4-00 



S. J. SHAND 



if the stage of the microscope happens to be ruled with cross-lines, 

 as is often the case. 



The method having been described, it remains to add some 

 details about the construction of the instrument. In making the 

 first model, we took the micrometer screws out of two small spherom- 

 eters; these had just the right length (about i^ inches) and pitch 



Fig. 4. — Photograph of original micrometer 



(0.5 mm.) and were used almost without modification. The num- 

 bers on the graduated disk run in the correct direction for screw L, 

 but must be reversed for screw R. These screws proved, on trial, to 

 be unsatisfactory, having been taken from a cheap type of spherom- 

 eter (the only kind obtainable in South Africa at the time), and 

 better ones have since been substituted for them by Messrs. Swift 

 & Son, London. It is of course essential that the screws shall be 

 machine-cut with the highest degree of accuracy; that the axis 

 shall be perfectly straight; and the graduated disks truly plane 

 and set exactly at right angles to the axis. The precise pitch of the 



