the same as given" by Dr. 

 Dallinger, as the proportion in Bacterium termo, the diameter 

 would be only the -o-q'oVoo' °^ an mcn * ^ ie thickness of the 

 Spirillum I have verified by measurement, but must leave the 

 proportion of the flagellum as an open question. One thing is 

 certain, it must be very minute. To prove the great importance of 

 this advance in the definition of dry objectives I beg to quote the 

 following from a letter to " Nature," by Dr. Dallinger, written 

 about the time when the first oil-immersions were brought out : — 

 " Even water-immersion lenses are of limited service in observa- 

 tions continuously conducted upon minute living organisms in fluid. 

 The fluid under the cover is in danger every moment of being 

 flooded by going too near the edge of the cover, thus rendering 

 the observation void." He further says, " That the production of 

 oil-immersion lenses should not lead the best opticians to abandon 

 efforts for the still greater improvements of dry lenses." 



With the aid of the new optical glass I claim that the improve- 

 ment Dr. Dallinger asked for has been effected, and that not only 

 in the more expensive Apochromatic lenses produced by Zeiss, but 

 also in the ordinary student's lenses, such as the one I show to- 

 night. 



About the beauty of the image produced by the new Apochro- 

 matics there can be no doubt, corrected as they are up to the same 

 point both for chromatic and spherical aberration. 



With the old glass this was impossible ; if you corrected too 



