55 



as it is one which is easily overlooked and 

 is often the cause of considerable mischief. 

 If the light comes through a window a well defined 

 image of the sash is reflected by the mirror, and with 

 a lower power objective this can easily be seen ; 

 unless the mirror is correctly adjusted the field will 

 appear to be crossed by dark bands. In the case of 

 lamp light the flame is reflected and has a similar 

 effect. With high powers this fault is not so easily 

 noticed, and for this reason require the more care ; 

 proper resolution may in this manner be partially or 

 totally destroyed. The remedy is either by shifting 

 the mirror or by varying its distance from the object. 

 More information on this subject is given under the 

 head of Sub-stage Illumination. 



Attaching High- Power Objectives. As the 



difficulty of properly getting an object or a certain 

 portion of it in the field, increases with the magnifying 

 power, it is a good rule to use the lower power objec- 

 tive as a " finder;" after getting the point to be further 

 examined in the center of the field, remove the objec- 

 tive and attach the higher power, and after following 

 the procedure of focusing as with the low power, 

 except that the objective should be brought almost 

 in contact with the cover, the point will be seen in 

 the field or will be found to be close to it. 



This plan of focusing as suggested above is always 

 a good one to follow and is observed by many of the 

 most expert manipulators. In many cases, however, 

 the focus is obtained without this precaution, by 



