OBJECTIVES AND OCULARS 15 



or lowering it until the bright circle seen upon the glass or paper 

 attains its minimum diameter. 



Oculars to be used on the chemical microscope should have 

 the plane of the eye-circle at such a distance above the eye-lens 

 as to permit the adjustment of drawing or other prisms to the 

 position of maximum brightness and diameter of field. 



Compensating or Compensation oculars are eyepieces specially 

 designed for use with apochromatic objectives. They are so 

 called because of the fact that they aid in the correcting of 

 chromatic aberration. 



Oculars are said to be par-focal when they are so constructed 

 as to permit their interchange on the microscope without dis- 

 turbing the focus of the instrument. 1 



Compensating oculars are usually par-focal. 



Projection Oculars, as their name implies, are used in photog- 

 raphy or with the projection microscope. Their purpose is the 

 projection of a bright and clear image upon a screen whose dis- 

 tance from the ocular may be varied. This is accomplished by 

 having the eye-lens of the ocular movable in the mount, thus 

 changing the distance between eye-lens and ocular diaphragm. 



Goniometer oculars are eyepieces provided with cross-hairs and 

 graduated circle. They are used for the measurement of crystal 

 angles and may be substituted for a rotating graduated stage 

 and thus permit angular measurements on any microscope whose 

 tube they fit. 



The Care of Oculars. — In general the suggestions made with 

 respect to objectives on pages 10 and n apply with equal 

 force to eyepieces. 



To remove cross-haired oculars grasp them firmly between the 

 fingers by the milled head and first lift them free from any slot 

 into which a stud upon them may fit, then remove them by a 

 screw motion. 



Dust on the ocular lenses may be located by raising and turn- 

 ing the entire ocular, then by unscrewing and turning first the 

 field lens, then the eye-lens. If both lenses are clean and the 



1 For a consideration of the conditions to be fulfilled in their construction, see 

 Gage, The Microscope, p. 47. Tenth ed. 



