G. GENERAL NATURAL HISTORY AND ZOOLOGY. 265 



G. GENERAL NATURAL HISTORY AND ZOOLOGY. 



PEOFESSOR ABBE OX THE MICIIOSCOPE. 



Mr. W. E. Fripp has translated for the " Proceedings of 

 the Bristol Naturalists' Society " Professor Abbe's papers 

 on the microscope. The whole is very obscurely worded, 

 and, either from the defects of the original or the miscon- 

 ception of the translator, it is so difficult of comprehension 

 that it will prove of small value to the working optician. 

 There are, however, many valuable suggestions, and some 

 very extraordinary and positive statements ; e. g., that an 

 adequate compensation of spherical aberration is, as a mat- 

 ter of fact, impossible in a dry lens when the angular aper- 

 ture exceeds 110; and that hence it must be concluded 

 that a dry objective will be less suited for ordinary scientific ^ 

 use in proportion as it renders visible such finer systems of 

 lines as exceed, the limits of resolving power answering to 

 that angle. And he states that the greatest possible in- 

 crease of resolving power can be obtained in a rational Avay 

 only by means of immersion objectives. The immersion lens 

 may be made, according to Professor Abbe, with an aperture 

 of about 100 in water, somewhat more than w^ould corre- 

 spond to 180 in air a statement directly opposed to what 

 Mr. Wenham has claimed. Professor Abbe states positively 

 that there exists no microscope in which there has been seen, 

 or will be seen, any structure which really exists in the ob- 

 ject and is inherent in its nature, that a normal eye can not 

 recognize with a sharply defining immersion lens magnify- 

 ing eight hundred times (diameters ?). 



LIMIT OF VISIBILITY IN THE MICROSCOPE. 



In his recent annual address to the Microscopical Society 

 of London, the president, Mr. H. C. Sorby, F.R.S., discusses 

 the relation between the limits of the powers of the micro- 

 scope and the size of the ultimate molecules of matter. As 

 the combined result of observation and theory, he concludes 

 that the normal limit of distinct visibility w^ith the most per- 

 fect microscope is one half of the wave-length of the light. 



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