SOME ACCESSORIES AND THEIR USES 39 



hj interposing a bull's-eye between the flame and 

 the mirror. 



It is not usually considered necessary to use a 

 condenser with objectives of lower power than |-inch, 

 but if it is desired to use it with such, the top lens 

 of the condenser will need to be removed so as to 

 secure a larger field of illumination. The worker 

 should also note that any lens of proper size and 

 suitable focus fitted into the under-stage collar will 

 act as a condenser and improve the image. 



Certain objects of a highly transparent character 

 are examined to the greatest advantage with what 

 is known as dark-ground illumination, by means of 

 which the object is beautifully lighted and at the 

 same time thrown up clearly against a black back- 

 ground. Condensers are usually fitted with a carrier 

 into which " stops " of blackened metal can be 

 inserted. The carrier is swung out, a suitable stop 

 inserted, and swung back into position. The 

 circular stop is thus placed below the centre of the 

 back lens of the condenser, cutting off the central 

 rays of light which would ordinarily enter the objec- 

 tive. The black background is thus obtained, and 

 when an object is placed in the field the rays, which 

 would not in the usual course enter the objective, 

 are caught by the object and diffracted, so that the 

 latter is exquisitely illuminated. Many creatures 

 coming under the term " pond-life " are seen in 

 remarkable beauty by this method of lighting ; the 

 same may be said of some marine zoophytes and 

 radiolarians, foraminifera and diatoms, also hairs of 

 plants. 



Until condensers with carriers for stops for dark- 



