being negative, the others positive. ' Another kind is 

 the projection eyepiece which is intended for projecting an 

 image on a photographic plate, or on a screen or wall. 



Defects. As has been stated many eyepieces are 

 carelessly constructed and possess defects which inter- 

 fere with obtaining a distinct image. These defects 

 do not show easily with low power objectives, but can 

 readily be seen with high powers. The most frequently 

 occurring fault is the lack of perfect grinding and 

 polishing of the lens surfaces. If the former, it will 

 show itself as spots in the field, and if the latter, as a 

 series of streaks and shadows, usually circular in form 

 as if the lens had been wiped with greasy fingers. 



Another defect may be in the glass itself, in the 

 so called striae, which will be indicated by dark and 

 light streaks across the field. 



Care must be taken not to confound small particles 

 of dust which are apt to fall upon the field lens and 

 which at times are very prominent in the field, with 

 imperfections of the surface. These can be dis- 

 tinguished from other defects only by wiping, or using 

 a camel's hair brush, and even with the utmost care 

 some particles are liable to remain. 



The eyepiece often fits too closely in the tube and 

 when making observations with high powers, a change 

 of eyepiece is apt to disturb the object. It should 



