JOURNAL 



OF THE 



ROYAL MICROSCOPICAL SOCIETY. 



APEIL, 1879. 



TEANSACTIONS OF THE SOCIETY. 



IX.—TJie President's Address. By H. J. Slack, F.G-.S. 

 (Bead 12th Februanj, 1879.) 



I AM, unfortunately, quite unable to follow in the footetejjs of 

 Mr. Sorby, who, upon two occasions, brought before you in his 

 Annual Addresses important original work, highly gratifying to 

 those who value and desire to increase the scientific standing of this 

 Society. Failing in this, it seemed most desirable to select a few 

 points of interest for your consideration, arising out of recent 

 invention and observation. 



First, I would mention the introduction of the oil lenses sug- 

 gested by Mr. Stephenson, and constructed under the direction of 

 Professor Abbe by Herr Zeiss, of Jena. The objects of this inven- 

 tion are twofold ; first, to do away with the troublesome necessity 

 for making corrections with the screw collar introduced by Andrew 

 Eoss, and secondly, to obtain the largest angle of aperture with 

 a good working distance. By selecting an oil, or mixture of oils 

 having the same refractive power as the covering glass, it wa» 

 expected that a fixed correction would suffice for any thickness. 

 When the cover was thicker, as the working distance of the 

 objective remained unchanged, there would be a thinner stratum 

 of oil used upon the immersion principle, and when the cover 

 was thinner the oil stratum would be thicker. After many trials. 

 Professor Abbe found oil of cedar wood had very nearly the requi- 

 site properties when the illumination was with oblique light, and 

 was improved for direct light by an admixture with oil of fennel 

 seed. The glass made for Mr. Stephenson fully answered expec- 

 tation. It had according to his description a balsam angle of 

 113^, its power was rather more than one-ninth, and the only 

 correction it needed was a change in the length of the Microscope 

 tube from 10 inches to 12 when very thin covers were em- 

 ployed. One result of using objectives of this construction, taken 

 in connection with Professor Abbe's account of the way in which 



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