120 SELECTION AND USE 



the freedom from twist or apparent displacement of the object. 

 In many microscopes, when a high power is used, and the 

 body is moved up or down for the purpose of adjusting the 

 focus, the object is actually thrown out of the field of view. 

 Such a microscope should be at once condemned. 



Whatever be the nature of the fine or coarse adjustment, see 

 that the body has sufficient range of motion to allow of the use 

 of objectives of, considerable working distance. 



The Diaphragm. Nothing tends so much to obscure our 

 view of the finer points of structure in any object as to have 

 them " drowned " in a superabundance of light, consequently 

 iu order to regulate the amount of light which passes through 

 the object, a diaphragm is employed. As ordinarily con- 

 structed, it is simply a metal plate placed below the stage, and 

 pierced with holes of various sizes, which may be brought ex- 

 actly under the field of view, the small holes allowing but a 

 small amount of light to pass, while the large ones admit a full 

 stream. Considerable difference of opinion exists amongst 

 microscopisis in regard to the proper position of the dia- 

 phragm. Thus Carpenter says (page 133) that unless placed 

 half an inch below the object it is comparatively inoperative. 

 Continental histologists, on the other hand, allege that it is 

 useless unless placed close up under the object. Microscopes 

 constructed according to both these plans are to be found in 

 market. Where the microscope is furnished with a sub-stage, 

 the distance of the diaphragm from the object is variable at will. 



It is obvious that when the diaphragm is placed at a consid- 

 erable distance below the object, the illumination is purified, 

 as it were, from all cross rays. When the diaphragm is placed 

 close to the object-slide, the illuminated field is contracted. 

 The action in this case, however, is somewhat complex, 

 owing to the influence of the slide in modifying the course of 

 the rays. 



Several very ingenious forms of Iris or graduating dia- 

 phragms have been devised, by which the size of the hole may 

 be changed without interrupting the observation. They are 

 exceedingly convenient, and present advantages which more 

 than counterbalance the cost. 



