Appendix A : The Microscope and Its Optical Principles 159 



ocular, or both, and after accurately centering the object in the 

 field, the high power is attached. In case a revolving nosepiece 

 is used, great care should be used in turning in the high power 

 not to strike the slide with the objective. This is very likely to 

 happen if the objectives are not parfocal. 



9. After the object is in focus give any further attention to 

 the illumination that is necessary (see illumination and mirror}. 

 If intensified illumination is desired, use the concave mirror, or 

 use the substage condenser and the plane-mirror. For ordinary 

 purposes the field should be evenly illuminated, although oblique 

 light is frequently useful. Manipulate the diaphragm until the 

 structure to be studied shows with the greatest distinctness. Too 

 much light "drowns" the object, and is hard on the eyes. (To 

 determine the proper distance at which the concave mirror should 

 stand below the stage, let direct sunlight shine upon the mirror, 

 and then adjust the latter so that the apex of the cone of light 

 comes just at the top of the stage where the object will rest.) 



10. In using oil-immersion objectives, a small drop of cedar 

 oil (specially prepared by the maker of the lens) is applied to 

 the front lens by means of a small rod or brush. It is very 

 important to keep the oil free from dust, and to see that it does 

 not contain air bubbles when applied to the lens. Carefully lower 

 the tube until the oil on the objective comes in contact with the 

 cover-glass. The operator should lower his head to the level of 

 the stage to observe this properly. Focus up as with a dry 

 objective. With a piece of lens paper or a soft cloth, clean the 

 immersion lens immediately after you have finished using it. 

 Likewise remove the oil from the cover-glass. 



11. The range of the fine adjustment is limited. Keep it as 

 near the middle point as possible. If the tube does not respond 

 to the movement of the screw you have probably gone beyond the 

 range of the fine adjustment. 



12. In working with the microscope keep both eyes open. The 

 eye which is not in use soon becomes accustomed to ignoring objects 

 in the field of vision. To avoid fatigue it is well to use first one 

 eye and then the other for observation. The eye should be placed 



