DRY OBJECTIVES OF GREAT VALUE STILL 8 1 



missed. It must always be an indispensable instrument in a large 

 part of the work done in the study of the life history of active 

 living organisms; and whatever accessories in research on such 

 subjects be employed, the main path of accurate and well correlated 

 discovery must be by ultimate and consecutive reference to tin- 

 changes of the living organism. But we cannot with any certainty 

 do this with either a water immersion or a homogeneous objective. 

 With an active organism under investigation, we desire, as far as 

 practicable, to limit the area of its excursions; a cover-glass of not 

 more than four-tenths or a quarter of an inch in diameter is large 

 enough when objectives from a ..^ inch to a - 1 ,, inch are used, 

 or when the recent 2 mm. objective with 27 eye-piece is employed. 

 To have oil or water on the top of the cover, between it and the 

 front lens of the objective combination, is, with almost inevitable 

 certainty, sooner or later, in following the object with counter move- 

 ments of the stage, to reach the edge of the cover, and cause the oil 

 or water above to mingle by capillarity with the minute drop of fluid 

 under observation, and thus to involve the whole in catastrophe. 



To do the main work of studying consecutively the life history 

 of unknown organisms, dry objectives will and must be used ; but 

 in all cases such work must be supplemented by the use of objectives 

 of great aperture. The details ami relations of minute structure 

 must be studied in one field, and their general origin and sequences 

 in another. The latter will be 'continuous,' the former will be 

 employed as necessity indicates. The diffraction theory of micro- 

 scopic vision does not invalidate, but in reality, under definable 

 conditions, directs the employment of 'narrow' apertures. All 

 depends on the minuteness of microscopic detail. The law has been 

 enunciated above : the minuter the dimensions of the structural 

 elements, the wider must be the aperture : the larger the details of 

 ultimate structure, the narrower the aperture that will suffice. This 

 is true in regard to objects of every kind ; there is no variation in 

 the conditions of microscopical delineation. 



The men engaged in microscopical research have different aims, 

 nay, the same worker at different times differs in the object pursued. 

 ' Ultimate structure ' is not the one consideration of the micro- 

 scnpist; he often, as indicated above, has to take a comprehensive 

 view of the whole object or objects of his research, apart from the 

 most complex and delicate details. 



It is folly to suppose that because great apertures have been 

 proved theoretically and practically to be able to open out minute 

 structure so perfectly, therefore there is no raison d'etre for small 

 apertures. Low amplification cannot render distinctly visible de- 

 tails beyond a certain limit of minuteness, and wide apertures 

 cannot be utilised unless there is a concurrent linear amplification 

 of the image which is competent to exhibit to the eye the smallest 

 dimensions which are by optical law . it/tin the reach and grasp of 

 such an aperture. 



In the same way great amplification will be useless if we have 

 small apertures which delineate details of dimensions only capable 

 of being distinctly seen in an image of much lower amplification. 



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