114 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



instrument and the main and draw tubes have the Society screw. 

 The coarse adjustment is effected by rack and pinion on the 

 " Jackson " principle, and has about 3 in. of motion. The fine 

 adjustment, which appears to be both simple and efficient, is effected 



Fig. 9. 



by the application of a worm-wheel and tangent-screw to the axis of 

 the pinion of the coarse adjustment. The worm-wheel is on this axis, 

 near the limb, and it is acted upon by the tangent-screw being sprung 

 against it, the milled head of which is shown behind the limb 

 in fig. 8. When the coarse adjustment is in use, a " snail "-shaped 



