is used. For counting blood corpuscles a 5mm (3-") or 

 a narrow angled 4mm (") with a long working distance 

 for working through the thick cover of the blood counter 

 is used. For entomological work and other low power 

 work a 4Omm (if") is very desirable. If bacteriological 

 work or special cytological work is to be done, an immer- 

 sion objective is indispensable. The 2mm (yVO 1S m ost 

 used. For extreme work the i.5mm (yVO ls called into 

 use. An immersion objective is one which is so corrected 

 that a drop of oil (water, if it is a water immersion) 

 must be used between the front lens and the cover glass. 

 They are used like any other objective excepting for the 

 placing of this drop of oil and for the fact that their 

 shorter working distance requires greater care in hand- 

 ling. It is best to place the drop of oil upon the front 

 lens of the objective with the little wire or rod which 

 accompanies every immersion oil bottle. Any bubbles 

 or dirt in the oil can be more easily detected in this way. 

 It is a little unhandy to do this where the objective is 

 screwed on the nosepiece. Many workers prefer to put 

 the drop on the cover below the objective. Great care 

 should be taken to exclude dirt or air bubbles. Dirt 

 particles are apt to scratch the lens, and bubbles set up 

 refractions which greatly interfere with the well working 

 of the lens. If the bubbles are present they can be easily 

 seen by removing the ocular and looking down into the 

 objective. They must be removed even though it be 

 necessary to wipe off all the oil and start over. The 

 bubbles are very apt to occur if the oil contact is broken 

 several times in an attempt to focus. If the full aper- 

 ture of the objective (or any aperture greater than N. A. 

 i.o) is to be used the condenser must also be immersed 

 as described on page 20. 



Where the very best possible resu.ts are demanded the 

 apochromatic objectives with compensating oculars are 

 24 



