336 



0. Hants — The Comparison of Two Screws. 



time to time, i. <?., to take a fresh start, so that the comparison 

 proceeded rather in sections. Moreover, the motion of the 

 slide not strictly parallel to itself appears in the data as an 

 error of the screw and virtually it is so. The discrepancy 

 may, however, he corrected without reestablishing a fresh zero, 

 by removing the effect of any slight rotation at one mirror, 

 due to an imperfect slide or other causes, by adjustment screws 

 at the tablet of the other mirror, until the sharp lines of the 

 ellipses are quite restored. In fact, this is a method of com- 

 pensation, by which excess of length due to incidental rotation 

 at one mirror is manual!}' imparted to the other. Care must 

 be taken to keep the angles a and a' unchanged. In other 

 respects the comparison so far as the experiment is concerned 

 was satisfactory (see fig. 4), very little difficulty in adjusting 



Fig. 4. 



14- 16 



and reversing being encountered. With two good slides the 

 work would have proceeded smoothly from end to end of the 

 screws, which were several inches long. 



The first screw had a pitch of *05 centimeter, the micrometer 

 reading to -00010 centimeter ; the second a pitch of -025 

 centimeter, the micrometer reading to -00005 centimeter. The 

 former was turned in steps of a single pitch, and the value 

 which brought the center of ellipses, originally coincident with 

 the given Fraunhofer line, back to coincide with it was read 

 off on the finer screw. 



