272 THE ELEMENTS OF MICROSCOPY. 



over to the right, so that they are at the ends of the base of a 

 triangle of which the object occupies the apex, and the effect will 

 be assisted by placing a black cloth under the microscope, so that 

 no light may be reflected from the table. Or the bull's-eye may 

 be brought into play by focussing the rays from the lamp obliquely 

 on the underside of the object. If the obliquity is not sufficient, 

 the field will not be quite dark, but it should not be greater than 

 is required, or the object will lose greatly in brightness. 



Other methods are the spot-lens and the paraboloid of Wenham. 

 In the former the centre of the lens is blackened, so that only the 

 marginal rays reach the object ; in the latter, the apex of a para- 

 boloid glass cylinder is cut off and the central portion blackened, 

 so that all the rays which pass through the parabola parallel to the 

 axis illuminate the object, being internally reflected, whilst the 

 latter is itself seen on a black background. Some very beautiful 

 effects are obtainable by the last appliance, and as its theory is 

 perfect it is a pity that it only works in practice with low powers. 

 The finest effects by far in dark-field illumination are obtained by 

 stopping out the central rays in the cone of light from the sub- 

 stage condenser. Even in the simplest form, above described, the 

 stops which are supplied with it give such perfect dark-field and 

 such brilliancy to the object, as are far beyond the reach of a 

 paraboloid or spot-lens, costing twice as much, and of no use for 

 any other purpose, whilst with the higher grades of substage con- 

 denser, an image obtained by stopping out the central rays is 

 almost as perfect as that given by the ordinary method, and com- 

 paratively high powers, as high as a }iih inch, can be used for 

 observation of it. Whichever method,^ however, is adopted, the 

 same remarks as before will apply to the necessity for bringing the 

 light to a focus upon the object, and in conclusion we would 

 impress the necessity of not using light of any greater obliquity 

 than will suffice to make the background dark. With some 

 extremely delicate objects there is very great advantage in slightly 

 darkening the field, as it may happen that the details are drowned 

 in a flood of light, and mere reduction of the light alone will have 

 to be carried almost to the point of invisibility before they are 

 apparent, whilst a very slight interruption of the central beam may 

 suffice to throw them into high relief This plan will often 



