﻿388 Dr. C. V. Burton on 



residual strips of light to be seen between the wire and the 

 lines of blackness on either side of it. 



5. If the wire is displaced laterally, one of these residual 

 strips will become brighter and the other fainter, and AE, 

 GB being small compared with AB, it is evident that a 

 displacement of the wire through a distance which is only a 

 small fraction of AB may cause the ratio of integral bright- 

 ness (area ADE : area FGB) to deviate considerably from 

 unity. It appears indeed that, when the illumination is homo- 

 geneous, the sensitiveness of the arrangement is limited only 

 by the intrinsic brightness of the source (cf. § 31). 



6. Some particulars of the actual apparatus may here be 

 given. The focal length of the achromatic objective 

 (fig. 1) was 55 cm. The mirror M was constituted by a 

 silvered glass flat, between which and the objective was a 

 screen pierced by a rectangular hole with bevelled edges. 

 The hole itself measured b'S mm. wide x 4 mm. high ; but 

 the screen was so supported that it could be turned about a 

 vertical axis into an oblique position, thus reducing the 

 width of the effective reflecting surface. In this way a 

 suitable relation could be secured between the scale of the 

 diffraction pattern in the focal plane WS and the thickness 

 of the wire W. Since W and S were some 5 cm. apart, the 

 distance of the perforated screen from the mirror, as well as 

 the obliquity of the screen, must be taken into account in 

 calculating the effective width of reflecting surface, which 

 was found to be 3'5 mm. The wire used was of copper and 

 0*176 mm. in diameter. E was either an ordinary eyepiece 

 of 16 mm. equivalent focal length, or preferably a cylin- 

 drical lens of crown glass of about 15 mm. equivalent focal 

 length. Stray light was excluded : the only light reaching 

 the eye through E was that which had originally passed 

 through S, and the observations were made in a fairly dark 

 cellar. 



7. The details of construction at the eye end of the 

 apparatus, though of rather a makeshift description, may 

 be worth mention, as the performance was, mechanically 

 speaking, quite satisfactory, and others may perhaps wish 

 to put a micrometer together on similar lines. In fig. 3, 

 A is a flat cast-iron plate, whose plane is vertical. F is a 

 Brown and Sharpe micrometer calliper, which is secured 

 between A and a clamping-plate (shown in chain-dot) 

 by means of the stud and nut Fj. To avoid straining 

 the frame of the calliper, a seat for it is formed by three 

 little bosses cast upon the plate A, and the clamping-plate 

 has three corresponding bosses upon its inner side. The 



