ILLUMINATING APPARATUS 33 



so that one or more selected beams of oblique light may be used to 

 illuminate the object. An experiment with finely ruled parallel 

 lines shows that if a small oblique beam of light is used to illu- 

 minate them at right angles to their length, finer lines can 

 be distinguished than with direct light ; but it is doubtful 

 whether this method is of any advantage for ordinary objects, 

 and it is liable to give rise to quite erroneous impressions of 

 structure. 



Dark-ground illumination is obtained by throwing light upon 

 the object in such a manner that the object is illuminated, but 

 that none of the light enters the microscope except that reflected 

 by the object itself. 



The illuminator must be capable of throwing light upon the 

 object at a greater angle than can be received by the object glass in 

 use. The illuminator must have a larger aperture than the 

 object glass. Dark-ground illumination with a substage con- park-ground 

 denser in which the object is illuminated by light that is so oblique '^'th^*^^^" 

 that it cannot enter the object glass is accomplished by placing substage 

 a glass with a central black patch below the condenser and by 

 opening the iris diaphragm to its full extent, as shown in Fig. 18, 

 page 27. With the Abbe form of condenser this method is useful 

 for low powers — IJ-inch (32-mm.) and 2/3-inch (16-mm.) — but is 

 not sufficiently corrected to cut off the central light with the 

 accuracy required for a high power. It will perform fairly well with 

 a 1/3-inch (8 mm.) achromatic, which has an aperture of '5 N.A., 

 but not for lenses with a larger aperture. With the achromatic 

 or immersion condenser, dark-ground illumination can with care 

 be used with a 1/6-inch (4-mm.) object glass, but it is better to use 

 a special liigh-power illuminator described later for the 1/6-inch 

 (4-mm.), 1/8-inch (3-mm.), and the 1/12-inch (2-mm.) object glass. 

 This is partly because the high-power illuminator is of shorter focal 

 length and gives more brilliant illumination, and partly because 

 the stop of a substage condenser is 

 some distance below the lenses and 

 allows some light to spread round the 

 stop employed. It does not produce 

 so black a background. 



For dark-ground illumination with ^^^^""^f^^;^ | pat'htp'ot 

 a condenser, an adjustable stop in- 

 vented by Mr. Traviss (Fig. 22), on 

 the principle of a reversed iris dia- 

 phragm, is a very convenient appliance. Fig. 22. — No. 3284, Tra- 

 By moving the handle the size of the viss Patch-stop, 



central patch is enlarged or diminished. 



The high-power dark-ground illuminator is a reflecting device High-power 



or o J! "11 ' n dark-ground 



by means of which a very small image or trie source or luurmna- iuomiuator. 

 tion is focussed upon the object, and this image is formed by rays 

 of light which fall upon the object at a very oblique angle. Fig. 23 



3 



