THE OPTICAL EQUIPMENT 59 



inch or 4-mm. apochromatic objective may be used with this 

 illuminator ; but if an oil-immersion objective such as a 

 one-twelfth-inch or 3- or 4-mm. apochromat is preferred, then 

 the objective itself must be reduced in numerical aperture. 

 This is most simply effected by putting in one of the small cone 

 fittings which drop inside the objective itself, and which have 

 already been described as a method of efficiently reducing the 

 numerical aperture of an objective. 



The dark-ground condenser of Leitz is shown diagrammati- 

 cally in Fig. 15 b, where the path of the rays is indicated. It will 

 be observed that the method depends on total reflection within 

 the apparatus itself, the light being ultimately condensed on 

 to the object ; the object, in fact, occupies such a position that 

 the whole of the light is focussed on to it. It is as sensitive 

 to all variations of object-slide, cover-glass, &c., as the Zeiss 

 apparatus, and in use all the points already enumerated should 

 be most carefully attended to. The results to be obtained with 

 it are in every respect most satisfactory, and, owing to its 

 construction, it is possible to obtain an even more exact 

 concentration of the rays and a higher degree of correction 

 than with the paraboloidal form. 



The ultra-microscopic method of Siedentopf is not so 

 clearly of use in photo-micrographic work. It is intended for 

 the illumination of ultra-microscopic particles only; and, as 

 these usually exhibit Brownian motion to a great degree, the 

 possibilities of obtaining photographic records are generally 

 slight. Further, as the images obtained are themselves not 

 representations of the structure or detail of the object, but 

 are mere diffraction discs, which only suffice to indicate the 

 presence of an object and not to give a representation of its 

 form, any further description of it here would appear to be 

 unnecessary. 



Another ingenious method of obtaining dark-ground 

 illumination is that designed by Mr. J. W. Gordon, which 

 differs essentially from those already described. In this 

 method, illumination of the object is effected by a solid cone of 

 light in the ordinary way, and the central rays are obstructed 

 either at a point between the objective and the eye-piece, or at 

 a position above the eye-piece known as the Kamsden disc. 

 Either arrangement will unquestionably result in obtaining 



