ARRANGEMENT FOR OPAQUE OBJECTS. 175 



well seen under the black-ground illumination ( 61, 62) ; for not 

 only does the brilliant luminosity which they then present, con- 

 trasting remarkably well with the dark ground behind them, 

 show their forms to extraordinary advantage ; but this mode of 

 illumination imparts to them an appearance of solidity, which 

 they do not exhibit by ordinary transmitted light ( 62) ; and it 

 also frequently brings out surface-markings, which are not other- 

 wise distinguishable. Hence, when any object is under exami- 

 nation, that can be supposed to be a good subject for this method, 

 the trial of it should never be omitted. For the low powers, the 

 use of the "spotted lens" will be found sufficiently satisfactory ; 

 for the higher, the paraboloid should be employed ( 61). Similar 

 general remarks may be made, respecting the examination of ob- 

 jects by polarized light. Some of the most striking effects of 

 this kind of illumination, are produced upon bodies whose par- 

 ticles have a crystalline aggregation ; and hence it may often be 

 employed with great advantage to bring such bodies into view, 

 when they would not otherwise be distinguished ; thus, for ex- 

 ample, the raphides of Plants are much more clearly made out by 

 its means, in the midst of the vegetable tissues, than they can be 

 by any other. But the peculiar effects of polarized light are also 

 exerted upon a great number of other organized substances, both 

 Animal and Vegetable ; and it often reveals differences in the 

 arrangement or in the relative density of their component par- 

 ticles, the existence of which would not otherwise have been 

 suspected ; hence, the Microscopist will do well to have recourse 

 to it, whenever he may have the least suspicion that its use can 

 give him an additional power of discrimination. 



91. Arrangement for Opaque Objects. Although a large pro- 

 portion of the objects best suited for Microscopic examination 

 are either in themselves sufficiently transparent to admit of being 

 viewed by light transmitted through them, or may be made so by 

 appropriate means, and although that method (where it can be 

 adopted) is generally the one best fitted for the elucidation of the 

 details of their structure, yet there are many objects of the most 

 interesting character, the opacity of which entirely forbids the 

 use of this method, and of which, therefore, the surfaces only 

 can be viewed, by means of the incident rays which they reflect. 

 These are, for the most part, objects of comparatively large di- 

 mensions, for which a low magnifying power suffices ; and it is 

 specially important, in the examination of such objects, not to 

 use a lens of shorter focus than is absolutely necessary for dis- 

 cerning the details of the structure ; since, the longer the focus 

 of the objective employed, the less is the indistinctness produced 

 by inequalities of the surface, and the larger, too, may be its aper- 

 ture, so as to admit a greater quantity of light, to the great im- 

 provement of the brightness of the image. It is surprising how 

 little attention has been given by Opticians to the construction of 

 objectives suitable for this purpose. In their zeal for the im- 



