THE FINE ADJUSTMENT 



plant the direct-action screw, where the form of the microscope may 

 appear to make that a necessity. This has been the case with the 

 Continental model. It was applied by its inventor to a microscope 

 made by himself, and was brought before the Quekett Club by Mr. 

 E. M. Xelson. 



It is very simple, and is made by cutting two threads in the 

 micrometer screw. Fig. 126 will illustrate the exact method. D is 

 the milled head of the direct-acting screw. The upper part, S, of 

 the screw has (say) twenty threads to the inch, and the lower part, T, 

 twenty-fix < threads to the inch. B is the fixed socket forming part 

 of the limb of the microscope, and H is the travelling socket con- 

 nected with the support of the body-tube. The revolution of I) 

 s the screw thread S to move up and down in B at the rate of 



FIG. 126. Campbell's 

 differential screw 

 fine adjustment 

 (1886). 



PIG. 127. Zeiss's usual ' new ' fine adjustment (1886) 



twenty turns to the inch, whilst the screw thread T causes the 

 travelling socket H to move in the reverse direction at the rate of 

 twenty-five turns to the inch. The combined effect, therefore, of 

 turning D twenty revolutions is to raise or lower T, and with it the 

 body-tube |th of an inch, or ^th of an inch for each revolution. 

 The spiral spring below H keeps the bearings in close contact. 



Of course any desired speed can be attained by proper combina- 

 tion of the threads : thus 32 and 30 would give ^th of an inch 

 for each revolution, and 31 and 30 would give ^-j^th of an inch. 



This screw has provided for the Continental model what Swift's 

 vertical lever has done for the Jackson model ; Mr. Baker, of 

 Holborn, has adopted it and with very satisfactory results ; for it 

 has passed through that most crucial of tests for a fine adjustment, 

 its employment in photo-micrography, with excellent results ; and 



