ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



115 



lever on the right of the limb (handle shown beneath the large milled 

 head) forces the tangent-screw from contact with the worm-wheel, 

 a spring latchet locking it in position. By releasing the "snail" 

 lever the tangent-screw is pressed into the worm-wheel, and acts upon 

 the coarse adjustment so slowly that objectives of high power can be 

 focussed with it. It will of course be understood that when the 



Fig. 10. 



tangent-screw is sprung against the worm-wheel the coarse adjust- 

 ment is no longer operative, which Mr. GriflSth considers to be a pro- 

 tection against breakage of slides. A similar system of fine focussing 

 was adopted in England many years ago, and is still used in some of 

 Plossl's models. The stage clips are supported on a bar above the 

 stage, allowing the slide to make almost a complete revolution. The 



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