A. ASHE ON TWO-SPEED FINE ADJUSTMENTS. 



135 



turned together the movement would be reduced to -^^j- inch. 

 Of course other ratios may be given, but whatever they may be 

 it is obvious that the central screw must be coarser than the 

 outer, as otherwise a right- or left-handed rotation of A would 

 not produce a motion in the same direction as that of A and b 

 together. 



It will be seen that, quite apart from the question of the 

 two speeds, the foregoing design of fine adjustment has all the 

 theoretical advantages of the differential screw, while, being 

 simpler in construction, it ought to prove more satisfactory. 



Fig. 3. 



There is yet another common type of instrument in which the 

 limb, actuated by a lever, moves in a slot cut in the pillar. To 

 this microscope the adjustment just described cannot be applied, 

 and as the differential screw is unnecessary for the purpose, I 

 removed and substituted for it a fine micrometer screw, lea\dng 

 the rest intact as in Fig. 1. Learning, however, that Messrs. 

 R. and J. Beck were also making experiments with the same end 

 in view, I placed myself in communication with them, with the 

 result that they entered most readily into the subject ; and I 

 now wish to thank them most heartily for the trouble they have 

 taken in making experiments to ensure success. 



