326 Address by the President. [Sess. 



lenses approach nearer the front lenses, according to the thick- 

 ness of the cover-glass. The collar is marked with a num- 

 bered scale, but the correct position has to be found by ex- 

 periment. In the Continental form the collar is marked with 

 certain numbers corresponding to the usual thicknesses of 

 cover-glasses, so that if the thickness of the cover-glass is 

 known, you have only to set the collar to that number. Thus, 

 if the cover-glass is 0"20 mm. thick, the collar should be set 

 to the division marked 0'20, But as in the majority of cases 

 the thickness of the cover-glass is not known, the correct 

 position will also have to be found by experiment. 



Immersion Lenses. — Hitherto we have been considering only 

 what are called dry object-glasses — that is, object-glasses in 

 which there is a film of air between the front lens of the same 

 and the thin glass covering the object. It was, however, 

 early perceived that a drop of water interposed between the 

 front lens and the cover-glass improved the performance of 

 the object-glass, but, at the same time, it altered the refraction 

 of the rays of light, so that the lenses had to be corrected in 

 accordance with the new conditions. When this was done a 

 class of object-glasses was constructed termed water-immer- 

 sion, which gave very good results. More light was admitted 

 into the object-glass, and hence a brighter held of view was 

 obtained in comparison with the dry objective of the same 

 angle of aperture. The reason of this will appear from the 

 remarks which were made under Fig. 1, Plate XXV., and 

 from those about the critical angle of reflection. Under Fig. 1 

 it was seen that rays of light which passed into water were 

 much more compressed than when they passed into air; again, 

 that the critical angle of light passing from glass to air was 

 41", but from glass to water it was 48°. Here there was a 

 great increase of light which could pass into the object-glass. 

 In using such glasses the front lens is brought into continuity 

 with the cover-glass by means of a drop of distilled water. 



A further and more important step was soon made in the 

 construction of immersion object-glasses. The thought oc- 

 curred to Mr John Ware Stephenson, London, of using instead 

 of water some denser medium — a medium the refractive index 

 of which would be as near as possible that of glass. He com- 

 municated his thoughts to the late Professor Abbe of Jena, 



