76f DARK-GROUND ILLUMINATION WITH HIGH POWERS [Cn. II 



spot of light giving the indication when the mirror is in the right 

 position. Slight changes only will then be needed on looking into 

 the microscope to get the best effect. 



131. Practical application of dark-field microscopy. In the 

 practical application of dark-field microscopy it is almost self-evident 

 that it can be used successfully only with scattered objects, that is 

 objects with blank space between them. If the objects covered the 

 whole field, then the field would all be bright, and there would be 

 no dark-ground effect. It is not then applicable to the study of 

 microscopic sections, for they cover many times over the entire field 

 with high powers. But in Biology, using the word in the compre- 

 hensive sense employed by Huxley, it is applicable and will also 

 surely give much information in the following cases: 



04) In the study of unicellular organisms in both the plant and 

 the animal kingdoms (i.e., bacteria and all other plant micro-organ- 

 isms, and the animal micro-organism). 



(B) In the study of multicellular organisms among both plants and 

 animals it is especially applicable to their fluid parts, and to their 

 individual cells when properly isolated. In the vertebrates, in- 

 cluding man, tnis would apply especially to the blood and lymph 

 with their granular contents, the tissue fluids and the fluids of the 

 natural spaces like the pleural and pericardial cavities, the peri- 

 toneal cavity, and the liquids found in the cavities of the central ner- 

 vous system, the joint cavities and the tendon sheaths. It is also 

 of great help in the study of the liquids found in mucous containers, 

 as milk, bile, urine, saliva and indeed in the study of all the liquids 

 of the body. 



Dr. Chamot points out its help in the study of foods, fibers, crys- 

 tallization phenomena, sub-microscopic particles and colloids. He 

 adds further (p. 40): " This method is invaluable for demonstrating 

 the presence of very minute bodies or those whose index of refraction 

 is so nearly the same as that of the medium in which they occur as 

 to cause them to escape detection when illuminated by transmitted 

 light," i.e., the ordinary light used for bright-field microscopy. 



132. Summary of steps necessary for successful dark-field 

 observation. (i) A powerful source of light must be provided. 



