182 THE USE OF THE MICROSCOPE 



also keeps dust from eyepieces and objectives. The rack- 

 work should be tightened enough to hold up, without 

 any slip, the nosepiece with perhaps four objectives, and 

 a binocular attachment, or a Phoku or similar attachable 

 camera. The fine motion by micrometer screw should be 

 strong enough to be uninjured by the above weights, and 

 should not be exposed to pressure or shock. The best 

 and most-used methods for fine motion are : a lever plus a 

 screw; a differential screw with coarse threads; a combina- 

 tion of a worm, or similar incHned plane, and a screw; and 

 toothed wheels plus a lever. 



Care of the Eyepiece. — The eyepiece should be kept 

 free from dust, and, hence, if the whole microscope is not 

 covered by a bell jar, cloth cap, cardboard cover, etc., at 

 least a cap should be kept over each eyepiece when not 

 being looked through. Such caps could be made of alumi- 

 num, papier mdche, cardboard, or even cloth. The eye- 

 pieces should not be dropped, especially those with 

 cemented lenses. Hence there is perhaps an advantage 

 in having them ''sprung" into the drawtube. The sides 

 of the "sprung-in" eyepieces can be cleaned with alcohol 

 occasionally with advantage, or slightly smeared with 

 vaselin. If the upper surface of the eyelens is at all 

 smeared, there will be a corresponding loss of definition, 

 and an optical smear is often only visible with a hand lens. 



Dust in the Microscope. — On focusing down, with the 

 low power and a dusty microscope, there are seen besides 

 the dust on the eyepiece (and prism, in a binocular), 

 first the dust on the two surfaces of the accessory lens 

 below the condenser, then that on the surfaces of the 

 reflecting prism (or mirror), and finally the dust on the 

 surfaces of the color screen. On continued focusing 

 down, the two surfaces of the cover-glass are passed, and 

 the dust on the upper and lower surfaces of the slide is 

 seen. Lastly, dust on the upper surface of the top con- 

 denser lens may come into focus. 



On looking at the back of the objective, smears of oil on 

 the low-power objective are visible; bubbles in the immer- 



