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.^ 
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 lo and ii 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 
^ For a consideration of the conditions to be fulfilled in their construction, see 
Gage, The Microscope, p. 47. Tenth ed. 
