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better view is obtained than with the other and 

 use the one giving the best results. 



As already stated, make it a habit at the outset 

 to keep both eyes open. 



There is one point over the lens called the eye- 

 point at which the rays cross within the smallest 

 compass and this is the proper position for the eye, 

 as the largest number of rays enter it. When 

 above or below this point the size of field will be 

 either reduced, or shadows or colors will appear in 

 it. In low power eyepieces the eye-point is farther 

 than the lens ; in high powers quite close in fact, 

 in some so close that the eyelashes may rest upon 

 the lens and may sometimes appear to be in the 

 field as dark bars. Generally speaking the best 

 point is where the entire field is seen and its mar- 

 gin (diaphragm) sharply defined. 



What Objects to Use. Suitable objects to 

 test the capacity of objectives are also valuable in 

 leading the student to a skillful use of the instru- 

 ment and in giving him proper judgment. 



Low Powers Proboscis of blow-fly. This should 

 be flat and transparent. For 1 inch, f and | inch 

 objectives the scales from Lepisma sacharina. 



Medium Powers Pleurosigma angulatum, dry, 

 stained Bacteria and Micrococci. 



High Powers Oil immersion ^ inch and -^ inch 

 objectives, Amphiplerua pelucida, Surirella gemma 



