134 Dr. Watson on the Determination of the 



which was tested by obtaining Newton's rings between the 

 metal surface and a piece of plane glass. The ends were 

 slightly rounded ; but as this did not amount to more than 

 about five half wave-lengths of sodium light, and this only 

 near the edge, the accuracy o£ the measurements did not 

 necessitate any correction being applied *. 



To measure the lengths of the bars two cylindrical palpers 

 having the same diameter as the bars were prepared, one end 

 o£ each being turned to a spherical surface. A fiducial line 

 was scratched on one cylinder. These cylinders rested in a 

 V-groove in the iron bed of a comparator fitted with micro- 

 meter-microscopes, one palper bedding against a stop. The 

 two palpers being placed in contact, by means o£ a force of 

 70 grams weight, the right microscope was adjusted to the 

 fiducial mark on the left-hand palper. The bar to be mea- 

 sured having been introduced between the palpers, which 

 were kept pressed together by the same force as before, the 

 left- 1) and microscope was adjusted to the fiducial mark, on the 

 left-hand palper. The micrometers having been read, the 

 bar was turned through 90° and the adjustment repeated. In 

 this way four measurements were taken, each measurement 

 consisting of four readings of the micrometers, the bar being 

 turned through a right angle between each measurement. To 

 obtain the distance between the micrometers a nickel steel 

 (Invar) metre which had been calibrated at the International 

 Bureau at Sevres was substituted for the bars, and the micro- 

 meter -readings taken for two divisions 10 centimetres apart, 

 care being taken not to displace either micrometer more than 

 necessary from the position it occupied when setting on the 

 bars. 



In the case of the bars with optically worked ends, the length 

 was also measured by setting the micrometers halfway between 

 the point of a needle and its image as seen reflected in the 

 end surface of the bar. At first this method gave values 

 which were consistently 0*001 centimetre lower than those 

 obtained by the contact method. This difference was found 

 to be due to the objectives of the microscopes not being 

 aplanatic, for in this method only half of the objective of the 

 microscope is used. On replacing the objectives with others 

 which were optically much better, and taking care to focus 

 accurately, the measurements obtained by the two methods 

 were in exact agreement. 



* This question Las also been tested at the National Physical Labora- 

 tory, where they measured the lengths of the generators of two of 

 these cylinders across two diameters, and found that the lengths were 

 constant except at the very edge, and that there the " rounding " did not 

 amount to more than "OOOG cm. 



