L90 



THE AMERICAN MONTHLY 



[October, 



forms numbered among the millions, 

 and an enumeration seemed impossi- 

 ble. We concluded then that the net 

 contained 3,000 An. foliacea, 2,400 

 An. longispina., and 18,000 Cera- 

 tium hiriiiidinella. Besides these 

 armies there were the Cladocera and 

 Copepada. To capture these alone 

 we have employed a wider meshed 

 net. We have drawn the net in the 

 open water and near shore, in rough 

 and smooth water, in cloudy days and 

 in sunshine, at various times of day 

 and night, and always reached about 

 the same results. 



The same net was used in the river 

 Limmat, which was found also to con- 

 tain an infinite multitude of the Dino- 

 bryon forms washed down out of 

 the Zurich lake. But the Sihl, on 

 the other hand, contained no trace 

 (' keine spur') of these organisms. 

 In a pond connected with the lake 

 by the Wehren-bach they were found, 

 but in much smaller numbers. 



The Dinobryon forms and their al- 

 lies then appear to be in particular the 

 dwellers of the great still waters. 



We present this preliminary notice 

 with all reserve. Whether this con- 

 dition is permanent and similar to that 

 of other waters will be shown as the 

 result of future investigations. 



Zurich, Mav 20, 18S6. 



Recent Improvements in Micro- 

 scope Objectives. 



BY ROMYN HITCHCOCK, F. R. M. S. 



Scarcely ten years have passed since 

 Professor E. Abbe, of Jena, presented 

 to the scientific world his theory of 

 vision with the microscope, which 

 resulted from a long series of inves- 

 tigations conducted mainly by Helm- 

 holtz and himself. It is not my in- 

 tention to enter upon a general dis- 

 cussion of this theory, but rather to 

 present, as briefly as possible, an ac- 

 count of the practical results to which 

 it has led. It may be well, however, 

 to briefly allude to some of the funda- 

 mental facts underlying the theory, 

 since the subject is not very generally 



understood by persons not especially 

 conversant with microscopical litera- 

 ture. 



So rapidly, indeed, have advances 

 been made in the construction of mi- 

 croscope objectives that even inves- 

 tigators in histology and in other 

 branches of microscopic research 

 are, in many instances, unacquaint- 

 ed with the highest results of the 

 optician's skill, and are firmly con- 

 vinced that their favorite lenses of 

 twenty years ago are still the best 

 that can be made. Such persons 

 are still ready to fight over again 

 the battle of the glasses which raged 

 long ago betvs'een one set of observ- 

 ers who believed in the wide angu- 

 lar aperture lenses, and another set 

 who upheld narrow angle lenses, 

 utterly unconscious of the fact that at 

 the present time the qualities of any 

 microscope objective can be mathe- 

 matically calculated and numerically 

 stated. 



Did time permit, I would be 

 pleased to review the progress that 

 has been made during the last 

 twenty or thirty years, but I must 

 refrain. The microscope is capable 

 of separately defining lines or mark- 

 ings as close together as the i-i 15000 

 of an inch. This is about the limit 

 of resoluticni with white light, the 

 theoretical limit being somewhat 

 higher. The length of a vibration 

 of red light is about 1-39000 of an 

 inch. How is it possible to resolve 

 a band composed of lines ruled so 

 much closer than a wave-length of 

 light.? Obviously, such minute 

 spaces cannot be imaged by the 

 dioptric method illustrated in the 

 text-books in explanation of the ac- 

 tion of the microscope. The effect 

 of such a band is to break up the 

 rays by diffraction, and Professor 

 Abbe has shown that, in order to 

 resolve a band of lines as close or 

 closer than 1-39000 of an inch, it 

 is necessary that the several diffrac- 

 tion spectra produced by the illu- 

 minating pencils be taken up by 

 the object-glass. These spectra are 



