85 SELECTION AND USE 



object is actually thrown out of the field of view. Such a 

 microscope should be at once condemned. 



It is a common fault with instruments in which the coarse 

 adjustment is made by rack and pinion, that the body cannot 

 be raised high enough to admit the use of low power objec- 

 tives. This defect should be avoided. Avoid, also, all instru- 

 ments in which the coarse adjustment is made by moving the 

 stage by means of a rack and pinion. 



Tlie 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 

 in 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 of view is con- 

 tracted. The action in this case, however, is somewhat com- 

 plex, owing to the action 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. 



