System of Homogeneous Immersion. By Prof. E. Ahhe. 2o9 



results which Dr. Koch, of Wollstein, obtained when examining 

 bacteria,* viz, by employing a full cone of rays filhng the entire 

 aperture of the objective, a method of illumination quite unheard 

 of as applied to such objects and with such an angular aperture. 

 With this illumination, which can only be eflfected by the aid 

 of a condenser of large aperture, the preparation is simultaneously 

 penetrated in all directions by the incident rays. As a result, 

 the . delineation of such parts as stand out in mutual contrast 

 through difference in refractive power (tissue structures, &c,), is 

 almost completely suppressed, and there remain visible only those 

 elements which act as absorbents through staining. On the other 

 hand, the essential advantages of oblique illumination are retained, 

 although the illumination remains central in name, in consequence 

 of the co-operation of the rays incident at a large angle towards 

 the axis of the Microscope, Very small and closely clustered 

 elements, as in prejDarations of bacteria, must certainly on both 

 these accounts become capable of a more thorough resolution than 

 with central illumination of the usual kind ; if, however, this 

 ingenious method of observation is to show corresponding results, 

 the defining properties of the objective must stand a most severe 

 test, and this test will be the more severe in proportion to the 

 magnitude of the angular aperture employed. 



As regards the nature of the immersion fluid, it is of course on 

 optical grounds a matter of indifference what is selected, so long 

 only as it is homogeneous and transparent, and equal, or very nearly 

 equal, to crown glass in refraction and dispersion. Experiment has 

 taught, however, that this condition of homogeneous immersion 

 leaves a much smaller choice than might be anticipated. At the 

 outset I examined over one hundred fluids of the most varied kinds — 

 essential and fatty oils and artificial chemical preparations— which 

 I either tested myself or caused to be examined with the refracto- 

 meter, to determine their refractive and dispersive indices, and 

 lately the investigation has been carried still further by Dr, 'J'opel, 

 who, under my guidance, determined the optical constants of nearly 

 two hundred chemical combinations from the collection in the 

 laboratory of the Jena University, which Professor Geuther was 

 kind enough to place at our disposal. Among all these, however, 

 not one was found which from its other properties could be used ; 

 which either alone or mixed with other fluids attained the refractive 

 index of crown glass (1'515 to 1-520 for sodium hght) without at 

 the same time more or less exceeding the disjiersion of crown glass, 

 A few only of the substances examined satisfied the necessary 

 conditions with sufl&cient accuracy to permit the deviation to be 

 regarded as unimportant. 



The most suitable fluid that has at present been discovered, is 



* ' Aetiologic del- AVundinfektions-Kriiiikheiten.' Leipsic, 1871). 



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