OPTICAL PRINCIPLES OF THE MICROSCOPE. . 17 



angle are used ' dry;' for as the amount of ' negative aberration ' ( 17) 

 is far smaller when the rays which emerge from the covering-glass pass 

 into water than when they pass into air, variations in its thickness pro- 

 duce a much less disturbing effect. And thus it is found practically that 

 * immersion ' objectives can be constructed with magnifying powers suffi- 

 ciently high, and angular apertures sufficiently large, for all the ordinary 

 purposes of scientific investigation, without any necessity for cover- 

 adjustment; being originally adapted to give the best results with a 

 covering-glass of suitable thinness, and small departures from this in 

 either direction occasioning very little deterioration in their performance. 

 For ' water-immersion ' objectives of the very largest aperture, however, 

 to be used upon the most difficult objects, exact cover-correction is still 

 necessary. Whilst 'immersion '-objectives constructed on the original 

 plan can only be employed 'wet '(that is with the interposition of 

 water), Messrs. Powell and Leland followed by other makers have so 

 arranged their combinations, that by a change in the front lens they may 

 be used 'dry/ as in the ordinary manner. And in Mr. Wenham's sys- 

 tem not even this change is required, the change from ' wet' to i dry/ 

 and vice versa, being accomplished by an alteration in the distance of the 

 front lens from the middle triplet, made by the screw-collar, as in ordi- 

 nary cover-correction. 



20. The ' immersion system ' has recently undergone a still further 

 development, by the practical application of a method originally sug- 

 gested by Mr. Wenham ' (but never carried by him into operation), and 

 independently suggested by Mr. Stephenson 2 to Prof. Abbe of Jena, 

 under whose scientific direction it has been worked out by the very able 

 German optician, Zeiss, with complete success. This method consists in 

 the replacement of the water previously interposed between the covering- 

 glass and the front surface of the objective, by a liquid having the same 

 refractive and dispersive power as crown-glass; so that the rays issuing at 

 any angle from the upper plane surface of the covering-glass, shall enter 

 the plane front of the objective without any change either by refraction 

 or dispersion, and without any sensible loss by reflection even the most 

 oblique rays proceeding in their undeflected course, until they meet the 

 convex back surface of the front lens. It is obvious that all the advan- 

 tages derivable from the system of water immersion are obtainable with 

 still greater completeness by this system of homogeneous immersion, pro- 

 vided that a fluid can be found which meets its requirements. After a 

 long course of experiments, Prof. Abbe found that oil of cedar-wood so 

 nearly corresponds with glass, alike in refractive and in dispersive power, 

 that it serves the purpose extremely well, except when it is desired to 

 take special advantage of the most divergent or marginal rays, oil of fen- 

 nel being then preferable. Objectives of -th, T Vth, and y\th inch focal 

 length have been constructed on this plan by Zeiss; and it appears cer- 

 tain that by its means a larger angle of aperture can be effectively ob- 

 tained, than on any other construction. Whether any tests can be re- 

 solved by its use, on which other objectives fail, is a point not yet 

 satisfactorily determined. But there can be no doubt that the system of 

 ( homogeneous immersion ' will greatly facilitate the use of objectives pos- 

 sessing the largest angular aperture, and capable of affording the highest 

 magnifying power, for the ordinary purpose of scientific research. It is 



1 ' Monthly MicroscopicalJournal," Vol. iii. (1870), p. 303. 



2 " Journ. of Royal Microsc. Society," Vol. i. (1878), p. 51. 



