We therefore get the greatest effectiveness out of an 

 objective, the largest field, the greatest penetration, and 

 the best illumination, by using the lowest magnifica- 

 tion which makes all the detail in the image visible. If 

 we increase the magnification beyond this point we do 

 so at the expense of other good qualities. 



Lengthening the tube increases the magnification 

 proportionately. 



FINAL HINTS 



Sometimes the worker may have faithfully carried out 

 all the directions heretofore given and been assured 

 that his lenses possess the above named qualities as they 

 ought, yet be unable to obtain the desired results. He 

 may be working with a water mount and his dry objec- 

 tive become "immersed" in some water which has 

 worked to the top of the cover glass. His objective may 

 be dirty from a previous "immersion," or it may have 

 some other dirt upon the front lens. The field may be 

 covered with specks which revolve when the ocular 

 is turned. The field may be dim or hazy, due to dirt 

 on the back of the objective or a film on the inner sur- 

 faces of the lenses of the ocular, or because of moisture 

 settling on the lenses because they have just been brought 

 from a cold into a warm room. He may see great streaks 

 on his field, which are due to his own eye lashes, or he 

 may see small, slowly moving bodies floating across the 

 field. With the exception of this last, the ailment has 

 only to be mentioned to suggest the remedy. The 

 muscae volitantes, as these last named bodies are called, 

 are little specks or shreds in the vitreous humor of the 

 eye which cannot be removed, but which can easily be 

 disregarded. 



In water mounts and fresh balsam mounts one is apt 

 to find air bubbles. Tc be sure, that the ^object is an 

 29 -. 



