160 MANAGEMENT OF THE MICROSCOPE. 



directions. A good example of the variety of appearances which 

 the same object may exhibit when illuminated in different 

 modes is shown in Fig. 88, which represents portions of a valve 

 of Pleurosigma formosum as seen under a power of 1300 

 diameters ; the markings shown at A, b, and c are brought out 

 by oblique light in different directions ; whilst at d is shown the 

 effect of central illumination with the Achromatic Condenser. 



121. There are many kinds of Transparent Objects — especially 

 such as either consist of thin plates, disks, or spicules of Siliceous 

 or Calcareous matter, or contain such'. bodies — which are peculiarly 

 well, seen under the Black-ground illumination (§§ 84, 85) ; for 

 not only does the brilliant luminosity which they then present, in 

 contrast 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 ordi- 

 nary transmitted light (§ 8b') ; and it also frequently brings out 

 surface-markings which are not otherwise distinguishable. Hence, 

 when any object is under examination that can be supposed to be a 

 good subject for this method, the ti'ial of it should never be omitted. 

 For the low powers, the use of the Spot-Lens or the Webster 

 Condenser with the central stop, will be found sufficiently satis- 

 factory ; for the higher, the Paraboloid should be employed. — 

 Similar general remarks may be made respecting the examination 

 of objects 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 example, 

 the Paphides 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 particles, 

 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 addi- 

 tional power of discrimination. 



122. Arrangement for Opaque Objects. — There are many objects 

 of the most interesting character, the opacity of which entirely for- 

 bids the transmission of light through them, and of which, there- 

 fore, the surfaces only can be viewed by means of the incident rays 

 which they reflect. These are, for the most part, objects of com- 

 paratively large dimensions, 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 discerning the details of the structure ; since, the longer the 

 focus of the Objective employed, the less is the indistinctness pro- 



