238 ON THE TnEOIlY OF THE MICROSCOPE. 



taken up when direct illumination was used now enter more 

 completely and work with greater effect, whilst the direct rays 

 are relatively less operative. Apart, however, from this, there 

 frequently occur, during ordinary observations, such accidental 

 moments of oblique ill'imination as may produce the effects 

 described in paragraph v.; consequently in every object which 

 presents two sets of strias fairly homogeneous with each other 

 several additional sets may, by a change in the incidence of light, 

 be brought into view and become visible in different directions, 

 provided the angular aperture of the objective employed has a 

 suitable relation to the finen"ss of striation, as clearly appears in 

 the case of vaiious Diatoms. Even the forms of illumination 

 which produce effect s''(sucli as are described in paragraph 4) may 

 occur unintentionally. In this way, for instance, must be ex- 

 plained the appearance of fine longitudinal lines between the 

 coarser real lines of Hipparchia Janeira which high power 

 objectives exhibit under certain positions of the mirror. 



XYII. The facts here recounted appear sufficient, when taken 

 in connection with incontestible laws of the theory of undulation, 

 to warrant a series of most important conclusions which affect the 

 doctrine of microscopic vision, as well as the composition and 

 manipulation of the microscope. 



Firstly as respects the vision of objects under the microscope. 

 Any part of a microscopic preparation which, either from its 

 being isolated (e.g., granules, single threads, or fibres), or from 

 its relatively large dimensions (namely, to vrave length of light), 

 produces no perceptible diffractive effect, is delineated in the 

 field of the microscope as an image formed according to the usual 

 dioptric laws of rays concentrating in a focal plane. Such an 

 image is entirely negative^ being dependent on an unequal 

 transmission of light which partial absorption of the rays (cg.^ 

 colored rays), or divergence of the rays (from refraction), or 

 diffraction of the rays (produced by pai tides of internal structure), 

 severally occasion. The absorption image thus produced is an 

 unquestionable similitude of the object itself, and if correctly inter- 



