240 American Quarterly Microscopical Journal. 



If my language will bear this construction, I offer whatever apology 

 may be necessary. The question after all is one of performance, and I 

 can hardly believe that Dr. Abbe, when he acknowledges that the 

 objective I examined is below the standard angle, 116°, and, as he says,- 

 only 107° to 109", really means what he does say (p. 158), that this 

 increase of angle, 7° or more, would not make any notable difference 

 in the performance of the objective. Such is not my experience ; 

 other things being equal, each degree of gain in angle is a positive 

 advantage, and certainly 7' would show a superiority in resolving 

 power, and, indeed, a general superiority which no careful observer 

 could overlook. Why parade conspicuously this large angle, if an 

 objective of 107°, or for that matter 100°, will perform so nearly equal 

 that no notable difference can be perceived? But I do not think this 

 is really Dr. Abbe's unqualified opinion. He has taken too much pains 

 to elaborate the formula to obtain this large angle, and M. Zeiss has 

 expended too much skill in the construction to insist upon such a 

 statement. I, for one, am exceedingly indebted to these gentlemen 

 for the demonstration of the fact that so large a balsam-angle can be 

 obtained, and I have reason to believe that two objectives, a >s-inch 

 and a jVi^ch, now in the hands of my distinguished friend, Lt. CoL 

 J. J. Woodward, at Washington, are both, and notably the latter, supe- 

 rior to the objective that I examined, and because they art' " standard " 

 objectives of 116° balsam-angle, or 1.27 according to Prof. Abbe's 

 notation. 



As I had not the " circular " by me when I examined the >s-inch 

 Zeiss " Oil Immersion," I did not pay so much attention to the length 

 of tube as I ought to have done. It is quite evident that a non-adjust- 

 able glass, of very wide angle like this is rigidly confined (or within nar- 

 row limits) to a definite ratio for the conjugate foci for its best perform- 

 ance, and the tube I used was, perhaps, half an inch too short, if I 

 remember rightly the distance named on the circular, which I had 

 seen long before. How far this affected the performance of the 

 ^-inch, I cannot now judge ; I do not imagine that it would make 

 a very great difference. Whatever this would have been would be so 

 much in M. Zeiss's favor. 



H. L. Smith. 



NOTES. 



— Dr. H. Lenz, of Lubeck, has devised a means of aerating the water 

 in aquaria, which has given great satisfaction. A tube conducting the 

 air to the bottom is expanded at the end and stuffed with fine sponge. 

 This causes the air to rise through the water in very minute bubbles. 



— Maxime Cornu has made some experiments on the germination of 

 the spermatia of Ascomycetes, and succeeded in causing the spermatia 



