TEXT-BOOK OF BACTERIOLOGY. 29 



is well known, rays of light in passing- from one optical medium to 

 another suffer a refraction and reflection which causes much loss 

 of light. If, therefore, rays proceed from the object and through 

 the thin glass cover into the air between it and the objective, 

 they lose as much illuminative power here as on their leaving the 

 air to enter the glass medium of the lens. 



This fact is also of essential importance for the definition, and 

 with reference to it Abbe has drawn up the final rule : the definition 

 of every microscopic object-glass depends on its " numerical aper- 

 ture. " This, however, is equal to the sine of half the angle of aper- y 

 ture multiplied by the index of refraction of the stratum whict 

 separates the object and the first lens of the object-glass. 



It follows from this that in the ordinary dry systems, such as 

 were almost exclusively in use for a long time, in which the index of 

 refraction of that intermediate stratum, the air, is an invariable 

 amount, the numerical aperture i.e., the definition cannot be in- 

 creased beyond a certain limit, which is only determined by the size 

 of the angle of aperture. 



The introduction of immersion in water by Amici was, therefore, 

 a very great improvement, since it brought between objective and 

 object a medium of considerably stronger refractive power i.e., 

 water. 



But a still greater progress was seen in the systems with homo- 

 geneous immersion as first introduced by Stephenson and after- 

 wards improved by Abbe. 



These two investigators also completed their theoretical elabo- 

 ration and calculation, thus contributing essentially to their general 

 adoption. 



Between the object and the objective was placed, instead of the 

 stratum of air, a medium of nearly equal refractive power with 

 glass namely, oil, and especially a certain kind of cedar oil. 



After this it was possible to give to the field of vision an unusu- 

 ally large quantity of light; for the losses which otherwise take place 

 at the separating surfaces by different media were now obviated. 

 The effects of immersion in oil may be illustrated by a simple ex- 

 periment. Into an empty test-tube place a moderately thick glass 

 rod. There will be no difficulty in perceiving it, for the differences 

 of refraction between the surrounding air and the glass allow the 

 latter to be seen clearly enough. 



Now pour some water into the glass : water stands nearer to 

 glass in point of refractive power, and it will be more difficult* to 

 distinguish the glass rod. If, however, the glass be filled with cedar 

 oil instead of water, the rod, as far as it lies in the oil, will immecli- 



