a b c 



Illumination and Illuminating Apparatus 99 



oil contact to be maintained. The distance between the 

 condenser and the object will vary according to the thick- 

 ness of the slip on which the object is mounted, and the 

 intermediate contact glass will have to be selected accord- 

 ingly. To use an oil immersion condenser effectively the 

 object must be mounted in some medium, and not dry upon 

 the cover-glass. 



As before mentioned, with the majority of condensers 

 stops are supplied having the centres blocked out, as shown 

 in Fig. 33, by means of which dark-ground and oblique illu- 

 minations are obtained. Dark- « 6 

 ground illumination gives a most 

 beautiful effect to very trans- 

 parent objects, such as infusoria, 

 pond-life specimens, etc. In the ^'<^- ^^--^ZIIb^"^ ^°''' 

 form of carrier for condensers 



shown on page 91, a cell is provided just above the iris 

 diaphragm to carry the stops. One similar to a (Fig. 33) 

 is placed in the cell, the iris diaphragm is opened com- 

 pletely, the condenser having been previously adjusted in 

 the usual way, when it will be found that the object will 

 be illuminated, but the ground on which it is seen will be 

 black. Different objectives require stops of special sizes, 

 which may be readily made of blackened cardboard, cut to 

 the most suitable size for working with the objective. 



These stops can be further used for strengthening the 

 contrast in the image with large cones of illumination and 

 objectives having high apertures. This method does not 

 minimize in any way the effective working of the objective, 

 for with objectives of large aperture rays may be present 

 which only impart brightness to the field, but do not con- 

 tribute to making visible the fine detail upon the object. 

 If less than half of the lateral spectra, as shown in Fig. 32, 

 are seen on looking down the tube at the back lens of the 

 object glass with a striated object in focus, then the central 

 portion of the direct beam or central disc has no lateral 

 image corresponding to it in the portions of the spectra 



7—2 



