MANAGEMENT OF THE MICROSCOPE. 



14:7 



PIG.' 113. 



it be confidently asserted that the solution of the most difficult Biological 

 problems to which the Microscope has been yet applied, has been attained 

 by arrangements by no means the most favorable to the discernment 

 of the markings on~ Diatom-valves or the lines on Nobert's test-plate; and 

 that, conversely, the arrangements specially 

 effective for the ' resolution' of the most 

 difficult lined ' tests ' have not, as yet, been 

 shown to have much value in Biological 

 investigation ( 158). 



147. There are many kinds of Transpa- 

 rent objects especially such as either con- 

 sist 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 ( 

 104, 105); for not only does the brilliant 

 luminosity which they then present, in 

 contrast with the dark ground behind them, 

 show their forms to extraordinary advan- 

 tage; but this mode of illumination imparts 

 to them an appearance of solidity which 

 they do not exhibit by ordinary transmitted 

 light ( 103); and it also frequently brings 

 out surface-markings which are not other- 

 wise distinguishable. Hence when any ob- 

 ject is under examination that can be sup- 

 posed to be a good subject for this method, 

 the trial of it should never be omitted. For 

 low powers, the use of the Spot-lens or the 

 Webster Condenser with the central stop 

 will be found sufficiently satisfactory; for 

 the higher, the Paraboloid or the Keflex Il- 

 luminator should be employed. Similar 

 general remarks may be made respecting 



the examination of objects by Polarized Valve of Pleurosigma form osum, 



light (8110). Some Of the most Striking with portions A, B,C, D, showing diverse. 



effects of this kind of illumination are efl 



produced upon bodies whose particles 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 raphides of Plants are much more clearly made 

 out by its means, in the midst of the 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 den- 

 sity of their component particles, the existence of which would not other- 

 wise 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. 



148. Arrangement for Opaque Objects. There are many ofrjects of 

 the most interesting character, the opacity of which entirely forbids the 

 transmission of light through them, 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 dimensions, 



