﻿386 Dr. C. V. Barton on 



maximum error of 0*1 second of arc, and with a probable error, 

 in a single setting, of 0*042 second. It should be said at 

 once that the flatness of the mirror was in this case prac- 

 tically perfect, the reflecting surface being that of a silvered 

 glass flat, with a stop placed before it ; on the other hand, 

 an acetylene flame was used as the source of illumination, 

 and this is greatly inferior in intrinsic brightness to various 

 incandescent filaments emitting white light. As will appear 

 later, the intrinsic brightness of the source has an important 

 influence on the attainable precision of reading. 



2. It might at first sight appear that the results just 

 referred to were not easily to be reconciled with known 

 optical principles, according to which the most accurately 

 focussed image is a system of diffraction patterns, and mag- 

 nification carried beyond certain limits ceases to be of use, 

 owing to the proportionate increase of diffuseness of the 

 image. But it is by taking advantage of the peculiarities of 

 diffraction patterns that increased refinement in fixing the 

 azimuth of a mirror may be secured ; and to this end it is 

 found convenient to use high magnifications. 



m £" 



Fig. 1. 



S .L P 



3. Fig. 1 is a diagrammatic plan view of the apparatus 

 used. F is a small acetylene flame, an image of which is 

 focussed by the lens L upon a narrow vertical slit S. This 

 is at the principal focus of the object-glass ; light which 

 has passed through S and being reflected from a plane 

 mirror M of rectangular outline, and refocussed in the 

 neighbourhood of the vertical wire W. On looking through 

 an eyepiece E *, a diffraction pattern is seen, consisting of 

 vertical bands : a central white band, separated by very 

 dark almost black intervals from the coloured bands which 

 fade off on each side. Either the slit St, or the frame 



* As will appear below (§30), this is preferably a cylindrical lens 

 •with its generating- lines vertical. 



t Before the slit S a refracting prism (not shown) was fixed, so that 

 the illuminating arrangements could be kept clear of the observer's head. 

 The condensing lens L was carried upon a bracket which moved with 

 the slit, while the flame F was burnt from a jet which could be 

 independently moved along a slide parallel to WS until the best illumi- 

 nation of the field was secured. 



