and it is easily recognized when better definition is 

 obtained in this way. 



Do not use so large an opening that there will be 

 an uncomfortable glare, nor so small that undue 

 exertion is required to see structure. 



When oblique light is used there should be no 

 obstruction to the course of light. If a cap diaphragm 

 is used with central illumination, it should be entirely 

 removed with oblique illumination. With the iris 

 diaphragm the full opening should be employed. In 

 the case of the revolving diaphragm which generally is 

 some distance from the object it should be removed if 

 possible, and if not, the largest opening should be used. 



Which Eye to Use. The right eye is generally 

 used for observations, but while the manipulator may 

 from habit be inclined to use this, it may be possible 

 that in some cases the left can be used to better 

 advantage and with less fatigue. It is a fact well 

 known to oculists and opticians that many eyes are 

 defective, of which fact the possessors may not be 

 aware. Short or long sightedness has little or no 

 influence in viewing an object, except to require a 

 different adjustment, but so-called astigmatism, a defect 

 of the eye which makes it impossible to see in a certain 

 axis distinctly, may influence the best results. If this 

 error is corrected by wearing glasses and these are used 

 while making observations, either eye can be used. 



13* 



