no ELEMENTARY TECHNOLOGY. 



j. The mechanism of the microscope. 



Mechanical contrivances are for holding the optical parts in 

 proper position and for convenience in manipulation. Many 

 forms in use. Three classes. 



First class, made of hard metal so that screws and racks 

 allow more work without "backlash." Most elaborate every 

 way. Not mere expensive luxuries, since the most delicate 

 work requires the best instruments. 



Second class, or students' microscopes, have stands of 

 fair workmanship but reduced to great simplicity. May have 

 objectives equal to the first class but adapted to moderate 

 powers and everyday work. 



Third class, or educational, are cheap stands and lenses, fit 

 for school and family demonstrations but not for research. 



' i. The base. Two principal forms, the horseshoe and the 

 tripod. The latter best. Must be firm with the instrument in 

 any position. 



2. The arm carrying the body must be free from vibration. 

 English and American microscopes have two forms, the " Ross 

 Model," with end of tube on arm, and the "Jackson," with 

 arm on side of tube. The Powell & Lealand stands have the 

 Ross Model and are good, but a cheap stand on Ross Model 

 is worst of all. Jackson form most used, but the supporting 

 arm is often too short. 



j. Inclination of body. French and German students' 

 instruments can only be used perpendicularly. An inclinable 

 body and stage are best. 



^. Coarse and fine adjustment. The coarse adjustment is 

 often by rack and pinion. Sliding tubes are fairly efficient if 

 cloth lined. The fine adjustment is by various screw mechan- 

 ism in different instruments. If attached to the stage it is only 

 useful if well made, and with moderate powers. 



5. Telescope body tube is a sliding combination, or draw tube, 

 useful for varying magnifying power and to adjust for different 

 objectives or thickness of cover glass. 



6. Stage should be large and firm, yet thin, to allow oblique 

 illumination. Mechanical stages are useful but not necessary. 



7. Mirror should be flat on one side and * concave on the 



