1,22 PRO< 1 I DINGS OF THE SOCIETY. 



in the ulist, luir values, for from the mean of all the measures the ratio of 



j-son's millimetre to Grayson's inch was Ion ml to be 25* 369 : ] instead 

 of being 25 • 400 : 1, which is the correcl ratio, Grayson's ratio being 

 wrong to the extent of 1 pan in 800. 



It would Beem desirable, however, Mr. Spitta added, seeing so small a 

 pari of the ruling would be used at any time, and that the rulings were 

 otherwise so perfecl and regular, that a portion should be examined at 

 theNational Physical Laboratory, when they were prepared for the work, 

 as be felt suresuch rulings would lie of greal use for Fellows to have a- a 

 means of calibrating their own micrometers. He wished to acknowledge 

 the bind assistance of Mr. Conrady, who had reduced and treated the 

 observation mathematically, so as to obtain therefrom all the information 

 ble. 



.Mr. Conrad Beck said that Mr. Grayson had. during the last year 

 or so, been occupied in making a new screw. The difficulty in making 

 the rulings absolutely correct lay in the fact that there was no existing 

 standard to which it was easy to refer. The Board of Trade were at 

 the present time conducting investigations in Westminster in regard to 

 the measurement of the standard yard, a bar of bronze kept in West- 

 minster, and looked at once in twenty years for the purpose of making 

 trite copies. It was extremely difficult to realize that the standard was 

 purely arbitrary, and so far there had been nothing to refer it to, and 

 that if that standard were by any chance destroyed, our standard yard 

 would be lost. The object of the Board of Trade at the present time 

 was to measure how many wave-lengths of a particular coloured light 

 there were in our standard yard, and for this purpose they had set up 

 an ap] aratus in Westminster to calibrate the standard. He thought 

 that when a standard was subdivided into small amounts it would be 

 extremely difficult to be certain no errors had arisen. He had himself 

 made up a somewhat simpler form of interferometer, and after some 

 experimenting he found that it would be quite possible for an ordinary 

 person to measure a stage micrometer in terms of the wave-length of 

 light. The interferometer depended upon the arrangement of two 

 minors reflecting a beam of light. A beam of light was split into two. 

 and one-half reflected from each mirror. The length of path of the 

 two beams of light could be made identically the same. When in the 



ighbourhood of equality and the length of' the two paths were within 

 a few wave-lengths of light, and the light was recombined and brought 

 hack into the eve. the phenomenon of interference was produced. On 

 the shifting of one of the mirrors, the hands produced by interference 

 we,,, seen to travel across the field, and could be counted. For every 

 motion of one of these hands across the screen, the mirror had to he 

 moved a wave-length of light. If white light were employed, the 

 interference obtained lasted only a very short period, hut in the use of 

 monochromatic light the period over which the motion of the bands 

 could be counted was very considerable— sufficient to measure the whole 

 of Grayson's micrometer : if possible, cadmium light should be used. It 

 would qoj be so easyto measure such lone' micrometers as those reported 

 "ii by Dr. Spitta. He hoped at some future time to be able to show 

 principles of his apparatus, though the difficulties attendant upon 



