14 ELEMENTARY CHEMICAL MICROSCOPY 



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 

 distance 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 9, 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 

 turning the entire ocular, then by unscrewing and turning first 

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

 the objective is clean yet the field shows specks of dirt and ap- 

 pears blurred, the dust and dirt will be found to be on the disk 

 carrying the cross-hairs or micrometer scale. Exceeding great 

 care is required in cleaning cross-hairs and micrometer plates 

 resting upon the diaphragm of the ocular and should only be 

 undertaken by a person having patience, care and steady nerves. 



Use low oculars first and confine the work whenever possible 

 to medium powers. Have recourse to high power oculars only 

 as a last resort, since they cut down the light to such an ex- 

 tent as to cause fatigue and eye-strain. 



Always look into a microscope with both eyes open. 



In the study of flat preparations between slides and cover 

 glasses, the general rule is to obtain the proper magnification 

 chiefly by means of the objective, using a low power ocular. 

 But in the case of irregular surfaces or curved and heaped up 

 drops of liquid, the reverse is essential and low power objectives 



