﻿392 



Dr. 0. V. Barton on 



the left, and from PB towards the right. Such shutters 

 would, of course, be fixed relatively to the wire HK, LM, 

 and would thus partake of any lateral displacement im- 

 parted micrometrically to the wire ; the observer's view of 

 the diffraction pattern being accordingly restricted to two 

 narrow slits NA, HK and LM, PB. I cannot say that such 

 an arrangement carefully constructed would not be helpful ; 

 but from a trial made with a hastily improvised pair of 

 shutters, it did not appear likely that the gain of sensitive- 

 ness would outweigh the disadvantage of thus obliterating 

 the lateral portions of the pattern y for in this case one could 

 not be sure at a glance that it was indeed the strips AE, 

 GB, which were in view. Possibly a useful compromise 

 might be found in the iise of shutters formed of dark, but 

 not wholly opaque, glass < 



14. The following tables give an analysis of readings 

 taken to test the consistency with which settings could be 

 made. Before each setting, the fine-adjustment head was 

 turned backf at random, leaving a wide discrepancy between 

 the wire and the centre of the diffraction pattern * The 

 adjustment in the forward f direction was then continued 

 until the setting appeared to be centrah and the micrometer 

 reading taken. The discrepancies in the readings must be 

 attributed to errors of setting, combined with such as may 

 have arisen from imperfections of the micrometric motion. 



Table T. 



Eyepiece : a Cylindrical lens of 15 mm. equivalent focal length. 

 Effective surface of mirror 3-5 mm. wide X 4 mm. highs 

 Focal length of object-glass 55 cm. 

 Source of light : an acetylene flame. 



Analysis of 10 successive 



settings^ 





In position of reflected image : — 



(a) In millimetres .... 



(b) In fractional parts of the 1 



width * of the central I 

 bright band j 



Greatest 



deviation 



from mean. 



Quadratic 

 mean devia- 

 tion from 

 mean. 



Probable 



error of a 



single setting. 



•00053 

 1/854 

 010 



•00033 

 1/584 

 0062 



00023 

 1865 

 0042 



In azimuth of mirror (seconds) ... 



* Calculated for X, 



f The terms " hack " and " forward" are used conventionally. 



