On Immersion Ohjeciives. 217 



pictures of Navicula Lyra with the objective, prints of which I send 

 you herewith. Both were taken with the same eye-piece and dis- 

 tance, simply the front was removed before making the second 

 negative. The first is magnified 765 diameters, the second 450. I 

 have sent copies of them to Mr. Brakey, who may perhaps learn 

 from this lesson to be more cautious in describing objectives he has 

 . not seen. 



I may say that since I first described this objective, Mr. ToUes 

 has somewhat modified its front, so that now, with sensibly the same 

 balsam angle, it nevertheless defines still better than before, and 

 has a greater working distance. He has also sent me two immer- 

 sion iths, constructed on the same general plan, which deserves a 

 moment's mention. In each of these objectives the front and second 

 systems are fastened permanently together, and remain stationary 

 while the two posterior systems are moved backward and forward by 

 the screw collar. One of these glasses has a maximum balsam angle 

 of 95°, the other of 92° ; both define Am])hi]jleura pellucida in a 

 very satisfactory manner, and the latter has the capability, when 

 properly adjusted, of working, with good definition through covers 

 rather thicker than the ^Vth of an inch. 



I will conclude this paper by mentioning that I notice in a 

 recent number of Max Schultze's ' Archiv fiir Mikroskopische Ana- 

 tomic' (Bd. ix., Heft 3), a preliminary paper on the theory of the 

 microscope, by Professor E. Abbe, of Jena, in which he also claims 

 that it is possible in properly-constructed immersion lenses to correct 

 the spherical aberration for greater angles of aperture than those 

 corresponding to the geometrical maximum for dry objectives. He 

 promises a more detailed paper, giving the grounds for his state- 

 ments in the ' Jenaischen Zeitschrift fiir Medicin und Naturwissen- 

 schaft,' Bd. viii. This, however, I have not yet seen, but refer to it 

 as of probable interest. 



War Depaetment, Surgeon-General's Office, 

 Washington, D.C, September 29, 1873. 



S 2 



