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utmost precision and smoothness and with abso- 

 lutely no lost motion. 



Revolving Stage.- -This is also a great con- 

 venience in all work, while being a necessity in 

 some directions, and when provided with centering 

 screws may be used to some extent as a mechani- 

 cal stage with only a limited movement however. 

 It should work freely with the rolling motion of 

 one finger, without any side play and without 

 throwing the object out of the plane of focus 

 during revolution. 



Substage.--This is an absolute necessity in a 

 modern microscope, except perhaps for students 

 use in primary work. It should have a vertical 

 adjustment and preferably with rack and pinion. 

 If possible select the complete substage attachment. 



Substage Condenser. If means will permit, 

 purchase this, as it is in all work most convenient 

 and in some directions, like bacteriology, abso- 

 lutely necessary. The Abbe condenser is the 

 cheapest form giving good results and one with 

 numerical aperture of 1.20 is sufficient in all cases 

 unless oil immersion objectives of the greatest 

 aperture are used. 



Objectives and Eyepieces. It is hoped that 

 the information given of the various qualities in 

 an objective will aid to make a suitable selection of 



